Chapter V · (continued 2)

Even in the rest of his work, which is full of refined and curious speculation, he uses facts, not to demonstrate his conclusions, but to illustrate them. He, therefore, selected those facts which suited his purpose, leaving the others untouched. And this, which many critics would call unfair, was not unfair in him; because he believed, that he had already established his principles without the aid of those facts. The facts might benefit the reader, by making the argument clearer, but they could not strengthen the argument. They were more intended to persuade than to prove; they were rather rhetorical than logical. Hence, a critic would waste his time if he were to sift them with a minuteness which would be necessary, supposing that Hume had built an inductive argument upon them. Otherwise, without going far, it might be curious to contrast them with the entirely different facts which Cudworth, eighty years before, had collected from the same source, and on the same subject. Cudworth, who was much superior to Hume in learning, and much inferior to him in genius,[719] displayed, in his great work on the Intellectual System of the Universe, a prodigious erudition, to prove that, in the ancient world, the belief in one God was a prevailing doctrine. Hume, who never refers to Cudworth, arrives at a precisely opposite conclusion. Both quoted ancient writers; but while Cudworth drew his inferences from what he found in those writers, Hume drew his from what he found in his own mind. Cudworth, being more learned, relied on his reading; Hume, having more genius, relied on his intellect. Cudworth, trained in the school of Bacon, first collected the evidence, and then passed the judgment. Hume, formed in a school entirely different, believed that the acuteness of the judge was more important than the quantity of the evidence; that witnesses were likely to prevaricate; and that he possessed, in his own mind, the surest materials for arriving at an accurate conclusion. It is not, therefore, strange, that Cudworth and Hume, pursuing opposite methods, should have obtained opposite results, since such a discrepancy is, as I have already pointed out, unavoidable, when men investigate, according to different plans, a subject which, in the existing state of knowledge, is not amenable to scientific treatment.

The length to which this chapter has already extended, and the number of topics which I have still to handle, will prevent me from examining, in detail, the philosophy of Reid, who was the most eminent among the purely speculative thinkers of Scotland, after Hume and Adam Smith, though, in point of merit, he must be placed far below them. For, he had neither the comprehensiveness of Smith, nor the fearlessness of Hume. The range of his knowledge was not wide enough to allow him to be comprehensive; while a timidity, almost amounting to moral cowardice, made him recoil from the views advocated by Hume, not so much on account of their being false, as on account of their being dangerous. It is, however, certain, that no man can take high rank as a philosopher, who allows himself to be trammelled by considerations of that kind. A philosopher should aim solely at truth, and should refuse to estimate the practical tendency of his speculations. If they are true, let them stand; if they are false, let them fall. But, whether they are agreeable or disagreeable, whether they are consolatory or disheartening, whether they are safe or mischievous, is a question, not for philosophers, but for practical men. Every new truth which has ever been propounded, has, for a time, caused mischief; it has produced discomfort, and often unhappiness, sometimes by disturbing social or religious arrangements, and sometimes merely by the disruption of old and cherished associations of thought. It is only after a certain interval, and when the framework of affairs has adjusted itself to the new truth, that its good effects preponderate; and the preponderance continues to increase, until, at length, the truth causes nothing but good. But, at the outset, there is always harm. And, if the truth is very great, as well as very new, the harm is serious. Men are made uneasy; they flinch; they cannot bear the sudden light; a general restlessness supervenes; the face of society is disturbed, or perhaps convulsed; old interests, and old beliefs, are destroyed, before new ones have been created. These symptoms are the precursors of revolution; they have preceded all the great changes through which the world has passed; and while, if they are not excessive, they forebode progress, so if they are excessive, they threaten anarchy. It is the business of practical men to moderate such symptoms, and to take care that the truths which philosophers discover, are not applied so rashly as to dislocate the fabric, instead of strengthening it. But the philosopher has only to discover the truth, and promulgate it; and that is hard work enough for any man, let his ability be as great as it may. This division of labour, between thinkers and actors, secures an economy of force, and prevents either class from wasting its power. It establishes a difference between science, which ascertains principles, and art, which applies them. It also recognizes, that the philosopher and the practical man, having each a separate part to play, each is, in his own field, supreme. But it is a sad confusion for either to interfere with the other. In their different spheres, both are independent, and both are worthy of admiration. Inasmuch, however, as practical men should never allow the speculative conclusions of philosophers, whatever be their truth, to be put in actual operation, unless society is, in some degree, ripe for their reception; so, on the other hand, philosophers are not to hesitate, and tremble, and stop short in their career, because their intellect is leading them to conclusions subversive of existing interests. The duty of a philosopher is clear. His path lies straight before him. He must take every pains to ascertain the truth; and, having arrived at a conclusion, he, instead of shrinking from it because it is unpalatable, or because it seems dangerous, should on that very account, cling the closer to it, should uphold it in bad repute, more zealously than he would have done in good repute; should noise it abroad far and wide, utterly regardless what opinions he shocks, or what interests he imperils; should, on its behalf, court hostility and despise contempt, being well assured, that, if it is not true, it will die, but that, if it is true, it must produce ultimate benefit, albeit unsuited for practical adoption by the age or country in which it is first propounded.

But Reid, notwithstanding the clearness of his mind and his great powers of argument, had so little of the real philosophic spirit, that he loved truth, not for its own sake, but for the sake of its immediate and practical results. He himself tells us, that he began to study philosophy, merely because he was shocked at the consequences at which philosophers had arrived. As long as the speculations of Locke and of Berkeley were not pushed to their logical conclusions, Reid acquiesced in them, and they were good in his eyes.[720] While they were safe and tolerably orthodox, he was not over-nice in inquiring into their validity. In the hands of Hume, however, philosophy became bolder and more inquisitive; she disturbed opinions which were ancient, and which it was pleasant to hold; she searched into the foundation of things, and by forcing men to doubt and to inquire, she rendered inestimable service to the cause of truth. But this was precisely the tendency at which Reid was displeased. He saw that such disturbance was uncomfortable; he saw that it was hazardous; therefore, he endeavoured to prove that it was groundless. Confusing the question of practical consequences with the totally different question of scientific truth, he took for granted that, because to his age the adoption of those consequences would be mischievous, they must be false. To the profound views of Hume respecting causation, he gravely objects, that if they were carried into effect, the operation of criminal law would be imperilled.[721] To the speculations of the same philosopher concerning the metaphysical basis of the theory of contracts, he replies, that such speculations perplex men, and weaken their sense of duty; they are, therefore, to be disapproved of, on account of their tendency.[722] With Reid, the main question always is, not whether an inference is true, but what will happen if it is true. He says, that a doctrine is to be judged by its fruits;[723] forgetting that the same doctrine will bear different fruits in different ages, and that the consequences which a theory produces in one state of society, are often diametrically opposed to those which it produces in another. He thus made his own age the standard of all future ones. He also trammelled philosophy with practical considerations; diverting thinkers from the pursuit of truth, which is their proper department, into the pursuit of expediency, which is not their department at all. Reid was constantly stopping to inquire, not whether theories were accurate, but whether it was advisable to adopt them; whether they were favourable to patriotism, or to generosity, or to friendship;[724] in a word, whether they were comfortable, and such as we should at present like to believe.[725] Or else, he would take other ground, still lower, and still more unworthy of a philosopher. In opposing, for instance, the doctrine, that our faculties sometimes deceive us,—a doctrine which, as he well knew, had been held by men whose honesty was equal to his own, and whose ability was superior to his own,—he does not scruple to enlist on his side the prejudices of a vulgar superstition; seeking to blacken the tenet which he was unable to refute. He actually asserts, that they who advocate it, insult the Deity, by imputing to the Almighty that He has lied. Such being the consequence of the opinion, it of course follows that the opinion must be rejected without further scrutiny, since, to accept it, would produce fatal results on our conduct, and would, indeed, be subversive of all religion, of all morals, and of all knowledge.[726]

In 1764, Reid published his Inquiry into the Human Mind; and in that, and in his subsequent work, entitled Essays on the Powers of the Mind, he sought to destroy the philosophy of Locke, Berkeley, and Hume. And as Hume was the boldest of the three, it was chiefly his philosophy which Reid attacked. Of the character of this attack, some specimens have just been given; but they rather concern his object and motives, while what we have now to ascertain is, his method, that is, the tactics of his warfare. He clearly saw, that Hume had assumed certain principles, and had reasoned deductively from them to the facts, instead of reasoning inductively from the facts to them. To this method, he strongly, and perhaps fairly, objects. He admits that Hume had reasoned so accurately, that if his principles were conceded, his conclusions must likewise be conceded.[727] But, he says, Hume had no right to proceed in such a manner. He had no right to assume principles, and then to argue from them. The laws of nature were to be arrived at, not by conjecturing in this way, but by a patient induction of facts.[728] Discoveries depended solely on observation and experiment; and any other plan could only produce theories, ingenious, perhaps, and plausible, but quite worthless.[729] For, theory should yield to fact, and not fact to theory.[730] Speculators, indeed, might talk about first principles, and raise a system by reasoning from them. But, the fact was, that there was no agreement as to how a first principle was to be recognized; since a principle which one man would deem self-evident, another would think it necessary to prove, and a third would altogether deny.[731]

The difficulties of deductive reasoning are here admirably portrayed. It might have been expected, that Reid would have built up his own philosophy according to the inductive plan, and would have despised that assumption of first principles, with which he taunts his opponents. But it is one of the most curious things in the history of metaphysics, that Reid after impeaching the method of Hume, follows the very same method himself. When he is attacking the philosophy of Hume, he holds deduction to be wrong. When he is raising his own philosophy, he holds it to be right. He deemed certain conclusions dangerous, and he objects to their advocates, that they argued from principles, instead of from facts: and that they assumed themselves to be in possession of the first principles of truth, although people were not agreed as to what constituted a first principle. This is well put, and hard to answer. Strange, however, to say, Reid arrives at his own conclusions, by assuming first principles to an extent far greater than had been done by any writer on the opposite side. From them, he argues; his whole scheme is deductive; and his works scarcely contain a single instance of that inductive logic, which, when attacking his opponents, he found it convenient to recommend. It is difficult to conceive a better illustration of the peculiar character of the Scotch intellect in the eighteenth century, and of the firm hold, which, what may be called, the anti-Baconian method, had upon that intellect. Reid was a man of considerable ability, of immaculate honesty, and was deeply convinced that it was for the good of society that the prevailing philosophy should be overthrown. To the performance of that task, he dedicated his long and laborious life; he saw that the vulnerable point of the adverse system was its method; he indicated the deficiencies of that method, and declared, perhaps wrongly, but at all events sincerely, that it could never lead to truth. Yet, and notwithstanding all this, such was the pressure of the age in which he lived, and so completely did the force of circumstances shape his understanding, that, in his own works, he was unable to avoid that very method of investigation which he rebuked in others. Indeed, so far from avoiding it, he was a slave to it. The evidence of this I will now give, because, besides its importance for the history of the Scotch mind, it is valuable as one of many lessons, which teach us how we are moulded by the society which surrounds us; how even our most vigorous actions are influenced by general causes of which we are often ignorant, and which few of us care to study; and, finally, how lame and impotent we are, when, as individuals, we try to stem the onward current, resisting the great progress instead of aiding it, and vainly opposing our little wishes to that majestic course of events, which admits of no interruption, but sweeps on, grand and terrible, while generation after generation passes away, successively absorbed in one mighty vortex.

Directly Reid, ceasing to refute the philosophy of Hume, began to construct his own philosophy, he succumbed to the prevailing method. He now assures us, that all reasoning must be from first principles, and that, so far from reasoning to those principles, we must at once admit them, and make them the basis of all subsequent arguments.[732] Having admitted them, they become a thread to guide the inquirer through the labyrinth of thought.[733] His opponents had no right to assume them, but he might do so, because to him they were intuitive.[734] Whoever denied them, was not fit to be reasoned with.[735] Indeed, to investigate them, or to seek to analyze them, was wrong as well as foolish, because they were part of the constitution of things; and of the constitution of things no account could be given, except that such was the will of God.[736]

As Reid obtained his first principles with such ease, and as he carefully protected them by forbidding any attempt to resolve them into simpler elements, he was under a strong temptation to multiply them almost indefinitely, in order that, by reasoning from them, he might raise a complete and harmonious system of the human mind. To that temptation he yielded with a readiness, which is truly surprising, when we remember how he reproached his opponents with doing the same thing. Among the numerous first principles which he assumes, not only as unexplained, but as inexplicable, are the belief in Personal Identity;[737] the belief in the External World;[738] the belief in the Uniformity of Nature;[739] the belief in the Existence of Life in Others;[740] the belief in Testimony;[741] also in the power of distinguishing truth from error,[742] and even in the correspondence of the face and voice to the thoughts.[743] Of belief generally, he asserts that there are many principles,[744] and he regrets that any one should have rashly attempted to explain them.[745] Such things are mysterious, and not to be pried into. We have also other faculties, which being original and indecomposable, resist all inductive treatment, and can neither be resolved into simpler elements, nor referred to more general laws. To this class, Reid assigns Memory,[746] Perception,[747] Desire of Self-Approbation,[748] and not only Instinct, but even Habit.[749] Many of our ideas, such as those concerning Space and Time, are equally original;[750] and other first principles there are, which have not been enunciated, but from which we may reason.[751] They, therefore, are the major premisses of the argument; no reason having yet been given for them, they must be simple; and not having yet been explained, they are, of course, inexplicable.[752]

All this is arbitrary enough. Still, in justice to Reid, it must be said, that, having made these assumptions, he displayed remarkable ability in arguing from them, and that, in attacking the philosophy of his time, he subjected it to a criticism, which has been extremely serviceable. His lucidity, his dialectic skill, and the racy and masculine style in which he wrote, made him a formidable opponent, and secured to his objections a respectful hearing. To me, however, it appears, that notwithstanding the attempts, first of M. Cousin, and afterwards of Sir William Hamilton, to prop up his declining reputation, his philosophy, as an independent system, is untenable, and will not live. In this I may be mistaken; but what is quite certain is, that nothing can be more absurd than to suppose, as some have done, that he adopted the inductive, or, as it is popularly called, Baconian method. Bacon, indeed, would have smiled at such a disciple, assuming all sorts of major premisses, taking general principles for granted with the greatest recklessness, and reserving his skill for the task of reasoning from propositions for which he had no evidence, except that on a cursory, or, as he termed it, a common-sense, inspection, they appeared to be true.[753] This refusal to analyze preconceived notions, comes under the head of what Bacon stigmatized as the anticipatio naturæ, and which he deemed the great enemy of knowledge, on account of the dangerous confidence it places in the spontaneous and uncorrected conclusions of the human mind. When, therefore, we find Reid holding up the Baconian philosophy, as a pattern which it behoves all inquirers to follow;[754] and when we, moreover, find Dugald Stewart, who, though a somewhat superficial thinker, was, at all events, a careful writer, supposing that Reid had followed it,[755] we meet with fresh proof of how difficult it was for Scotchmen of the last age to imbibe the true spirit of inductive logic, since they believed, that a system which flagrantly violated its rules, had been framed in strict accordance with them.

Leaving mental philosophy, I now come to physical science, in which, if anywhere, we might expect that the inductive plan would predominate, and would triumph over the opposite, or deductive, one. How far this was the case, I will endeavour to ascertain, by an examination of the most important discoveries which have been made by Scotchmen concerning the organic and inorganic world. And, as my object is merely to indicate the turn and character of the Scotch mind, I shall avoid all details respecting the practical effects of those discoveries, and shall confine myself to such a narration as will exhibit their purely scientific aspect, so as to enable the reader to understand what additions were made to our knowledge of the laws of nature, and in what way the additions were made. The character of each discovery, and its process, will be stated, but nothing more. Neither here, nor in any part of this Introduction, do I pretend to investigate questions of practical utility, or to trace the connexion between the discoveries of science and the arts of life. That I shall do in the body of the work itself, where I hope to explain a number of minute social events, many of which are regarded as isolated, if not incongruous. For the present, I solely aim at those broad principles, which, by marking out the epochs of thought, underlie the whole fabric of society, and which must be clearly apprehended before history can cease to be a mere empirical assemblage of facts, of which the scientific basis being unsettled, the true order and coherence must be unknown.

Among the sciences which concern the inorganic world, the laws of heat occupy a conspicuous place. On the one hand, they are connected with geology, being intimately allied, and, indeed, necessarily bound up, with every speculation respecting the changes and present condition of the crust of the earth. On the other hand, they touch the great questions of life, both animal and vegetable; they have to do with the theory of species, and of race; they modify soil, food, and organization; and to them we must look for valuable help towards solving those great problems in biology, which, of late years, have occupied the attention of the boldest and most advanced philosophers.

Our present knowledge of the laws of heat, may be briefly stated as branching into five fundamental divisions. These are: latent heat; specific heat; the conduction of heat; the radiation of heat; and, finally, the undulatory theory of heat; by which last, we are gradually discarding our old material views, and are accustoming ourselves to look upon heat as simply one of the forms of force, all of which, such as light, electricity, magnetism, motion, gravitation, and chemical affinity, are constantly assuming each other's shape, but, in their total amount, are incapable either of increase or of diminution.[756] This grand conception, which is now placing the indestructibility of force on the same ground as the indestructibility of matter, has an importance far above its scientific value, considerable as that undoubtedly is. For, by teaching us, that nothing perishes, but that, on the contrary, the slightest movement of the smallest body, in the remotest region, produces results which are perpetual, which diffuse themselves through all space, and which, though they may be metamorphosed, cannot be destroyed, it impresses us with such an exalted idea of the regular and compulsory march of physical affairs, as must eventually influence other and higher departments of inquiry. Our habits of thought are so connected and interwoven, that notions of law and of the necessary concatenation of things, can never be introduced into one field of speculation, without affecting other fields which lie contiguous to it. When, therefore, the modern doctrine of conservation of force,[757] becomes firmly coupled with the older doctrine of conservation of matter, we may rest assured that the human mind will not stop there, but will extend to the study of Man, inferences analogous to those already admitted in the study of Nature. Having once recognized that the condition of the material universe, at any one moment, is simply the result of every thing which has happened at all preceding moments, and that the most trivial disturbance would so violate the general scheme, as to render anarchy inevitable, and that, to sever from the total mass even the minutest fragment, would, by dislocating the structure, bury the whole in one common ruin, we, thus admitting the exquisite adjustment of the different parts, and discerning, too, in the very beauty and completeness of the design, the best proof that it has never been tampered with by the Divine Architect, who called it into being, in whose Omniscience both the plan, and the issue of the plan, resided with such clearness and unerring certainty, that not a stone in that superb and symmetrical edifice has been touched since the foundation of the edifice was laid, are, by ascending to this pitch and elevation of thought, most assuredly advancing towards that far higher step, which it will remain for our posterity to take, and which will raise their view to so commanding a height, as to insure the utter rejection of those old and eminently irreligious dogmas of supernatural interference with the affairs of life, which superstition has invented, and ignorance has bequeathed, and the present acceptance of which betokens the yet early condition of our knowledge, the penury of our intellectual resources, and the inveteracy of the prejudices in which we are still immersed.

It is, therefore, natural, that the physical doctrine of indestructibility applied to force as well as to matter, should be essentially a creation of the present century, notwithstanding a few allusions made to it by some earlier thinkers, all of whom, however, groped vaguely, and without general purpose. No preceding age was bold enough to embrace so magnificent a view as a whole, nor had any preceding philosophers sufficient acquaintance with nature to enable them to defend such a conception, even had they desired to entertain it. Thus, in the case now before us, it is evident, that while heat was believed to be material, it could not be conceived as a force, and therefore no one could grasp the theory of its metamorphosis into other forces; though there are passages in Bacon which prove that he wished to identify it with motion. It was first necessary to abstract heat into a mere property or affection of matter, and there was no chance of doing this until heat was better understood in its immediate antecedents, that is, until, by the aid of mathematics, its proximate laws had been generalized. But, with the single exception of Newton, whose efforts, notwithstanding his gigantic powers, were, on this subject, very unsatisfactory, and who, moreover, had a decided leaning towards the material theory, no one attempted to unravel the mathematical laws of heat till the latter half of the eighteenth century, when Lambert and Black began the career which Prevost and Fourier followed up. The mind, having been so slow in mastering the preliminaries and outworks of the inquiry, was not ripe for the far more difficult enterprise of idealizing heat itself, and so abstracting it, as to strip it of its material attributes, and leave it to nothing but the speculative notion of an immaterial force.

From these considerations, which were necessary to enable the reader to appreciate the value of what was done in Scotland, it will be seen how essential it was that the laws of the movement of heat should be studied before its nature was investigated, and before the emission theory could be so seriously attacked as to allow of the possibility of that great doctrine of the indestructibility of force, which, I make no doubt, is destined to revolutionize our habits of thought, and to give to future speculations a basis infinitely wider than any previously known. In regard to the movements of heat, we owe the laws of conduction and of radiation chiefly to France and Geneva, while the laws of specific heat, and those of latent heat, were discovered in Scotland. The doctrine of specific heat, though interesting, has not the scientific importance which belongs to the other departments of this great subject; but the doctrine of latent heat is extremely curious, not only in itself, but also on account of the analogies it suggests with various branches of physical inquiry.

What is termed latent heat, is exhibited in the following manner. If, in consequence of the application of heat, a solid passes into a liquid, as ice, for instance, into water, the conversion occupies a longer time than could be explained by any theory which had been propounded down to the middle of the eighteenth century. Neither was it possible to explain how it is, that ice never rises above the temperature of 32° until it is actually melted, no matter what the heat of the adjacent bodies may be. There were no means of accounting for these circumstances. And though practical men, being familiar with them, did not wonder at them, they caused great astonishment among thinkers, who were accustomed to analyze events, and to seek a reason for common and every-day occurrences.

Soon after the middle of the eighteenth century, Black, who was then one of the professors in the University of Glasgow, turned his attention to this subject.[758] He struck out a theory which, being eminently original, was violently attacked, but is now generally admitted. With a boldness and reach of thought not often equalled, he arrived at the conclusion, that whenever a body loses some of its consistence, as in the case of ice becoming water, or water becoming steam, such body receives an amount of heat which our senses, though aided by the most delicate thermometer, can never detect. For, this heat is absorbed; we lose all sight of it, and it produces no palpable effect on the material world, but becomes, as it were, a hidden property. Black, therefore, called it latent heat, because, though we conceive it as an idea, we cannot trace it as a fact. The body is, properly speaking, hotter; and yet its temperature does not rise. Directly, however, the foregoing process is inverted, that is to say, directly the steam is condensed into water, or the water hardened into ice, the heat returns into the world of sense; it ceases to be latent, and communicates itself to the surrounding objects. No new heat has been created; it has, indeed, appeared and disappeared, so far as our senses are concerned; but our senses were deceived, since there has, in truth, been neither addition nor diminution.[759] That this remarkable theory paved the way for the doctrine of the indestructibility of force, will be obvious to whoever has examined the manner in which, in the history of the human mind, scientific conceptions are generated. The process is always so slow, that no single discovery has ever been made, except by the united labours of several successive generations. In estimating, therefore, what each man has done, we must judge him, not by the errors he commits, but by the truths he propounds. Most of his errors are not really his own. He inherits them from his predecessors; and if he throws some of them off, we should be grateful, instead of being dissatisfied that he has not rejected all. Black, no doubt, fell into the error of regarding heat as a material substance, which obeys the laws of chemical composition.[760] But this was merely an hypothesis, which was bequeathed to him, and with which the existing state of thought forced him to encumber his theory. He inherited the hypothesis, and could not get rid of his troublesome possession. The real service which he rendered is, that, in spite of that hypothesis, which clung to him to the last, he, far more than any of his contemporaries, contributed towards the great conception of idealizing heat, and thus enabled his successors to admit it into the class of immaterial and supersensual forces. Once admitted into that class, the list of forces became complete; and it was comparatively easy to apply to the whole body of force, the same notion of indestructibility, which had previously been applied to the whole body of matter. But it was hardly possible to effect this object, while heat stood, as it were, midway between force and matter, yielding opposite results to different senses; amenable to the touch, but invisible to the eye. What was wanting, was to remove it altogether out of the jurisdiction of the senses, and to admit that, though we experience its effects, we can only conceive its existence. Towards accomplishing this, Black took a prodigious stride. Unconscious, perhaps, of the remote tendency of his own labours, he undermined that doctrine of material heat, which he seemed to support. For, by his advocacy of latent heat, he taught that its movements constantly baffle, not only some of our senses, but all of them; and that, while our feelings make us believe that heat is lost, our intellect makes us believe that it is not lost. Here, we have apparent destructibility, and real indestructibility. To assert that a body received heat without its temperature rising, was to make the understanding correct the touch, and defy its dictates. It was a bold and beautiful paradox, which required courage as well as insight to broach, and the reception of which marks an epoch in the human mind, because it was an immense step towards idealizing matter into force. Some, indeed, have spoken of invisible matter; but that is a contradiction in terms, which will never be admitted, as long as the forms of speech remain unchanged. Nothing can be invisible, except force, mind, and the Supreme Cause of all. We must, therefore, ascribe to Black the signal merit that he first, in the study of heat, impeached the authority of the senses, and thereby laid the foundation of every thing which was afterwards done. Besides the relation which his discovery bears to the indestructibility of force, it is also connected with one of the most splendid achievements effected by this generation in inorganic physics; namely, the establishment of the identity of light and heat. To the senses, light and heat, though in some respects similar, are in most respects dissimilar. Light, for instance, affects the eye, and not the touch. Heat affects the touch, but, under ordinary circumstances, does not affect the eye. The capital difference, however, between them is, that heat, unlike light, possesses the property of temperature; and this property is so characteristic, that until our understandings are invigorated by science, we cannot conceive heat separated from temperature, but are compelled to confuse one with the other. Directly, however, men began to adopt the method followed by Black, and were resolved to consider heat as supersensual, they entered the road which led to the discovery of light and heat being merely different developments of the same force. Ignoring the effects of heat on themselves, or on any part of the creation, which was capable of feeling its temperature, and would therefore be deceived by it, nothing was left for them to do, but to study its effects on the inanimate world. Then, all was revealed. The career of discovery was fairly opened; and analogies between light and heat, which even the boldest imagination had hardly suspected, were placed beyond a doubt. To the reflection of heat, which had been formerly known, were now added the refraction of heat, its double refraction, its polarization, its depolarization, its circular polarization, the interference of its rays, and their retardation; while, what is more remarkable than all, the march of our knowledge on these points was so swift, that before the year 1836 had come to a close, the chain of evidence was completed by the empirical investigations of Forbes and Melloni, they themselves little witting that every thing which they accomplished was prepared before they were born, that they were but the servants and followers of him who indicated the path in which they trod, and that their experiments, ingenious as they were, and full of resource, were simply the direct practical consequence of one of those magnificent ideas which Scotland has thrown upon the world, and the memory of which is almost enough so to bribe the judgment, as to tempt us to forget, that, while the leading intellects of the nation were engaged in such lofty pursuits, the nation itself, untouched by them, passed them over with cold and contemptuous indifference, being steeped in that deadening superstition, which turns a deaf ear to every sort of reason, and will not hearken to the voice of the charmer, charm he never so wisely.

By thus considering the descent and relationship of scientific conceptions, we can alone understand what we really owe to Black's discovery of latent heat. In regard to the method of the discovery, little need be said, since every student of the Baconian philosophy must see, that the discovery was of a kind for which none of the maxims of that system had provided. As latent heat escapes the senses, it could not obey the rules of a philosophy, which grounds all truth on observation and experiment. The subject of the inquiry being supersensual, there was no scope for what Bacon called crucial experiments and separations of nature. The truth was in the idea; experiments, therefore, might illustrate it, might bring it up to the surface, and so enable men to grasp it, but could not prove it. And this, which appears on the very face of the discovery, is confirmed by the express testimony of Dr. Thomson, who knew Black, and was, indeed, one of the most eminent of his pupils. We are assured by this unimpeachable witness, that Black, about the year 1759 began to speculate concerning heat; that the result of those speculations was the theory of latent heat; that he publicly taught that theory in the year 1761; but that the experiments which were necessary to convince the world of it were not made till 1764,[761] though, as I need hardly add, according to the inductive method, it was a breach of all the rules of philosophy to be satisfied with the theory three years before the experiments were made, and it was a still greater breach, not only to be satisfied with it, but to have openly promulgated it as an original and unquestionable truth, which explained, in a new manner, the economy of the material world.

The intellect of Black belonged to a class, which, in the eighteenth century, was almost universal in Scotland, but was hardly to be found in England, and which, for want of a better word, we are compelled to call deductive, though fully admitting that even the most deductive minds have in them a large amount of induction, since, indeed, without induction, the common business of life could not be carried on. But for the purposes of scientific classification, we may say, that a man or an age is deductive, when the favourite process is reasoning from principles instead of reasoning to them, and when there is a tendency to underrate the value of specific experience. That this was the case with the illustrious discoverer of latent heat, we have seen, both from the nature of the discovery, and from the decisive testimony of his friend and pupil. And a further confirmation may be found in the circumstance, that, having once propounded his great idea, he, instead of instituting a long series of laborious experiments, by which it might be verified in its different branches, preferred reasoning from it according to the general maxims of dialectic; pushing it to its logical consequences, rather than tracking it into regions where the senses might either confirm or refute it.[762] By following this process of thought, he was led to some beautiful speculations, which are so remote from experience, that even now, with all the additional resources of our knowledge, we cannot tell whether they are true or false. Of this kind were his views respecting the causes of the preservation of man, whose existence would, he thought, be endangered, except for the power which heat possesses of lying latent and unobserved. Thus, for example, when a long and severe winter was followed by sudden warmth, it appeared natural that the ice and snow should melt with corresponding suddenness; and if this were to happen, the result would be such terrible inundations, that it would be hardly possible for man to escape from their ravages. Even if he escaped, his works, that is, the material products of his civilization, would perish. From this catastrophe, nothing saves him but the latent power of heat. Owing to this power, directly the ice and snow begin to melt at their surface, the heat enters their structure, where a large part of it remains in abeyance, and thus losing much of its power, the process of liquefaction is arrested. This dreadful agent is lulled, and becomes dormant. It is weakened at the outset of its career, and is laid up, as in a storehouse, from which it can afterwards emerge, gradually, and with safety to the human species.[763]

In this way, as summer advances, a vast magazine of heat is accumulated, and is preserved in the midst of water, where it can do man no injury, since, indeed, his senses are unable to feel it. There the heat remains buried, until, in the rotation of the seasons, winter returns, and the waters are congealed into ice. In the process of congelation, that treasury of heat, which had been hidden all the summer, reappears; it ceases to be latent; and now, for the first time, striking the senses of man, it tempers, on his behalf, the severity of winter. The faster the water freezes, the faster the heat is disengaged; so that, by virtue of this great law of nature, cold actually generates warmth, and the inclemency of every season, though it cannot be hindered, is softened in proportion as the inclemency is more threatening.[764]

Thus, again, inasmuch as heat becomes latent, and flies from the senses, not only when ice is passing into water, but also when water is passing into steam, we find in this latter circumstance, one of the reasons why man and other animals can live in the tropics, which, but for this, would be deserted. They are constantly suffering from the heat which is collected in their bodies, and which, considered by itself, is enough to destroy them. But this heat causes thirst, and they consequently swallow great quantities of fluid, much of which exudes through the pores of the skin in the form of vapour. And as, according to the theory of latent heat, vapour cannot be produced without a vast amount of heat being buried within it, such vapour absorbs and carries off from the body, that which, if left in the system, would prove fatal. To this we must add, that, in the tropics, the evaporation of water is necessarily rapid, and the vapour which is thus produced, becomes another storehouse of heat, and a vehicle by which it is removed from the earth, and prevented from unduly interfering with the economy of life.[765]

From these and many other arguments, all of which were so essentially speculative, and dealt with such hidden processes of nature, that even now we are not justified either in confidently admitting them or in positively denying them, Black was led to that great doctrine of the indestructibility of heat,[766] which, as I have pointed out, has, in its connexion with the indestructibility of force, a moral and social importance even superior to its scientific value. Though the evidence of which he was possessed was far more scanty than what we now have, he, by the reach of his commanding intellect, rather than by the number and accuracy of his facts, became so penetrated with a conviction of the stability of physical affairs, that he not only applied that idea to the subtle phenomena of heat, but, what was much harder to do, he applied it to cases in which heat so entirely escapes the senses, that man has no cognizance of it, except through the medium of the imagination. According to his view, heat passes through an immense variety of changes, during which it appears to be lost; changes which no eye can ever see, which no touch can ever experience, and which no instrument can ever measure. Still, and in the midst of all these changes, it remains intact. From it nothing can be taken, and to it nothing can be added. In one of those fine passages of his Lectures, which, badly reported as they are,[767] bear the impress of his elevated genius, Black, after stating what would probably happen, if the total amount of heat existing in the world were to be diminished, proceeds to speculate on the consequences of its being increased. Were it possible for any power to add to it ever so little, it would at once overstep its bounds; the equilibrium would be disturbed; the framework of affairs would be disjointed. The evil rapidly increasing, and acting with accumulated force, nothing would be able to stop its ravages. It must continue to gain ground, till all other principles are absorbed and conquered. Sweeping on, unhindered, and irresistible, before it, every animal must perish, the whole vegetable world must disappear, the waters must pass into vapour, and the solid parts of the globe be merged and melted, until, at length, the glorious fabric, loosened and dissolved, would fall away, and return to that original chaos out of which it had been evolved.[768]

These, like many other of the speculations of this great thinker, will find small favour with those purely inductive philosophers, who not only suppose, perhaps rightly, that all our knowledge is in its beginning built upon facts, but who countenance, what seems to me, the very dangerous opinion, that every increase of knowledge must be preceded by an increase of facts. To such men it will appear, that Black had far better have occupied himself in making new observations, or devising new experiments, than in thus indulging his imagination in wild and unprofitable dreams. They will think, that these flights of fancy are suitable, indeed, to the poet, but unworthy of that severe accuracy, and of that close attention to facts, which ought to characterize a philosopher. In England, especially, there is, among physical inquirers, an avowed determination to separate philosophy from poetry, and to look upon them, not only as different, but as hostile. Among that class of thinkers, whose zeal and ability are beyond all praise, and to whom we owe almost unbounded obligations, there does undoubtedly exist a very strong opinion, that, in their own pursuit, the imagination is extremely dangerous, as leading to speculations, of which the basis is not yet assured, and generating a desire to catch too eagerly at distant glimpses before the intermediate ground has been traversed. That the imagination has this tendency is undeniable. But they who object to it on this account, and who would, therefore, divorce poetry from philosophy, have, I apprehend, taken a too limited view of the functions of the human mind, and of the manner in which truth is obtained. There is, in poetry, a divine and prophetic power, and an insight into the turn and aspect of things, which, if properly used, would make it the ally of science instead of the enemy. By the poet, nature is contemplated on the side of the emotions; by the man of science, on the side of the understanding. But the emotions are as much a part of us as the understanding: they are as truthful; they are as likely to be right. Though their view is different, it is not capricious. They obey fixed laws; they follow an orderly and uniform course; they run in sequences; they have their logic and method of inference. Poetry, therefore, is a part of philosophy, simply because the emotions are a part of the mind. If the man of science despises their teaching, so much the worse for him. He has only half his weapons; his arsenal is unfilled. Conquests, indeed, he may make, because his native strength may compensate the defects of his equipment. But his success would be more complete and more rapid, if he were properly furnished and made ready for the battle. And I cannot but regard as the worst intellectual symptom of this great country, what I must venture to call the imperfect education of physical philosophers, as exhibited both in their writings and in their trains of thought. This is the more serious, because they, as a body, form the most important class in England, whether we look at their ability, or at the benefits we have received from them, or at the influence they are exercising, and are likely to exercise, over the progress of society. It cannot, however, be concealed, that they display an inordinate respect for experiments, an undue love of minute detail, and a disposition to overrate the inventors of new instruments, and the discoverers of new, but often insignificant, facts. Their predecessors of the seventeenth century, by using hypotheses more boldly, and by indulging their imagination more frequently, did certainly effect greater things, in comparison with the then state of knowledge, than our contemporaries, with much superior resources, have been able to achieve. The magnificent generalizations of Newton and Harvey could never have been completed in an age absorbed in one unvarying round of experiments and observations. We are in that predicament, that our facts have outstripped our knowledge, and are now encumbering its march. The publications of our scientific institutions, and of our scientific authors, overflow with minute and countless details, which perplex the judgment, and which no memory can retain. In vain do we demand that they should be generalized, and reduced into order. Instead of that, the heap continues to swell. We want ideas, and we get more facts. We hear constantly of what nature is doing, but we rarely hear of what man is thinking. Owing to the indefatigable industry of this and the preceding century, we are in possession of a huge and incoherent mass of observations, which have been stored up with great care, but which, until they are connected by some presiding idea, will be utterly useless. The most effective way of turning them to account, would be to give more scope to the imagination, and incorporate the spirit of poetry with the spirit of science. By this means, our philosophers would double their resources, instead of working, as now, maimed, and with only half their nature. They fear the imagination, on account of its tendency to form hasty theories. But, surely, all our faculties are needed in the pursuit of truth, and we cannot be justified in discrediting any part of the human mind. And I can hardly doubt, that one of the reasons why we, in England, made such wonderful discoveries during the seventeenth century, was because that century was also the great age of English poetry. The two mightiest intellects our country has produced are Shakspeare and Newton; and that Shakspeare should have preceded Newton, was, I believe, no casual or unmeaning event. Shakspeare and the poets sowed the seed, which Newton and the philosophers reaped. Discarding the old scholastic and theological pursuits, they drew attention to nature, and thus became the real founders of all natural science. They did even more than this. They first impregnated the mind of England with bold and lofty conceptions. They taught the men of their generation to crave after the unseen. They taught them to pine for the ideal, and to rise above the visible world of sense. In this way, by cultivating the emotions, they opened one of the paths which lead to truth. The impetus which they communicated, survived their own day, and, like all great movements, was felt in every department of thought. But now it is gone; and, unless I am greatly mistaken, physical science is at present suffering from its absence. Since the seventeenth century, we have had no poet of the highest order, though Shelley, had he lived, would perhaps have become one. He had something of that burning passion, that sacred fire, which kindles the soul, as though it came fresh from the altar of the gods. But he was cut off in his early prime, when his splendid genius was still in its dawn. If we except his immature, though marvellous, efforts, we may assuredly say, that, for nearly two hundred years, England has produced no poetry which bears those unmistakable marks of inspiration which we find in Spenser, in Shakspeare, and in Milton. The result is, that we, separated by so long an interval from those great feeders of the imagination, who nurtured our ancestors, and being unable to enter fully into the feelings of poets, who wrote when nearly all opinions, and, therefore, nearly all forms of emotion, were very different to what they now are, cannot possibly sympathize with those immortal productions so closely as their contemporaries did. The noble English poetry of the sixteenth and seventeenth centuries is read more than ever, but it does not colour our thoughts; it does not shape our understandings, as it shaped the understandings of our forefathers. Between us and them is a chasm, which we cannot entirely bridge. We are so far removed from the associations amid which those poems were composed, that they do not flash upon us with that reality and distinctness of aim, which they would have done, had we lived when they were written. Their garb is strange, and belongs to another time. Not merely their dialect and their dress, but their very complexion and their inmost sentiments, tell of bygone days, of which we have no firm hold. There is, no doubt, a certain ornamental culture, which the most highly educated persons receive from the literature of the past, and by which they sometimes refine their taste, and sometimes enlarge their ideas. But the real culture of a great people, that which supplies each generation with its principal strength, consists of what is learnt from the generation immediately preceding. Though we are often unconscious of the process, we build nearly all our conceptions on the basis recognized by those who went just before us. Our closest contact is, not with our forefathers, but with our fathers. To them we are linked by a genuine affinity, which, being spontaneous, costs us no effort, and from which, indeed, we cannot escape. We inherit their notions, and modify them, just as they modified the notions of their predecessors. At each successive modification, something is lost and something is gained, until, at length, the original type almost disappears. Therefore it is, that ideas entertained several generations ago, bear about the same relation to us, as ideas preserved in a foreign literature. In both cases, the ideas may adorn our knowledge, but they are never so thoroughly incorporated with our minds, as to be the knowledge itself. The assimilation is incomplete, because the sympathy is incomplete. We have now no great poets; and our poverty in this respect is not compensated by the fact, that we once had them, and that we may, and do, read their works. The movement has gone by; the charm is broken; the bond of union, though not cancelled, is seriously weakened. Hence, our age, great as it is, and, in nearly all respects, greater than any the world has yet seen, has, notwithstanding its large and generous sentiments, its unexampled toleration, its love of liberty, and its profuse, and almost reckless, charity, a certain material, unimaginative, and unheroic character, which has made several observers tremble for the future. So far as I can understand our present condition, I do not participate in these fears, because I believe that the good we have already gained, is beyond all comparison greater than what we have lost. But that something has been lost, is unquestionable. We have lost much of that imagination, which, though, in practical life, it often misleads, is, in speculative life, one of the highest of all qualities, being suggestive as well as creative. Even practically, we should cherish it, because the commerce of the affections mainly depends on it. It is, however, declining; while, at the same time, the increasing refinement of society accustoms us more and more to suppress our emotions, lest they should be disagreeable to others. And as the play of the emotions is the chief study of the poet, we see, in this circumstance, another reason which makes it difficult to rival that great body of poetry which our ancestors possessed. Therefore, it is doubly incumbent on physical philosophers to cultivate the imagination. It is a duty they owe to their own pursuits, which would be enriched and invigorated by such an enlargement of their resources. It is also a duty which they owe to society in general; since they, whose intellectual influence is already greater than that of any other class, and whose authority is perceptibly on the increase, might have power enough to correct the most serious deficiency of the present age, and to make us some amends for our inability to produce such a splendid imaginative literature as that which our forefathers created, and in which the choicest spirits of the seventeenth century did, if I may so say, dwell and have their being.

If, therefore, Black had done nothing more than set the example of a great physical philosopher giving free scope to the imagination, he would have conferred upon us a boon, the magnitude of which it is not easy to overrate. And it is very remarkable, that, before he died, that department of inorganic physics, which he cultivated with such success, was taken up by another eminent Scotchman, who pursued exactly the same plan, though with somewhat inferior genius. I allude, of course, to Leslie, whose researches on heat are well known to those who are occupied with this subject; while, for our present purpose, they are chiefly interesting as illustrating that peculiar method which, in the eighteenth century, seemed essential to the Scotch mind.

About thirty years after Black propounded his famous theory of heat, Leslie began to investigate the same topic, and, in 1804, published a special dissertation upon it.[769] In that work, and in some papers in his Treatises on Philosophy, are contained his views, several of which are now known to be inaccurate,[770] though some are of sufficient value to mark an epoch in the history of science. Such was his generalization respecting the connexion between the radiation of heat, and its reflection; bodies which reflect it most, radiating it least, and those which radiate it most, reflecting it least. Such, too, was another wide conclusion, which the best inquirers have since confirmed, namely, that, while heat is radiating from a body, the intensity of each ray is as the sine of the angle which it makes with the surface of that body.

These were important steps, and they were the result of experiments, preceded by large and judicious hypotheses. In relation, however, to the economy of nature, considered as a whole, they are of small account in comparison with what Leslie effected towards consolidating the great idea of light and heat being identical, and thus preparing his contemporaries for that theory of the interchange of forces, which is the capital intellectual achievement of the nineteenth century. But it is interesting to observe, that, with all his ardour, he could not go beyond a certain length. He was so hampered by the material tendencies of his time, that he could not bring himself to conceive heat as a purely supersensual force, of which temperature was the external manifestation.[771] For this, the age was barely ripe. We accordingly find him asserting, that heat is an elastic fluid, extremely subtle, but still a fluid.[772] His real merit was, that, notwithstanding the difficulties which beset his path, he firmly seized the great truth, that there is no fundamental difference between light and heat. As he puts it, each is merely a metamorphosis of the other. Heat is light in complete repose. Light is heat in rapid motion. Directly light is combined with a body, it becomes heat; but when it is thrown off from that body, it again becomes light.[773]

Whether this is true or false, we cannot tell; and many years, perhaps many generations, will have to elapse before we shall be able to tell. But the service rendered by Leslie is quite independent of the accuracy of his opinion, as to the manner in which light and heat are interchanged. That they are interchanged, is the essential and paramount idea. And we must remember, that he made this idea the basis of his researches, at a period when some very important facts, or, I should rather say, some very conspicuous facts, were opposed to it; while the main facts which favoured it were still unknown. When he composed his work, the analogies between light and heat, with which we are now acquainted, had not been discovered; no one being aware, that double refraction, polarization, and other curious properties, are common to both. To grasp so wide a truth in the face of such obstacles, was a rare stroke of sagacity. But, on account of the obstacles, the inductive mind of England refused to receive the truth, as it was not generalized from a survey of all the facts. And Leslie, unfortunately for himself, died too soon to enjoy the exquisite pleasure of witnessing the empirical corroboration of his doctrine by direct experiment, although he clearly perceived that the march of discovery, in reference to polarization, was leading the scientific world to a point, of which his keen eye had discerned the nature, when, to others, it was an almost invisible speck, dim in the distant offing.[774]

In regard to the method adopted by Leslie, he assures us, that, in assuming the principles from which he reasoned, he derived great aid from poetry; for he knew that the poets are, after their own manner, consummate observers, and that their united observations form a treasury of truths, which are nowise inferior to the truths of science, and of which science must either avail herself, or else suffer from neglecting them.[775] To apply these truths rightly, and to fit them to the exigencies of physical inquiry, is, no doubt, a most difficult task, since it involves nothing less than holding the balance between the conflicting claims of the emotions and the understanding. Like all great enterprises, it is full of danger, and, if undertaken by an ordinary mind, would certainly fail. But there are two circumstances which make it less dangerous in our time, than in any earlier period. The first circumstance is, that the supremacy of the human understanding, and its right to judge all theories for itself, is now more generally admitted than ever; so that there can be little fear of our leaning to the opposite side, and allowing poetry to encroach on science. The other circumstance is, that our knowledge of the laws of nature is much greater than that possessed by any previous age; and there is, consequently, less risk of the imagination leading us into error, inasmuch as we have a large number of well-ascertained truths, which we can confront with every speculation, no matter how plausible or ingenious it may appear.

On both these grounds, Leslie was, I apprehend, justified in taking the course which he did. At all events, it is certain, that, by following it, he came nearer than would otherwise have been possible, to the conceptions of the most advanced scientific thinkers of our day. He distinctly recognized that, in the material world, there is neither break nor pause; so that what we call the divisions of nature have no existence, except in our minds.[776] He was even almost prepared to do away with that imaginary difference between the organic and inorganic world, which still troubles many of our physicists, and prevents them from comprehending the unity and uninterrupted march of affairs. They, with their old notions of inanimate matter, are unable to see that all matter is living, and that what we term death is a mere expression by which we signify a fresh form of life. Towards this conclusion, all our knowledge is now converging; and it is certainly no small merit in Leslie, that he, sixty years ago, when really comprehensive views, embracing the whole creation, were scarcely known among scientific men, should have strongly insisted that all forces are of the same kind, and that we have no right to distinguish between them, as if some were living, and others were dead.[777]

We owe much to him, by whom such views were advocated. But they were then, and in a certain, though far smaller degree, they are now, so out of the domain of physical experience, that Leslie never could have obtained them by generalizing in the way which the inductive philosophy enjoins. His great work on heat was executed, as well as conceived, on the opposite plan;[778] and his prejudices on this point were so strong, that we are assured by his biographer, that he would allow no merit to Bacon, who organized the inductive method into a system, and to whose authority we in England pay a willing, and I had almost said a servile, homage.[779]

Another curious illustration of the skill with which the Scotch mind, when once possessed of a principle, worked from it deductively, appears in the geological speculations of Hutton, late in the eighteenth century. It is well known, that the two great powers which have altered the condition of our planet, and made it what it is, are fire and water. Each has played so considerable a part, that we can hardly measure their relative importance. Judging, however, from the present appearance of the crust of the earth, there is reason to believe, that the older rocks are chiefly the result of fusion, and that the younger are aqueous deposits. It is, therefore, not unlikely, that, in the order in which the energies of nature have unfolded themselves, fire preceded water, and was its necessary precursor.[780] But, all that we are as yet justified in asserting is, that these two causes, the igneous and the aqueous, were in full operation long before man existed, and are still busily working. Perhaps they are preparing another change in our habitation, suitable to new forms of life, as superior to man, as man is superior to the beings who occupied the earth before his time. Be this as it may, fire and water are the two most important and most general principles with which geologists are concerned; and though, on a superficial view, each is extremely destructive, it is certain that they can really destroy nothing, but can only decompose and recompose; shifting the arrangements of nature, but leaving nature herself intact. Whether one of these elements will ever again get the upper hand of its opponent, is a speculation of extreme interest. For, there is reason to suspect, that, at one period, fire was more active than water, and that, at another period, water was more active than fire. That they are engaged in incessant warfare, is a fact with which geologists are perfectly familiar, though, in this, as in many other cases, the poets were the first to discern the truth. To the eye of the geologist, water is constantly labouring to reduce all the inequalities of the earth to a single level; while fire, with its volcanic action, is equally busy in restoring those inequalities, by throwing up matter to the surface, and in various ways disturbing the crust of the globe.[781] And as the beauty of the material world mainly depends on that irregularity of aspect, without which scenery would have presented no variety of form, and but little variety of colour, we shall, I think, not be guilty of too refined a subtlety, if we say that fire, by saving us from the monotony to which water would have condemned us, has been the remote cause of that development of the imagination which has given us our poetry, our painting, and our sculpture, and has thereby not only wonderfully increased the pleasures of life, but has imparted to the human mind a completeness of function, to which, in the absence of such a stimulus, it could not have attained.

When geologists began to study the laws according to which fire and water had altered the structure of the earth, two different courses were open to them, namely, the inductive and the deductive. The deductive plan was to compute the probable consequences of fire and water, by reasoning from the sciences of thermotics and hydrodynamics; tracking each element by an independent line of argument, and afterwards coördinating into a single scheme the results which had been separately obtained. It would then only remain to inquire, how far this imaginary scheme harmonized with the actual state of things; and if the discrepancy between the ideal and the actual were not greater than might fairly be expected from the perturbations produced by other causes, the ratiocination would be complete, and geology would, in its inorganic department, become a deductive science. That our knowledge is ripe for such a process, I am far, indeed, from supposing; but this is the path which a deductive mind would take, so far as it was able. On the other hand, an inductive mind, instead of beginning with fire and water, would begin with the effects which fire and water had produced, and would first study these two agents, not in their own separate sciences, but in their united action as exhibited on the crust of the earth. An inquirer of this sort would assume, that the best way of arriving at truth would be to proceed from effects to causes, observing what had actually happened, and rising from the complex results up to a knowledge of the simple agents, by whose power the results have been brought about.

If the reader has followed the train of thought which I have endeavoured to establish in this chapter, and in the first volume, he will be prepared to expect that when, in the latter half of the eighteenth century, geology was first seriously studied, the inductive plan of proceeding from effects to causes became the favourite one in England; while the deductive plan of proceeding from causes to effects, was adopted in Scotland and in Germany. And such was really the case. It is generally admitted, that, in England, scientific geology owes its origin to William Smith, whose mind was singularly averse to system, and who, believing that the best way of understanding former causes was to study present effects, occupied himself, between the years 1790 and 1815, in a laborious examination of different strata.[782] In 1815, he, after traversing the whole of England on foot, published the first complete geological map which ever appeared, and thus took the first great step towards accumulating the materials for an inductive generalization.[783] In 1807, and, therefore, before he had brought his arduous task to an end, there was formed in London the Geological Society, the express object of which, we are assured, was, to observe the condition of the earth, but by no means to generalize the causes which had produced that condition.[784] The resolution was, perhaps, a wise one. At all events, it was highly characteristic of the sober and patient spirit of the English intellect. With what energy and unsparing toil it has been executed, and how the most eminent members of the Geological Society have, in the pursuit of truth, not only explored every part of Europe, but examined the shell of the earth in America and in Northern Asia, is well known to all who are interested in these matters; nor can it be denied, that the great works of Lyell and Murchison prove that the men who are capable of such laborious enterprises, are also capable of the still more difficult achievement of generalizing their facts and refining them into ideas. They did not go as mere observers, but they went with the noble object of making their observations subservient to a discovery of the laws of nature. That was their aim; and all honour be to them for it. Still, it is evident, that their process is essentially inductive; it is a procedure from the observation of complex phenomena, up to the elements to which those phenomena are owing; it is, in other words, a study of natural effects, in order to learn the operation of natural causes.

Very different was the process in Germany and Scotland. In 1787, that is, only three years before William Smith began his labours, Werner, by his work on the classification of mountains, laid the foundation of the German school of geology.[785] His influence was immense; and among his pupils we find the names of Mohs, Raumer, and Von Buch, and even that of Alexander Humboldt.[786] But the geological theory which he propounded, depended entirely on a chain of argument from cause to effect. He assumed, that all the great changes through which the earth had passed, were due to the action of water. Taking this for granted, he reasoned deductively from premisses with which his knowledge of water supplied him. Without entering into details respecting his system, it is enough to say, that, according to it, there was originally one vast and primeval sea, which, in the course of time, deposited the primitive rocks. The base of all was granite; then gneiss; and others followed in their order. In the bosom of the water, which at first was tranquil, agitations gradually arose, which, destroying part of the earliest deposits, gave birth to new rocks, formed out of their ruins. The stratified thus succeeded to the unstratified, and something like variety was established. Then came another period, in which the face of the waters, instead of being merely agitated, was convulsed by tempests, and, amid their play and collision, life was generated, and plants and animals sprung into existence. The vast solitude was slowly peopled, the sea gradually retired; and a foundation was laid for that epoch, during which man entered the scene, bringing with him the rudiments of order and of social improvement.[787]

These were the leading views of a system which, we must remember, exercised great sway in the scientific world, and won over to its side minds of considerable power. Erroneous and far-fetched though it was, it had the merit of calling attention to one of the two chief principles which have determined the present condition of our planet. It had the further merit of provoking a controversy, which was eminently serviceable to the interests of truth. For, the great enemy of knowledge is not error, but inertness. All that we want is discussion, and then we are sure to do well, no matter what our blunders may be. One error conflicts with another; each destroys its opponent, and truth is evolved. This is the course of the human mind, and it is from this point of view that the authors of new ideas, the proposers of new contrivances, and the originators of new heresies, are benefactors of their species. Whether they are right or wrong, is the least part of the question. They tend to excite the mind; they open up the faculties; they stimulate us to fresh inquiry; they place old subjects under new aspects; they disturb the public sloth; and they interrupt, rudely, but with most salutary effect, that love of routine, which, by inducing men to go grovelling on in the ways of their ancestors, stands in the path of every improvement, as a constant, an outlying, and, too often, a fatal obstacle.

The method adopted by Werner was evidently deductive, since he argued from a supposed cause, and reasoned from it to the effects. In that cause, he found his major premiss, and thence he worked downwards to his conclusion, until he reached the world of sense and of reality. He trusted in his one great idea, and he handled that idea with consummate skill. On that very account, did he pay less attention to existing facts. Had he chosen, he, like other men, could have collected them, and subjected them to an inductive generalization. But he preferred the opposite path. To reproach him with this is irrational; for, in his journey after truth, he chose one of the only two roads which are open to the human mind. In England, indeed, we are apt to take for granted that one road is infinitely preferable to the other. It may be so; but on this, as on many other subjects, assertions are current which have never been proved. At all events, Werner was so satisfied with his method, that he would not be at the pains of examining the position of rocks and their strata, as they are variously exhibited in different countries; he did not even explore his own country, but, confining himself to a corner of Germany, he began and completed his celebrated system, without investigating the facts on which, according to the inductive method, that system should have been built.[788]

Exactly the same process, on the same subject, and at the same time, was going on in Scotland. Hutton, who was the founder of Scotch geology, and who, in 1788, published his Theory of the Earth, conducted the inquiry just as Werner did; though, when he began his speculations, he had no knowledge of what Werner was doing.[789] The only difference between them was, that while Werner reasoned from the agency of water, Hutton reasoned from the agency of fire. The cause of this may, I think, be explained. Hutton lived in a country where some of the most important laws of heat had, for the first time, been generalized, and where consequently, that department of inorganic physics had acquired great reputation. It was natural for a Scotchman to take more than ordinary interest in a subject in which Scotland had been so successful, and had obtained so much fame. We need not, therefore, wonder that Hutton, who, like all men, felt the intellectual bent of the time in which he lived, should have yielded to an influence of which he was, perhaps, unconscious. In obedience to the general mental habits of his country he adopted the deductive method. In further obedience to the more special circumstances connected with his own immediate pursuits, he gathered the principles from which he reasoned from a study of fire, instead of gathering them, as Werner did, from a study of water.

Hence it is, that, in the history of geology, the followers of Werner are known as Neptunists, and those of Hutton as Plutonists.[790] And these terms represent the only difference between the two great masters. In the most important points, namely their method, they were entirely agreed. Both were essentially one-sided; both paid a too exclusive attention to one of the two principal agents which have altered, and are still altering, the crust of the earth; both reasoned from those agents, instead of reasoning to them; and both constructed their system without sufficiently studying the actual and existing facts; committing, in this respect, an error which the English geologists were the first to rectify.

As I am writing a history, not of science, but of scientific method, I can only briefly glance at the nature of those services which Hutton rendered to geology, and which are so considerable, that his system has been called its present basis.[791] This, however, is too strongly expressed; for, though Hutton was far from denying the influence of water,[792] he did not concede enough to it, and there is a tendency among several geologists to admit that the system of Werner considered as an aqueous theory, contains a larger amount of truth than the advocates of the igneous theory are willing to allow. Still, what Hutton did was most remarkable, especially in reference to what are now termed metamorphic rocks, the theory of whose formation he was the first to conceive.[793] Into this, and into their connexion, on the one hand, with the sedimentary rocks, and, on the other hand, with those rocks whose origin is perhaps purely igneous, I could not enter without treading on debatable ground. But, putting aside what is yet uncertain, I will mention two circumstances respecting Hutton which are undisputed, and which will give some idea of his method, and of the turn of his mind. The first circumstance is, that, although he ascribed to subterranean heat, as exhibited in volcanic action, a greater and more constant energy than any previous inquirers had ventured to do,[794] he preferred speculating on the probable consequences of that action, rather than drawing inferences from the facts which the action presented; he being on this point so indifferent, that he arrived at his conclusions without inspecting even a single region of active volcanoes, where he might have watched the workings of nature, and seen what she was really about.[795] The other circumstance is equally characteristic. Hutton, in his speculations concerning the geological effects of heat, naturally availed himself of the laws which Black had unfolded. One of those laws was, that certain earths owe their fusibility to the presence of fixed air in them before heat has expelled it; so that if it were possible to force them to retain their fixed air, or carbonic acid gas, as we now call it, no amount of heat could deprive them of the capability of being fused. The fertile mind of Hutton saw, in this discovery, a principle from which he could construct a geological argument. It occurred to him, that great pressure would prevent the escape of fixed air from heated rocks, and would thus enable them to be fused, notwithstanding their elevated temperature. He then supposed that, at a period anterior to the existence of man, such a process had taken place under the surface of the sea, and that the weight of so great a column of water had prevented the rocks from being decomposed while they were subjected to the action of fire. In this way, their volatile parts were held together, and they themselves might be melted, which could not have happened except for this enormous pressure. By following this line of argument, he accounted for the consolidation of strata by heat; since, according to the premisses from which he started, the oily, or bituminous parts, would remain, in spite of the efforts of heat to disperse them.[796] This striking speculation led to the inference, that the volatile components of a substance, and its fixed components, may be made to cohere, in the very teeth of that apparently irresistible agent whose business it is to effect their separation. Such an inference was contrary to all experience; or, to say the least, no man had ever seen an instance of it.[797] Indeed, the event was only supposed to happen in consequence of circumstances which were never met with on the surface of the globe, and which, therefore, were out of the range of all human observation.[798] The utmost that could be expected was, that, by means of our instruments, we might, perhaps, on a small scale, imitate the process which Hutton had imagined. It was possible, that a direct experiment might artificially combine great pressure with great heat, and that the result might be, that the senses would realize what the intellect had conceived.[799] But the experiment had never been tried, and Hutton, who delighted in reasoning from ideas rather than from facts, was not likely to undertake it.[800] He cast his speculation on the world, and left it to its fate.[801] Fortunately, however, for the reception of his system, a very ingenious and skilful experimenter of that day, Sir James Hall, determined to test the speculation by an appeal to facts; and as nature did not supply the facts which he wanted, he created them for himself. He applied heat to powdered chalk, while, at the same time, with great delicacy of manipulation, he subjected the chalk to a pressure about equal to the weight of a column of water half a mile high. The result was, that, under that pressure, the volatile parts of the chalk were held together; the carbonic acid gas was unable to escape; the generation of quicklime was stopped; the ordinary operations of nature were baffled, and the whole composition, being preserved in its integrity, was fused, and, on subsequently cooling, actually crystallized into solid marble.[802] Never was triumph more complete. Never did a fact more fully confirm an idea.[803] But, in the mind of Hutton, the idea preceded the fact by a long interval; since, before the fact was known, the theory had been raised, and the system which was built upon it had, indeed, been published several years. It, therefore, appears that one of the chief parts of the Huttonian Theory, and certainly its most successful part, was conceived in opposition to all preceding experience; that it pre-supposed a combination of events which no one had ever observed, and the mere possibility of which nothing but artificial experiment could prove; and, finally, that Hutton was so confident of the validity of his own method of inquiry, that he disdained to make the experiment himself, but left to another mind that empirical branch of the investigation which he deemed of little moment, but which we, in England, are taught to believe is the only safe foundation of physical research.[804]

I have now given an account of all the most important discoveries made by Scotland, in the eighteenth century, respecting the laws of the inorganic world. I have said nothing of Watt, because, although the steam-engine, which we owe to him, is of incalculable importance, it is not a discovery, but an invention. An invention it may justly be termed, rather than an improvement.[805] Notwithstanding what had been effected in the seventeenth century, by De Caus, Worcester, Papin, and Savery, and notwithstanding the later additions of Newcomen and others, the real originality of Watt is unimpeachable. His engine was, essentially, a new invention; but, under its scientific aspect, it was merely a skilful adaptation of laws previously known; and one of its most important points, namely, the economy of heat, was a practical application of ideas promulgated by Black.[806] The only discovery made by Watt, was that of the composition of water. Though his claims are disputed by the friends of Cavendish, it would appear that he was the first who ascertained that water, instead of being an element, is a compound of two gases.[807] This discovery was a considerable step in the history of chemical analysis, but it neither involved nor suggested any new law of nature, and has, therefore, no claim to mark an epoch in the history of the human mind.[808] There is, however, one circumstance connected with it which is too characteristic to be passed over in silence. The discovery was made in 1783, by Watt, the Scotchman, and by Cavendish, the Englishman, neither of whom seems to have been aware of what the other was doing.[809] But between the two there was this difference. Watt, for several years previously, had been speculating on the subject of water in connexion with air, and having, by Black's law of latent heat, associated them together, he was prepared to believe that one is convertible into the other.[810] The idea of an intimate analogy between the two bodies having once entered his mind, gradually ripened; and when he, at last, completed the discovery, it was merely by reasoning from data which others possessed besides himself. Instead of bringing to light new facts, he drew new conclusions from former ideas.[811] Cavendish, on the other hand, obtained his result by the method natural to an Englishman. He did not venture to draw a fresh inference, until he had first ascertained some fresh facts. Indeed, his discovery was so completely an induction from his own experiments, that he omitted to take into consideration the theory of latent heat, from which Watt had reasoned, and where that eminent Scotchman had found the premisses of his argument.[812] Both of these great inquirers arrived at truth, but each accomplished his journey by a different path. And this antithesis is accurately expressed by one of the most celebrated of living chemists, who, in his remarks on the composition of water, truly says, that while Cavendish established the facts, Watt established the idea.[813]

Thus much, as to what was effected by the Scotch in the department of inorganic science. If we now turn to organic science, we shall find that, there also, their labours were very remarkable. To those who are capable of a certain elevation and compass of thought, it will appear, in the highest degree, probable, that, between the organic and inorganic world, there is no real difference. That they are separated, as is commonly asserted, by a sharp line of demarcation, which indicates where one abruptly ends, and the other abruptly begins, seems to be a supposition altogether untenable. Nature does not pause, and break off in this fitful and irregular manner. In her works there is neither gap nor chasm. To a really scientific mind, the material world presents one vast and uninterrupted series, gradually rising from the lowest to the highest forms, but never stopping. In one part of that series, we find a particular structure, which, so far as our observations have yet extended, we, in another part, cannot find. We also observe particular functions, which correspond to the structure, and, as we believe, result from it. This is all we know. Yet, from these scanty facts, we, who, at present, are still in the infancy of knowledge, and have but skimmed the surface of things, are expected to infer, that there must be a point, in the chain of existence, where both structure and function suddenly cease, and, after which, we may vainly search for signs of life. It would be difficult to conceive a conclusion more repugnant to the whole march and analogy of modern thought. In every department, the speculations of the greatest thinkers are constantly tending to coördinate all phenomena, and to regard them as different, indeed, in degree, but by no means as different in kind. Formerly, men were content to ground their conviction of this difference in kind, on the evidence of the eye, which, on a cursory inspection, saw an organization in some bodies, and not in others. From the organization, they inferred the life, and supposed that plants, for instance, had life, but that minerals had none. This sort of argument was long deemed satisfactory; but, in the course of time, it broke down; more evidence was required, and, since the middle of the seventeenth century, it has been universally admitted, that the eye, by itself, is an untrustworthy witness, and that we must employ the microscope, instead of relying on the unaided testimony of our own puny and precarious senses. But the microscope is steadily improving, and we cannot tell what limits there are to its capacity for improvement. Consequently, we cannot tell what fresh secrets it may disclose. Neither can we say, that it may not be altogether superseded by some new artificial resource, which shall furnish us with evidence, as superior to any yet supplied, as our present evidence is superior to that of the naked eye. Even already, and notwithstanding the shortness of time during which the microscope has been a really effective instrument, it has revealed to us organizations, the existence of which no one had previously suspected. It has proved, that what, for thousands of years, had been deemed mere specks of inert matter, are, in truth, animals possessing most of the functions which we possess, reproducing their species in regular and orderly succession, and endowed with a nervous system, which shows that they must be susceptible of pain and enjoyment. It has detected life hidden in the glaciers of Switzerland; it has found it embedded in the polar ice, and, if it can flourish there, it is hard to say from what quarter it can be shut out. So unwilling, however, are most men to relinquish old notions, that the resources of chemistry have been called in, to ascertain the supposed difference between organic and inorganic matter: it being asserted, that, in the organic world, there is a greater complexity of molecular combination, than in the inorganic.[814] Chemists further assert, that, in organic nature, there is a predominance of carbon, and, in inorganic, a predominance of silicon.[815] But chemical analysis, like microscopic observation, is making such rapid strides, that each generation, I had almost said each year, is unsettling some of the conclusions previously established; so that, now, and for a long time hence, we must regard those conclusions as empirical, and, indeed, as merely tentative. Surely a permanent and universal inference cannot be drawn from shifting and precarious facts, which are admitted to-day, and may be overthrown to-morrow. It would, therefore, appear that, in favour of the opinion, that some bodies are living, and that others are dead, we have nothing, except the circumstance, that our researches, so far as they have yet gone, have shown that cellular structure, growth, and reproduction, are not the invariable properties of matter, but are excluded from a large part of the visible world, which, on that account, we call inanimate. This is the whole of the argument on that side of the question. On the other side, we have the fact, that our sight, and the artificial instruments, by whose aid we have arrived at this conclusion, are confessedly imperfect; and we have the further fact, that, imperfect as they are, they have proved, that the organic kingdom is infinitely more extensive than the boldest dreamer had ever imagined, while they have not been able to enlarge the boundaries of the inorganic kingdom to any thing like the same amount. This shows, that, so far as our opinions are concerned, the balance is steadily inclining in one given direction; in other words, as our knowledge advances, a belief in the organic is encroaching upon a belief in the inorganic.[816] When we, moreover, add, that all science is manifestly converging towards one simple and general theory, which shall cover the whole range of material phenomena, and that, at each successive step, some irregularities are explained away, and some inequalities are reduced, it can hardly be doubted, that such a movement tends to weaken those old distinctions, the reality of which has been too hastily assumed; and that, in their place, we must, sooner or later, substitute the more comprehensive view, that life is a property of all matter, and that the classification of bodies into animate and inanimate, or into organic and inorganic, is merely a provisional arrangement, convenient, perhaps, for our present purposes, but which, like all similar divisions, will eventually be merged in a higher and wider scheme.

Until, however, that step is taken, we must be content to reason according to the evidence supplied by our imperfect instruments, or by our still more imperfect senses. We, therefore, recognize the difference between organic and inorganic nature, not as a scientific truth, but as a scientific artifice, by which we separate in idea, what is inseparable in fact; hoping, in this way, to pursue our course with the greater ease, and ultimately to obtain results, which will make the artifice needless. Assuming, then, this division, we may refer all investigations of organic bodies to one of two objects. The first object is, to ascertain the law of those bodies, in their usual, healthy, or, as we somewhat erroneously phrase it, normal course. The other object is, to ascertain their law, in their unusual, unhealthy, or abnormal course. When we attempt to do the first of these things, we are physiologists. When we attempt to do the second, we are pathologists.[817]