PART FIRST. (continued 4)

5. That, during the circulation, the blood charges itself with carbonic hydrogen air, is an assertion which cannot be easily admitted. It has already been observed, that, by the air-pump, venous blood does not appear to contain any elastic fluid whatever; and it is also certain, that animals cannot bear any quantity of air injected into their veins. Dr. Girtanner himself tried several kinds, and all of them proved fatal. Having injected a considerable quantity of oxygen air into the jugular vein of a dog, the animal raised most terrible outcries, breathed very quickly, and with the utmost difficulty; by little and little his limbs became stiff, he fell asleep, and died in less than three minutes. On injecting into the vein of another dog a small quantity of phlogisticated air, the animal died in twenty seconds. With carbonic acid gas (fixed air) a third dog died in a quarter of an hour. A fourth was killed in six minutes by nitrous air.73 From these experiments, had no others ever been made on the subject, it seems very probable, that no species of air can be safely admitted into the blood in its elastic state. If any such therefore should naturally be produced in the body, it must either be instantly thrown out, or disease must ensue. Such objections to the Doctor’s theory are so natural, that we might have thought he would have foreseen and provided against them. Instead of this he grounds the whole upon such slender evidence as could not be admitted in the most trifling matter. “An incision (says he) was made in the jugular vein of a sheep, and the blood which came from it was received into a bottle filled with nitrous air. When the bottle was half filled, it was closed. The blood coagulated immediately, and a separation of a great quantity of blackish serum took place. The day after, on opening the bottle, a very strong smell of nitrous ether (dulcified spirit of nitre) was perceived, the nitrous air having been changed in part into nitrous ether by the carbonic hydrogen gas of the blood. This experiment proves, beyond a doubt, that the venous blood contains carbonic hydrogen air; and that this air is not very intimately mixed with it, but may be expelled with the greatest ease.”

On reading the Doctor’s account of this experiment; it must be very obvious, that, however decidedly he may be of opinion that it proves beyond a doubt the existence of hydrogen air in the venous blood, yet there is not one solid reason; from what he says, for supposing any such thing. How can any man determine from the mere smell of sheep’s blood taken out of the body of the animal, and mixed with a poisonous vapour, what is the composition of human blood in the living body? In the case of any substance suspected to contain elastic air, the air-pump will always afford an experimentum crucis. But we know that venous blood does not yield any elastic vapour by the pump: if instead of blood, however; we should fill a portion of vein with beer, cyder, or other fermented liquor, it would instantly discover, by its swelling up, that it really contained air in an elastic state. If then from the tumefaction of the vein when filled with fermented liquor we conclude that the latter contains fixed air, why should we not, from the non-tumefaction of it when filled with blood, conclude that the vital fluid contains no air? If Dr. Girtanner was so well assured that the venous blood contains hydrogen air, he ought to have expelled some of it from a portion of the blood, noted the difference between the blood which had lost its air, and that which had not, and then, by adding the air to it again, restored the blood to its former state. Nothing less then recomposition can prove the truth of a chemical analysis; as division can only be proved by multiplication, or multiplication by division.

From all that has been said, we may fairly conclude, that no proof can be brought sufficient to prove the existence either of oxygen air or any other species of aerial fluid, in its elastic state, in the blood. Neither can we prove that any part of the condensable part of oxygen air is received by the breath in the lungs. It is, however, probable that this condensable part may be received into the stomach with our food; that having passed through the various channels of circulation, and arrived at last at the surface, it there resumes its aerial nature by combining with the superfluous heat of the body, and is evaporated through the pores of the skin by insensible perspiration. The aerial vapour which passes off by these pores indeed has been discovered to partake of the nature of fixed air; but we know that this species of gas always contains the basis of oxygen, being indeed composed of it; and whether the oxygen be taken into the body in its pure state or not, the result would undoubtedly be the same; for an union would be formed between it and the carbonic particles to be thrown off from the body. But thus we can never suppose the basis of oxygen or any other air to be a permanent part of the composition of our bodies; nor can the quantity of it be augmented by breathing any kind of air. The readiest way to increase the quantity seems to be by drinking fermented liquors. Thus, if the body is too hot, the superfluous heat will have a proper subject to act upon, viz. the condensable part of the fixed air; and hence we may perhaps account for the very grateful and cooling sensation produced by drinking these liquors in some diseases. With respect to the existence of carbon, charcoal or hydrogen in the blood, it is probable that it exists in equal quantity at all times, being indeed the fundamental material of the whole body, and probably only a modification of that dust from whence man was originally taken.74 When the blood therefore grows very black, when the teeth are covered with a black sordes, the hands become foul, &c. we may say, indeed we too surely feel, that, in such cases, there is a propensity in the body to return to its original state of dissolution; but there is not one solid reason for supposing the proportion of its materials to be varied; that there is a collection of oxygen in one part, hydrogen in another, or in short that nature can admit of any such disproportion taking place.

6. We must now consider Dr. Girtanner’s account of the origin of animal heat, which is, that, “during the distribution of the oxygen through the system, the heat which was united with this oxygen escapes; hence the animal heat;” and, “that the great capacity of the arterial blood for heat is owing to the oxygen with which it is united in the lungs.”—This leads us to consider in a more particular manner the doctrine of heat, a subject hitherto much less investigated than the importance of the subject requires. What little we do know of this matter seems to be almost entirely owing to Dr. Black, who hath discovered some very remarkable phenomena unknown to former philosophers. His discovery here, as in that of fixed air, was accidental. Making experiments on the water of different temperatures, he found that the mixture would always be an arithmetical mean betwixt the two quantities mixed. Thus, on mixing water at 50 degrees with an equal quantity at 100, the temperature of the mixture would be 75 degrees; but if instead of using water only he took snow or ice for one of the quantities, the mixture was no longer an arithmetical mean betwixt the two temperatures, but greatly below it; so that a quantity of heat seemed to be totally lost and in a manner annihilated. His attention was engaged by this unexpected phenomenon, and, prosecuting his experiments, he found that, when water was converted into ice, it really became warmer than it was before; and, by keeping the fluid perfectly still during the time that cold was applied, he was able to cool it to 27 degrees of Fahrenheit’s thermometer, which is five degrees below the freezing point; but on shaking this water so cooled, it was instantly converted into ice, and the thermometer rose to 32. On reversing the experiment he found that mere fluidity in water is not sufficient to melt ice. A considerable degree of heat is necessary; and even when this is previously given to the water, the whole becomes as cold as ice by the time that the ice is melted. The result of his experiments in short was this: Water, when frozen, absorbs an hundred and thirty-five degrees of heat before its fluidity can be restored: that is, supposing a pound of ice at the temperature of 32 to be mixed with a pound of water at the temperature of 32, by adding 135 degrees, so that the temperature of the water is augmented to 167, the ice will indeed be melted, but the temperature of the whole quantity of liquid will be reduced to 32. In this case therefore the heat manifestly assumes two different modes of action: one in which it acts internally upon the substance of the body, without being sensible to the touch, while in its other state it hath no effect upon the internal parts, but affects bodies on the outside. The former state therefore the Doctor distinguished by the name of latent, the latter by that of sensible heat.

The same theory was applied to explain the doctrine of evaporation, and that in the most decisive and satisfactory manner. The Doctor found, that, in the distillation of water, much more heat was communicated to that in the worm-tub of the still, than could be supposed necessary to raise the water distilled to 212 degrees, which is the utmost that water can bear. In prosecuting the experiment he found the quantity of heat absorbed by the water, when raised into vapour, truly surprising; no less than a thousand degrees; an heat more than sufficient to have made the whole quantity of fluid that came over red hot. Some objections, however, were made to this theory, even by the Doctor’s friends. Mr. Watt, particularly, though he could not deny the theory derived from Dr. Black’s experiments, yet suggested one, which, had it proved successful, would have overthrown the whole. It was this: Let water be distilled in vacuo, where it boils with a heat of 97 degrees, and the operation must be carried on with much less fuel, and with much greater ease, than in the common mode. It was said that, in this experiment, Dr. Black was equally concerned with Mr. Watt; but, in a personal conversation with the Doctor himself, he assured me that he had no farther concern than foretelling that the experiment would not succeed, which it seems did not. The event was as follows: Mr. Watt, determining at all events to try the experiment, caused to be made a copper retort and receiver, joined together in one piece. In the receiver he pierced a small hole, and, heating both retort and receiver, plunged the latter into cold water. The consequence was, that a considerable quantity of water entered the vessel, and was easily poured back into the retort, as a subject for distillation. A fire being now applied, the water was soon raised into steam, which filled both retort and receiver, and in a great measure expelled the external air. The small orifice in the receiver being now closed, and the receiver itself plunged into cold water, the distillation went on in vacuo; for, as soon as any of the steam was condensed, the space which it had occupied (according to Dr. Black one thousand and sixty-six times more than the original water) was become absolutely empty, and more steam, rarefied, not by any quantity of sensible heat, but merely by that which it contained in a latent state, would occupy the place of the former. The event of the experiment showed the truth of Dr. Black’s theory. The water boiled, and steam was raised as well as if access had been given to the air; but with this difference, that the upper part of the distilling vessel was never heated above what the hand could easily bear. With the water in the cooler it was quite otherwise. It became hot as usual, and, by the quantity of heat it received, plainly demonstrated that the vapour, though destitute of most of its sensible heat, yet contained an immense quantity in a latent state. The saving of fuel therefore in the practice of distillation, which was Mr. Watt’s object in making the experiment, was quite trifling, and not equal to the trouble of filling the retort with liquid.

The doctrine of latent heat thus established, furnished a solution of many phenomena which could not formerly be explained in a satisfactory manner. Thus the melting of all kinds of substances was found to be owing to an absorption of heat, while their condensation was attended with the contrary. Fluidity in all cases was explained on the same principle; and the more heat that was absorbed, the more fluid the matter became. Thus water, when in a condensed or solid state, absorbs 135 degrees of heat before it becomes fluid. A thousand degrees more convert it into vapour, and at last, by passing through the intense heat of a glass-house furnace, it is converted into a brilliant flame, and augments the heat of the furnace to a great degree. Hence the practice in glass-houses of throwing water into the ash-hole, the vapour of which, by passing through the burning fuel, makes the furnace much hotter than it was. In a similar manner were explained the phenomena of crystallization, the ductility of metals, the heat produced by hammering them, and the hardness produced by the operation, as well as the operation of annealing, &c. One other phenomenon, a very curious one, shall be noticed, on account of its being connected with the subject of this treatise. It is this: Let a small vessel filled with vitriolic ether be put into a larger one of water, and both included in the receiver of an air-pump. On exhausting the air, the ether boils, and is converted into vapour, while the water freezes. This shows that heat does not always act equally upon surrounding bodies, but has a tendency to enter some in preference to others; and from other experiments it appears, that this property has a considerable connexion with the density of the bodies concerned.

Thus one step was gained, and it was universally admitted that heat, in some cases, entered bodies, and in others was thrown out of them; but now the question arose, What is heat; and by what laws is it regulated, or from what source is it derived? Here Dr. Black himself was at a loss; for, as he supposed cold to be a mere non-entity, and only to consist in a comparatively smaller degree of heat, some phenomena occurred which would not easily admit of solution upon such an hypothesis. With these Dr. Black did not meddle much, but others were bolder. Dr. Irving, Professor of Chemistry at Glasgow, undertook to explain the whole mystery of latent heat upon the single principle of attraction. One of the most puzzling phenomena in the way of Dr. Black’s theory had been, that in some cases heat and cold seemed to repel each other, and a very remarkable instance of this was, that, in the morning, a little before sunrise, when the rays of light pass through the atmosphere, a little above the surface of the earth, the air then becomes manifestly colder than even at midnight. Dr. Irving’s explanation of this was, that the sun’s rays attracted heat from the atmosphere, and thus rendered it colder. Such at least was the explanation given in an inaugural dissertation by Dr. Cleghorn, one of Dr. Irving’s scholars; for the Doctor himself delivered his opinions only to them. In other cases he supposed that different substances had different capacities for receiving heat; and, of consequence, should the form, or rather the internal constitution, of the body be changed, the capacity of it for receiving heat must also be changed; and as an attraction subsists, or is supposed to subsist, between heat and all other substances, it is plain that while this attraction subsists, if the capacity of any substance for receiving heat be augmented, it will imbibe much more than it would have done had its former constitution remained. Thus water in its liquid state contains a certain quantity of heat; we may therefore say that water has a capacity for receiving heat equal to one to ten, or what we please. Vapour has a capacity for containing heat ten times greater than water. Water therefore, when converted into vapour, will imbibe ten times the quantity of heat that the water contains; and, again, on being re-converted into water, the capacity becoming what it was before, the superfluous quantity must be thrown out, as in Dr. Black’s experiments. In like manner, when a metal is melted by the fire, the capacity of it for receiving heat is changed: of consequence a great quantity is imbibed, and again expelled by the change of capacity which takes place on its becoming solid; and thus, from the change of capacity, in different substances, every phenomenon was solved.

This doctrine of capacities did not give general satisfaction. Dr. Black himself said of it, that it was neither probable nor ingenious;75 notwithstanding which, it continued to be received, and even very generally adopted. Dr. Crawford, so well known for his writings on this subject, has adopted the idea, and Dr. Girtanner, in the passage above quoted from him, appears to be of the same opinion. The doctrine, however, had several opponents, among whom were the Monthly Reviewers. In their account of Nicholson’s First Principles of Chemistry, they express themselves in the following manner: “We only wish, that, in the doctrine of heat, he had avoided, which he might easily have done, Dr. Crawford’s idea of bodies having different capacities for heat. In the melting of ice, for instance, a quantity of heat is absorbed, without any increase of the temperature, that is, without making the water sensibly warmer than the ice before its liquefaction; which is said to be owing to the water having a greater capacity for heat, or being able to hold more of it, than the ice; and, in like manner, when converted into vapour, its capacity is further increased, or it can hold more still. This appears to us a very unchemical, and a very inadequate idea of the matter: for, admitting water to have a greater capacity than ice, how is the change from one state to the other to be effected? Can the properties which a body is found to possess, after a change has taken place, be assigned as a cause of the change itself? Or will it be said, that the heat first enlarges the capacity, and then hides itself in that capacity so enlarged? We should think it much better to say, consonantly with the phenomena of other combinations in chemistry, that a certain quantity of heat, uniting with the ice, first liquefies it, as a certain quantity of acid only neutralizes an alkali; that if any surplus quantity must be introduced, that surplus, remaining free and uncombined, must act and be sensible as heat in the one case, and acid other; and that different bodies require different quantities of heat or acid to be combined with them, for producing the changes in question.”76

Thus the Reviewers, as well as others, reasoned a priori, and several facts were adduced to prove that no such changes in capacity could take place. But however strong the arguments adduced, or however plain the experiments might be, little or no notice was taken of them, and the enlargement or diminution of capacities has been repeated, seemingly by rote, from one author to another, without the least inquiry or investigation. Dr. Girtanner indeed says that “the oxygen united with the arterial blood in the lungs” is the cause of the great capacity of the arterial blood for heat. But this is assigning a very doubtful cause for a very doubtful effect. He ought to have proved in the first place that arterial blood really has this capacity; for its being hotter than the blood of the veins, only shews that it parts with more heat to surrounding bodies than venous blood does; which is a proof that it contains less heat, if there be any difference, than that of the veins. But the truth is, that the capacity for containing heat depends neither on the oxygenation nor hydrogenation of a fluid, but upon its density. The more fluid and the more easily expansible into vapour that any substance is, the greater quantity of heat it is capable of containing, and vice versa. This has been fully ascertained by Mr. William Jones, an English clergyman, whose observations on the generally received system of philosophy contain many particulars worthy of attention. From his experiments it appears that a piece of red-hot iron, thrown into water, imparts much less sensible heat to it, and is itself much more effectually quenched, than by throwing it into an equal quantity of quick-silver of the same temperature with the water. As the quick-silver therefore becomes much hotter to the touch than water does upon throwing a piece of red-hot iron into it, and as the iron itself is much more imperfectly quenched by the metal than by the water, it follows that the latter is capable of containing much more heat than the former. But such experiments are not applicable to the blood. Though that of the arteries may be somewhat hotter than the venous blood, yet the reason is obvious. The heat is communicated directly to the arterial blood in the lungs; but during the circulation a part of it evaporates, and the farther distant any part is from the lungs, the more cool will the vital fluid be, without regard to any alteration of capacity, which indeed never can be shown to exist.

But the most decisive experiments against any supposed alteration in the capacities of bodies for containing heat are those lately tried by Count Rumford, and related in the Philosophical Transactions for 1798. His attention to this subject was engaged by observing the great degree of heat acquired by a brass gun during the time of boring it,77 and still more by the intense heat (much greater than that of boiling water) of the metallic chips separated from it by the borer. From a consideration of these things he was naturally led to the following inquiries. “Whence comes the heat actually produced in this mechanical operation? Is it furnished by the metallic chips which are separated by the borer from the solid mass of metal? If this were the case, then, according to the modern doctrine of caloric, the capacity for heat of the parts of the metal so reduced to chips, ought not only to be changed, but the change undergone by them be sufficiently great to account for all the heat produced. But no such change had taken place; for I found, that by taking equal quantities by weight of these chips, and of thin slips of the same block of metal, separated by means of a fine saw, and putting them at the same temperature, that of boiling water, and putting them into equal quantities of cold water (that is to say, at 59 1/2 of Fahrenheit) the portion of water into which the chips were put, was not, to all appearance, heated either less or more than the other portion in which the chips were put.”

From this experiment, several times repeated with the same result, Count Rumford inferred, that the heat could not possibly have been furnished at the expense of the latent heat of the metallic chips. He then proceeded to ascertain “how much heat was actually generated by friction, when a blunt steel borer being so forcibly shoved (by means of a strong screw) against the bottom of the bore of the cylinder, [of the machine in use] that the pressure against it was equal to the weight of about ten thousand lb. avoirdupois, the cylinder being turned round on its axis (by the power of horses) at the rate of about thirty-two times in a minute.” In this experiment the metallic dust or scaly matter detached from the cylinder by the borer weighed only 837 grains troy; but, says the author, “Is it possible that the very considerable quantity of heat produced in this experiment (a quantity which actually raised the temperature of above 113 lb. of gun-metal at least 70 degrees of Fahrenheit’s thermometer, and which of course would have been capable of melting 6 1/2 lbs. of ice, or making near five pounds of ice-cold water to boil) could have been furnished by so inconsiderable a quantity of metallic dust, and this merely in consequence of a change of its capacity for heat? As the weight of this metallic dust (837 grains troy) amounted to no more than one 948th part of that of the cylinder, it must have lost no less than 948 degrees of heat to have been able to raise the temperature of the cylinder one degree; and consequently it must have given off more than sixty-six thousand, three hundred and sixty degrees of heat to have produced the effects which were actually found to have been produced in this experiment.”

It was next considered whether the air did not contribute to the generation of this heat; and our author determined that this could not be the case; because the quantity of heat generated was not sensibly diminished when the free access of air was prevented. From another experiment it appeared that the generation of the heat was neither prevented nor retarded by keeping the apparatus immersed in water. Here the friction generated so much heat, that in one hour the temperature of the water surrounding the cylinder was raised from 60 to 107 degrees of Fahrenheit. In half an hour more it was raised to 142; at the end of two hours to 178; at two hours 20 minutes to 200; and in two hours and a half it boiled.78 On the whole, Count Rumford concludes, that “the quantity of heat, produced equably by the friction of the blunt borer against the bottom of the hollow metallic cylinder, was greater than that produced equably in the combustion of nine wax candles, each three quarters of an inch diameter, all burning at the same time with a clear, bright flame.” From all these experiments, however, our author does not draw any certain conclusion. “What is heat? (says he.) Is there any such thing as an igneous fluid? Is there any thing that can with propriety be called caloric? The heat produced, in the author’s experiments, by the friction of two metallic surfaces, was not furnished by small particles of metal, detached from the larger solid on their being rubbed together. It was not supplied by the air, because the machinery in three experiments was kept under water, and the access of atmospherical air completely prevented. It was not furnished by the water which surrounded the machinery, because this water was continually receiving heat from the machinery and could not at the same time be giving to and receiving heat from the same body; and because there was no chemical decomposition of any part of this water.” At last he observes, that the source of this heat, whatever it is, must evidently be inexhaustible, adding, that “any thing, which any insulated body, or system of bodies, can continue to furnish without limitation, cannot possibly be a material substance; and it appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of any thing capable of being excited and communicated, in the manner the heat was excited and communicated in these experiments, except it be MOTION.”

On this last paragraph, however, it is obvious to remark, that the whole force of the argument rests upon an insinuation, that the cylinder and borer were insulated, or cut off from all communication with any other material substance. Had this been the case, then no doubt it would follow that an endless supply of any thing material could not be furnished by them; but if, as Dr. Boerhaave and many other learned and intelligent persons have supposed, fire be an element universally present, and which becomes sensible to the touch only in consequence of a particular mode of action, it will follow, that no substance in nature can be insulated with respect to it; but, in whatever place, and for whatever length of time, any substance shall be affected in such a manner as to agitate this fluid, there we shall perceive a production of heat without limitation, even though heat itself be no more than the action of a fluid essentially material, though invisible to us.

Considerations of this kind occurred long ago to the writer of this treatise, when by the nature of his employment it was necessary for him to speculate upon these subjects. It could not then but appear to him that the theory of Dr. Black was far superior to any that had been published. The opinion of those who supposed fire to consist in the vibratory motion of the particles of solid bodies, seemed altogether untenable. It is impossible to explain the phenomena of heat upon ordinary mechanical principles, because, with respect to all terrestrial substances, heat constantly appears as an agent, while they are merely passive; and no man can explain the nature of a cause from its effect. Thus one of the most obvious effects of heat is expansion, or enlargement of bulk, in such bodies as are heated. But if from this fact we infer that the parts of elementary fire are repulsive of one another, our reasoning is certainly erroneous. In like manner, when we are not sensible of heat, we are not authorised to conclude that it is not present; for Dr. Black has demonstrated that it may be present in very great quantity, though indiscoverable either by our senses or by a thermometer.

But, with regard to the theory published by Dr. Black himself, it is evident that, though one part of it rests on the solid basis of experiment, the other is founded entirely upon hypothesis, and that too an hypothesis which cannot admit of being proved by any experiment, viz. that cold is a mere negative, and hath no real existence in nature. Among many phenomena which militate against this opinion, the following experiment of M. Geoffrey seems to be the most remarkable. He took a small bason filled with water, and set it on a support in the middle of a large tub of water, in such a manner that the temperature of the water in the tub might communicate itself to that in the bason. This being ascertained by a thermometer placed in the bason, he threw a quantity of burning coals into the tub. The effect of this, on the supposition that cold is a mere privation of heat, ought to have been, that the heat of the coals, communicated to the water in the large tub, would in a short time pervade the small bason, and affect the thermometer there. The latter would therefore rise; but instead of this it fell several degrees before it began to rise; for which it doth not appear that any other reason can be assigned than that the cold is partly repelled by the heat of the coals, and therefore, entering into the small bason of water, it causes the thermometer to sink previous to its rising. To the same purpose we may urge the phenomenon already taken notice of, viz. that the sun’s rays, when passing at some distance above the surface of the earth, cool the lower part of the atmosphere. The natural solution is, that the heat of the sun partly repels the cold downwards; and as for the doctrine of attracting heat from the atmosphere, Count Rumford has shown that this does not happen in a case where we might with much more probability expect it; not to mention the violence done to the common perceptions of mankind by supposing the sun’s rays, which are most evidently the source of heat, to have any occasion to attract heat from the atmosphere or any thing else.

Lastly, with regard to the capacities of bodies for containing heat, the doctrine appears to involve a radical error, of such enormous magnitude, that it is impossible to make any thing of it. This is no less than confounding the heat which flows out from bodies with that which they contain as an essential part of their composition, and which they cannot emit without being changed into some other form. Thus the capacity of aqueous vapour for containing heat, according to Dr. Black, is 1000 degrees; yet without decomposing the vapour it would have been impossible to have known this; for vapour is often extremely cold to the touch, and a thermometer immersed in it will sink greatly. In short, all that we can know about the capacity of bodies for retaining heat is, that they either continue to absorb it, or we may continue to force it into them, till they be reduced to vapour. It is doubtful whether they can receive more; for from the experiment with Papin’s digester, formerly mentioned, it appears that the additional quantity of heat, which the water was made to receive, very quickly left it as soon as the steam had room to expand.

But, to come to a conclusion upon this subject: If we will investigate the nature of heat, we must do it as in other cases, viz. by making the igneous fluid, caloric, or what we please to call it, the object of our senses; for we cannot reason fairly, or indeed come to any rational conclusion at all, by doing otherwise. In this investigation it is necessary to attend to the particulars mentioned by Count Rumford. The fluid must be omnipresent in its nature, infinite in its quantity, and equable, uniform and incessant in its action; as far as these epithets can be applied to any material being. There are only two fluids which we know that can answer to these characteristics. The one is the light of the sun, which pervades all the celestial spaces; the other the electric fluid, which penetrates every terrestrial substance. Both of these produce heat, unlimited in quantity, as well as in duration, provided their action be continually kept up. The mode in which both produce heat is exactly the same, viz. by converging into a focus; and the greater the quantity, the greater is the heat, and that without any limitation either as to intensity or duration. With regard to the solar rays, it has long been known that by concentration they would produce heat; nevertheless it was unaccountably doubted whether the rays themselves were the matter of heat. One objection to this was, that on the tops of high mountains the air is exceedingly cold, though the sun shines very bright. But this objection was founded upon an erroneous notion that, wherever the matter of heat exists, there we must feel it; which doth not follow any more than that wherever air exists there we must feel a wind blowing upon us. Wind is air in motion, and heat is a more subtile fluid in motion. One demonstration of this is, that, on the tops of the highest mountains, a burning lens or mirror will set fire to combustible bodies as readily as in the vallies at the foot of them. Neither has heat, properly so called, anything to do with air. The focus of a burning-glass will heat bodies in vacuo as well as in the open air; and Sir Isaac Newton has observed, that if a thermometer be included in the vacuum of an air-pump, it will acquire the temperature of the room nearly in the same time that another will when included in a similar glass without any exhaustion.

The science of electricity is but of late date; and most violent and hypothetical disputes have taken place concerning the nature of the fluid. Its luminous and burning properties naturally led a number of people to suppose that it was elementary fire; but this was opposed by others with as much violence as if there had been something criminal in the supposition. The opposition, however, was founded upon the same error with that about the solar light. It was imagined that wherever elementary fire existed, there heat must be felt; and it was especially urged, that electricity, though it produced light, did not produce any heat, except when it exploded with such violence as to penetrate the internal substance of bodies, agitating their particles, and by this agitation producing heat. It has now, however, been found, that the electric aura, as it is called, when made to converge in great quantity to the point of a needle, will heat it to such a degree as to set fire to gun-powder. This shows that heat is occasioned by the convergence of this fluid to a focus, and to its divergence from it. In the focal point, heat will always take place. From the experiments of Hauksbee, Beccaria and Priestley, it likewise appears, that electricity will render transparent the most opaque bodies, such as sealing-wax, pitch, &c. which even the most intense light of the sun cannot do. As to the intensity of the heat produced by it, experiments have shown, that it cannot be exceeded even by that of the most powerful mirror. Globules of gold have been vitrified, platina melted, and the most infusible substances reduced to glass, by means of the electric shock. From so many evidences, therefore, it appears to me impossible to conclude otherwise than that the light of the sun and the electric fluid are the same thing; and, according to the different modes in which they act, they produce the phenomena of heat and light in all their varieties, besides a multitude of other effects of which we cannot have any perception. We may indeed, if we please, suppose that some other thing exists which is heat itself, and that the light or electric fluid sets in motion, attracts, repels, or acts otherwise upon this unknown something; just as it comes into our heads to fabricate our system. But, until our senses can discover in some way or other this hidden substance, reason will always suggest that it has no existence. We may say that without such a supposition we cannot solve the phenomena of heat. But do we ever expect to solve these phenomena; or do we know all that the solar light and electric fluid can perform? If we do not know what they can do, neither do we know what they cannot; and the invention of other fluids must be accounted not only chimerical but useless.

But, to be more particular: on the subject of heat people have embarrassed themselves more with philosophical reveries than by any real difficulty, and rendered the matter more obscure than nature has made it. We have already observed, that by the convergence of light, or of electricity, heat is always produced. Here we can see the mode in which the fluid acts, viz. first by converging, and then diverging. When the light falls upon a solid body, it is evident, that if it be allowed to flow out as easily as it flows in, no internal agitation of the parts, or of any fluid contained in them, can take place. Transparent bodies therefore are never heated. Again, if the light be not allowed to enter the substance of a body, but is entirely reflected, the body cannot be heated; and hence it is very difficult to melt a polished metal even by a strong burning-glass. M. Macquer’s burning mirror, which vitrified flints, could not melt silver. But, when the light falls upon a body capable of allowing it to enter its substance, at the same time that it cannot get out without difficulty, it is plain that the force of the fluid will be exerted in order to overcome that difficulty; the body will be expanded in all directions; the fluid will be thrown out in the same manner, and the more that the internal action of the light prevails over that power by which the parts of the body cohere, the more will the phenomena of heat be perceptible.

Again, let us suppose that the etherial fluid enters the substance of any body capable of being dilated to a great degree, it is equally plain that the action of the fluid must for some time be directed only upon the internal parts, and consequently will be imperceptible on the outside. This then is called latent heat; and where the pressure on the outside balances that on the inside no heat will be perceptible to the touch. But by whatever means this balance is broken, heat will instantly be perceptible; and experiments show that the balance may be broken either by an increase of cold or heat. Thus, in the case of water, the internal pressure remains equal to the external, until the fluid is cooled to a few degrees below 32. The balance is then broken, and the internal action prevails; a quantity of what is called sensible heat escapes, and the water is converted into ice. Again, at the temperature of 32, little or none of the water evaporates; but by the addition of heat, by which the internal action of the subtile fluid we speak of becomes greater than the external, the water is converted into vapour; and it is remarkable that the same effect takes place on greatly augmenting the degree of cold; for the evaporation from ice, even in frosty weather, is found to be very considerable.

On the whole, from innumerable experiments it appears, that there exists in nature a certain invisible fluid, by the action of which, when diverging from a centre, heat is produced in the central point. By a certain other power this diverging force is limited, so that in some cases it is not perceptible beyond the surface of the body in which it acts, and then it is called latent heat. In other cases it is perceptible in a certain degree, and the degree in which it is perceptible hath been called the temperature or sensible heat of the body. On mixing different substances together it is found, that very often the proportions between the external and internal actions are varied. This has been already observed, when giving an account of Dr. Black’s discovery of latent heat, viz. that when snow and warm water are mixed together the temperature of the mixture differs very considerably from the arithmetical mean between the temperatures of the two substances employed. Dr. Crawford prosecuted the experiment further, and found that there were few substances which, on being mixed, did not shew a temperature different from that of the arithmetical mean between the temperatures of the two originally employed. This difference he unfortunately used as the foundation of a rule for determining the capacities of different substances for containing heat, and upon this erroneous principle has raised a superstructure, which upon no occasion can be of service to science, but must always produce obscurity and confusion wherever it is introduced.

With regard to the power which sets bounds to the expansion of the fluid acting as heat, it is natural to think that it can be no other than the same fluid acting in a contrary direction, or from a circumference towards a centre; and thus we shall always find that the same fluid, by limiting its own operations, may produce those phenomena which have been hitherto deemed so difficult of explanation. In what manner this limitation is in all cases effected, or indeed in any case, we cannot pretend to explain. It is sufficient to observe, that wherever there is a perpetual efflux of any thing, there must be also a perpetual influx at the same time, and in proportion to the one the other will be. These two are directly contrary to one another, and, as we suppose the fluid to be universal, it is evident, that if any part of it be put in motion in a particular direction, the rest will press towards that part where the motion is, in order to keep up the equilibrium. Hence we may easily account for the heat produced by percussion or by friction. By hammering a piece of iron, as Dr. Black justly observes, the fluid is forced out from between the parts of the metal. The emission of this fluid in all directions is heat itself; and no sooner is one quantity thrown out than another supplies its place with great rapidity, and so on, until the pressure of the rest in some way or other counteracts the emission of any more, and the heat ceases. Just so with friction. The heat produced by it is always in proportion to the pressure employed. By this pressure the parts of the two substances are forced into such close contact, that an agitation and emission of the fluid pervading their substance takes place. This agitation, as we have already noticed, is heat itself, and, as long as the friction is continued, more and more heat will be produced, without any limitation, as Count Rumford has observed.

Some bodies have a greater disposition than others to emit this subtile fluid; and these we say are naturally of a warmer temperature than others. The temperature is nothing else than the efflux of the fluid from them, continually kept up by the action of the surrounding fluid. By mixture with different substances the temperatures of various bodies may be changed; by some the influx, and by others the efflux, may be augmented. In the former case we say the body becomes colder, in the latter hotter, than before; and in not a few cases the agitation of the fluid becomes so great that the matter actually takes fire. In all these cases, however, we can discover nothing more than the bare fact, that so and so is the case. We know that the bodies do grow hot by the convergence of the etherial fluid towards them, and its emission from them; but why it should converge or diverge we know not.

Thus much with regard to heat in general. We must next consider another fluid which has very generally been accounted the source and fountain of heat, viz. air. This is indeed so much the source of heat in all our operations, that it was natural to think it the only one; but experiments have now determined that air itself is a mere creature of heat and light;79 for, by employing these in a proper manner, airs or gases of all kinds have been produced. Thus, by exposing water in a glass vessel for some time to the rays of the sun, a quantity of very pure oxygen air may be obtained; by concentrating the sun’s rays upon charcoal, inflammable air may be had; and by distilling, with a strong heat, substances of various kinds, we may obtain a great variety of aerial vapours. From all this we may reasonably conclude that heat, attached to some other substance, dissolved in it in such a manner as to become invisible, forms the substance of air. Heat therefore being the agent in the composition of air, it is reasonable to suppose that it is the agent in its decomposition also, or in its transformation from one species to another, of which the conversion of oxygen into fixed air by combustion is an instance. When air is taken into the lungs the blood is warmed by the action of that invisible fluid, which has already given elasticity to the air. In consequence of a considerable quantity of this fluid being then converted from a latent into a sensible state, part of the elastic principle must be lost, and the air diminished in bulk. The reason why this must constantly take place is, that part of the heat evaporates from the surface of the body, during the course of circulation. Were it not so, the quantity thrown out by the lungs would be exactly equal to that which the blood received, and consequently there could be no diminution between the bulk of the air expired and that which was inspired; but, on account of the waste just mentioned, the blood must always receive somewhat more than it gives out by the breath. Thus, while the air we breathe continues the same, and the organization of the body is not changed, the natural operations will go on smoothly, and health will continue; but, as we have formerly observed, by an alteration of either of these, disease must ensue; and we must now endeavour, from the principles laid down, to examine the mode in which epidemic diseases, and particularly the plague, may be produced.

The air is so evidently connected with human life, that it has been from the earliest ages accounted the source of pestilential diseases, though, as none of the more obvious qualities of it, such as heat, cold, moisture, or dryness, appeared to be connected with them, they were generally supposed to proceed from the action of some unknown natural cause, or from that of the Deity himself. Some, however, have also been of opinion that plagues might originate from the obvious qualities of the air in conjunction with certain effluvia from putrid vegetable or animal bodies. Thus, in several plagues mentioned in ancient history, we find swarms of dead locusts, grasshoppers, the carcases of those slain in battle, crowded houses, and filth of all kinds, assigned as causes. This opinion was adopted by Dr. Mead, and he gives the following account of the origin of the plague in Egypt. “Grand Cairo is crowded with inhabitants, who for the most part live very poorly and nastily; the streets are narrow and close; it is situate in a sandy plain, at the foot of a mountain, which, by keeping off the winds that would refresh the air, makes the heats very stifling. Through the midst of the city passes a great canal, which is filled with water at the overflowing of the Nile; and, after the river decreases, is gradually dried up: into this canal the people throw all manner of carrion, filth, &c. so that the stench which arises from this and the mud together is insufferably offensive. In this posture of things, the plague every year preys upon the inhabitants, and is only stopped when the Nile, by overflowing, washes away this load of filth; the cold winds, which set in at the same time, lending their assistance by purifying the air.” He then proceeds to account for the plagues in Ethiopia in the manner above related, viz. by the prodigious swarms of locusts, which sometimes occasion a famine by devouring the fruits of the earth, and, when they happen to be cast by the winds into the sea, occasion a pestilence; the putrefaction being heightened by the intemperance of the climate, which here is so great that it is infested with violent rains for three or four months together; and it is particularly observed of this country, that the plague usually invades it whenever rains fall during the sultry heats of July and August. He next takes notice of what the Arabians say of the origin of the plague in Ethiopia, viz. that it is brought on by unseasonable moistures, heats, and want of winds. But, whatever truth may be in the account given of the Ethiopic plagues, the testimonies already produced in this treatise are sufficient to render it very doubtful, at least, whether the plague ever does originate in Cairo, or any other place in Egypt. Besides, if we once admit the existence of any thing as a cause adequate to the production of a certain effect, wherever that cause exists the effect ought certainly to follow, unless where we plainly perceive something which prevents its action. It is not fair reasoning to say that the action of the cause is prevented by something unknown, for we might as well say that this unknown something is the cause originally, and acts only upon certain occasions, of when it thinks proper. Now, if the filth of the canal of Cairo be the cause of the plague in that city, it ought to recur annually at the season when that filth exists in greatest quantity, and in the most putrid slate. Nevertheless we have the express testimony of Mr. Eyles Irwin, that at the time he was in Cairo there had not been any plague for seven years. The account he gives of it is a kind of contrast to that above quoted from Dr. Mead. “Misir al Kaira, says he, or the City of Anguish, so called from the frequent visits which it has received from the plague, but commonly called Grand Cairo by us, is situated in lat. 30 degrees 3 minutes N. on an artificial branch of the Nile. Old Cairo nearly faces the river; but the new city is removed above a mile from it, and approaches to the range of mountains which runs through Upper Egypt, and abruptly breaks off here. It is undoubtedly one of the finest cities in the east; which, from the present style of architecture that reigns among the orientals, is but a faint commendation. The houses are in general built of stone, and, being elevated to several stories, would make a grand appearance, notwithstanding the inelegance of their structure, were not the effect destroyed by the excessive narrowness of the streets. This is one of the causes to which the ingenious Dr. Mead ascribes the birth of the plague in this capital; but experience evinces that it arises from foreign and adventitious causes. There has not been a plague here these seven years; which is rendered more remarkable by the commencement of the Russian war at the date of its cessation. No one can account for this; though a year seldom passed by before without a visit from it.”80

From this it plainly appears, that, however these putrid effluvia may concur with other circumstances in producing the plague, they are by no means the only cause; otherwise not a single year could have passed in Cairo without a pestilence; and the very same thing we shall find to hold good in every other, let us choose for a cause what we will. In order to investigate this matter fully, we must now consider what causes have been assigned by physicians for other epidemical diseases; and here, to avoid prolixity, we shall chiefly confine ourselves to those enumerated by Dr. Fordyce as the causes of fever; a gentleman whose very extensive experience must give the greatest weight to his testimony.81 The principal causes assigned by him are,

1. Infection, or “a peculiar matter generated in the body of a man in fever, which is carried by the atmosphere, and applied to some part of the body of a person in health, and which causes fever to take place in him.” That such a cause exists, he proves from observing that “of any number of men, one half of whom go near a person ill of a fever, and the other half do not, a greater number of the former will be infected, in a short period afterwards, than in those who do not.” He says he has known, in such circumstances, seven out of nine infected with the disease. This infection is not discoverable by smell or any other organ of sense; neither can the greatest attention to cleanliness disarm it of its malignity. Of this the Doctor says that he has known instances; nay, of a person going into a room where a feverish patient was, and bringing with him the infection, which was communicated to others in the room to which he came. He owns, however, that by allowing the air to stagnate in which feverish patients are, the infection will become extremely violent and fatal. This may naturally be supposed, even without having recourse to putrid effluvia; because, independent of these, the imperceptible infectious matter itself will undoubtedly be accumulated in the atmosphere of the room, and act more powerfully than it could have done had it been partly carried off and diluted by attention to cleanliness and ventilation. He also says, that “when a number of persons live in a small space, supposing even that they are kept as clean as possible, it happens frequently that fever arises in some, often in many of them. It has been in this case supposed, and is extremely probable, that some peculiar species of matter is produced, capable of producing fever, on being applied to the body.”

2. Effluvia from putrid animal or vegetable matters. Of this our author seems to be less fully ascertained than of the former, as he does not say that he has observed any instances of fevers arising from this cause; and he concludes by observing that “either the cause of fever, consisting of matter produced in the body of a person affected with this disease, seems probably different from that produced by putrefaction, or might be generated without any putrefaction taking place.”

3. Cold. Our author “is not disposed to allow that sudden exposure to cold occasions fever to take place, unless some symptom of the disease follows immediately. If a man had been suddenly exposed to cold, and continued in perfect health for twenty-four hours, the author would never allow that fever, or any other disease, was occasioned by it. In this case (exposure to cold) the evidence is much stronger than in that of infection; for the author (Dr. Fordyce) has seen many instances where, from exposure to cold, the commencement of the attack was instantaneous; and many are to be found in the records of medicine.”

4. Moisture. On this subject the Doctor observes, that the application of water to the body is not a cause of fever, unless the air has particles of water floating in it; in which case fever has ensued more frequently than in other cases. Water may exist in the atmosphere in three states. 1. In small drops suspended in it like dust in water. 2. In vapour. In this case the transparency of the air is not impaired, and a chemical combination, as it is called, between the air and water takes place. If the atmosphere be hot or dense, it is capable of combining chemically with a larger proportion of water. If therefore the atmosphere should in this manner be saturated with water, at any particular degree of heat or density, by diminishing either of these the vapour will be condensed, and the water reduced to the former state of suspension in small particles. 3. Water, heated to the boiling point, emits a steam, which combines chemically with the atmosphere, till the latter be saturated, after which it assumes the form of small particles; and this last is the only state which has been found to produce fever.

Moisture will also produce fever when applied to the body by wearing wet clothes. Those which imbibe or part with heat most slowly, are least apt to produce fevers on being heated. The warmer the atmosphere, the more liable people are to fevers from moisture.

It has been observed, that moisture from marshes, stagnating canals, or where the water runs very slowly, is more apt to produce fevers than what proceeds from the sea, lakes or rapid rivers. “This (says the Doctor) has given occasion to suppose that some other vapours proceed from such marshes beside water, and produce the disease. It certainly often happens, that a considerable degree of putrefaction takes place in marshy grounds, and more especially in warm climates; but it is by no means to be concluded that moisture in the atmosphere always produces fever in consequence of putrefaction. Putrefaction can only take place in vegetable or animal substances. If water therefore, not impregnated with either, should be in such a situation as to produce moisture in the atmosphere, no putrefaction can take place; therefore, if fevers ensue, they are certainly in consequence of moisture, not putrefaction. Many instances of this may be brought, as in the war which took place in Flanders, between the tenth and eleventh year of the present century, an army encamped upon sandy ground, in which water was found in digging less than a foot deep, and occasioned a great moisture in the air, which produced in a few days numbers of fevers, although the army was perfectly healthy before, and no more fevers were produced on shifting their ground. There are a vast many other instances of the same thing having taken place. Besides, fever has often arisen immediately in persons sitting in rooms, the floors of which had been just moistened with pure water.”

5. Certain kinds of food. On this Dr. Fordyce observes, that, though food of difficult digestion undoubtedly produces a number of diseases, he has never seen it productive of fever excepting once. Dr. Girtanner relates, that the emperor of Germany, having forced a number of his subjects to serve as soldiers, and sent them into an unwholsome part of Walachia, where he fed them with a kind of paste made of bread and water instead of meat, many of them died of the scurvy. The Doctor, however, does not ascribe this to any positive cause, but to three negatives, viz. the abstraction of the stimulus of nutriment, by feeding on the paste just mentioned; of the stimulus of oxygen in the corrupted atmosphere of Walachia; and lastly of the nervous stimulus, the most powerful of all; the greatest part being engaged by force against their will. This corroborates what Dr. Fordyce has said, that bad food is very seldom the cause of fever; for among so many, who used the imperial paste just mentioned, some would certainly have been affected by fevers, had it been capable of producing them; but, as it did not, it is most evident that the deficiency of stimuli is not the cause of fever.

6. Passions of the mind. These are looked upon by Dr. Fordyce to be among the less frequent causes of fever, though it is certain that they have been productive of multitudes of diseases, and even of sudden deaths; and Dr. Falconer, in his Prize Dissertation, ascribes to the passions very considerable effects in fevers, and even in the plague itself. “Contagious fevers (says he) afford strong instances of the influence of mental affections, both as prophylactics and remedies. The plague is a remarkable example, and the same reasoning extends to other disorders of a febrile, contagious nature. Fear, it is well observed by Dr. Cullen, by weakening the body, and thereby increasing its irritability, is one of the causes which, concurring with contagion, render it more certainly active, which he ascribes to its weakening effect on the body, by which its irritability is increased. Against this therefore he directs the mind to be particularly fortified, which is best done by giving people a favourable idea of the power of preservative means, and by destroying the opinion of the incurable nature of the disorder, by occupying the mind with business or labour, and by avoiding all objects of fear, as funerals, passing-bells, and any notice of the death of particular friends. Even charms might be used with good effect, could we promote a strong prepossession of their efficacy, either by the confidence they inspire, or by their engrossing the attention of the mind. It is no less certain, that a studious regard to promote hope and confidence in recovery, is equally necessary for the cure as for the prevention of such disorders. We know that contagious fevers have a peculiar tendency to diminish the energy of the brain, and of course to debilitate the whole system; and that this is especially the case with the plague, which produces the most considerable effects in weakening the nervous82 system or moving powers, and in disposing the fluids to a general putrescency; and Dr. Cullen is of opinion that to these circumstances, as the proximate causes of the plague, regard should chiefly be had, both for the prevention and cure of this disorder. It must therefore be highly necessary, during the course of this disease, to attend to the support of the spirits, as on these the vital principles greatly depend; and they can by no means be so effectually kept up as by inspiring a confidence of recovery.”

Dr. Zimmerman presents us with a great number of examples of the influence of the passions in producing diseases, or death itself; some of the most remarkable of which follow. “All the passions (says he) when carried to excess, bring on very formidable diseases. Sometimes they occasion death, or bring us at least into imminent danger. The most reputable physicians agree in opinion that terror may occasion apoplexy, and death; and indeed they consider apoplexy as the most common effect of violent passion. Without being carried to excess, a passion will sometimes occasion a difficulty of breathing, together with a sense of stricture in the breast, and an hesitation to speak; the tongue remaining as it were immoveably fixed on the palate. Hysterical and hypocondriacal affections are sometimes the effects of grief in the most healthy people. Joy is much more dangerous to life than sudden grief. Sophocles died through joy at being crowned on account of a tragedy he had composed in his old age. The famous Fouquet died on being told that Louis XIV had restored him to liberty. The niece of the celebrated Leibnitz, not suspecting that a philosopher would hoard up treasure, died suddenly on finding under her uncle’s bed a box containing sixty thousand ducats. Violent anger has sometimes produced hæmorrhages and subcutaneous extravasations; or, some vessel of the brain being ruptured by these transports, a fatal apoplexy has taken place. There have been instances of excessive anger being succeeded by epilepsy, colic, or a violent degree of fever. Sometimes it has occasioned an increased flow of bile. In some this produces vomiting; in others it goes off downwards, and causes diarrhœa; or being retained, from a stricture of the gall-ducts, will perhaps be absorbed, and occasion jaundice. In cases where anger has been succeeded by extreme grief, obstructions have taken place in the liver. The effects of terror are similar to those of anger, but in general more violent. Sometimes excessive terror seems to give to men a preternatural strength, as is the case with madmen and drowning persons. In some cases it has not only excited immediate convulsions, but caused them to return periodically. Fear has been said to make the hair stand upright, and to contract the pores from which the hairs issue in the same manner as cold does. There are instances in authors even of the colour of the hair being changed by excessive fright. Philip V died suddenly on being told that the Spaniards had been defeated, and, on opening him, his heart was found ruptured. Timid people are more liable than others to fall sick. A firmness of mind is one of the best preservations against contagion. Willis has very well observed, that they who fear the small-pox the most are generally the first to be attacked with it. Cheyne assures us that fear is extremely prejudicial in all epidemical diseases. Dr. Rogers remarks, that fear constantly increases the ravages of a contagious disease. Rivinus attributed the propagation of the plague at Leipsic wholly to fear. The French physicians, who wrote on the plague at Marseilles, went so far as to deny its being contagious, and ascribed its propagation chiefly to fear.”

As for the cause of the plague itself, Dr. Fordyce supposes it to be produced by an infection of a particular kind. That which takes place in Syria and Egypt, he says, has only been clearly described by Dr. Russel; and it cannot be gathered from the accounts whether this may be originally produced without having been propagated as the first class of infections above mentioned are. “That disease (says he) called the plague, which ravaged this country (England) on considering the histories of the disease, seems to have been a fever,83 produced by infections of the first class which have been enumerated.” Dr. Moore has given an account of the origin of a plague, which, if it could be depended upon, would decide the question concerning the origin of this dreadful distemper without previous infection. This passage is extracted from the History of the Royal Medical Society for the years 1777 and 1778. “Dr. Mitchell, physician to the hospital at Smyrna, appears, according to a memoir of which he is the author, sent by M. le Baron de Tott to the Medical Society to believe in the spontaneity of the plague (or that it arises of itself without any predisposing cause in the body) for proof of which he cites the following circumstance: A solitary shepherd, having no communication with any body, fell sick while he was tending his flocks; he went into an inhabited part, where he communicated the plague with which he found that he was attacked. This circumstance would prove much, if it was certain that the shepherd had no communication with others; if it were known how long, and with what precaution, he had been secluded from company: but the proofs of these are too difficult to be established to allow of any conclusion to be drawn from the fact. We are obliged therefore to acknowledge [it to be a doubtful matter] whether it is in fact a country that is the cradle of the plague; what country this is, supposing that such an one exists; or, finally, whether it sometimes appears spontaneously, and whether the first whom it attacks becomes the focus from whence it emanates.”

Dr. Fordyce, in treating of the origin of fever, seems inclined to think that it may arise without any predisposing cause; and after having enumerated the various causes already mentioned, and fully considered them, gives it as his opinion, that “there must undoubtedly be other causes than those which give occasion to the disease, but which are at present totally unknown.” In like manner Dr. Moore, speaking of the nervous fever, sums up what may be known concerning the cause of it in the following words. “Upon the whole, we know that people of delicate, exhausted and sickly constitutions, and those whose minds are saddened by depressing passions, are greatly predisposed to this disease, the immediate seeds of which, we also know, may be generated in places where human effluvia are collected and confined. And this is the most essential part of our knowledge respecting the cause of this disease; and even this little is disturbed with uncertainty: for we sometimes meet with instances of people of robust constitutions, who are seized with the disease in all its malignity, when they are under no depressing passion, when the disease is not epidemic, to whom we cannot trace it from any place where the human effluvia could be confined in any uncommon degree, or from any person in the disease, of which perhaps there is no other person ill in the neighbourhood for several miles round; and, in short, when we cannot connect it with any of the causes supposed to be the sources of the distemper. On extraordinary occasions of this kind we have nothing for it but to suppose that, notwithstanding the apparent vigour of the patient, his body has been peculiarly predisposed to catch the infection, and that some contagion, not forcible enough to infect any other person, has by some means, unobserved, been conveyed to him; or, if so many suppositions displease, we may suppose at once that there is in some cases a source of this fever which has not been suspected. For, although the numerous observations that have been made give us the strongest reason to think that human effluvia produce this disease, we have no right to infer that it cannot arise also from some other source.”

To the same purpose I subjoin the very respectable opinion and testimony of Dr. Patrick Russel. “In some epidemical distempers, the sudden alternations of the air have constant and manifest influence; in others, though the influence of the air must be equally admitted, it seems not to depend on sensible alteration or succession in the common properties of the atmosphere, but on some inexplicable combination, some occult, new, unknown quality. Amongst epidemics of this last kind must be reckoned the plague. . . . Should ever that state of the air, without the concurrence of which the contagion of the pestilence never spreads, or ceases to act, be discovered, and ascertained by unequivocal marks, the dread of the plague, universally prevalent, would be greatly diminished; more effectual means of preservation would be found out, and the application of them might safely be limited to certain seasons.

“Experience in Turky, where, generally, no precautions are taken in the times of pestilence, clearly evinces, that, in a certain state of the air, a communication with infected places may subsist without any material consequence. The return of the plague at Aleppo happens at irregular periods; the intervals are of considerable, but unequal, length; and in those the commerce with Egypt, Constantinople and Smyrna remains uninterrupted. In the intervals between 1744 1760, and from 1762 to 1780, the plague raged several times in the places now mentioned, without affecting Aleppo; and even in two or three years subsequent to 1762, though it was at Marash, as well as other places not far distant, with which Aleppo has continual intercourse, no instances were discovered of communicated infection: if such happened, they must have escaped my utmost vigilance; and the daily exercise of my profession led me to be very much among the natives of all ranks. At the same time I have reason to suspect that infected families from some of those places took refuge in Aleppo; and I know, with certainty, that not only some merchants of that city, who happened to be at Marash when the plague broke out there, returned to their families in the summer of 1763, but that caravans of various merchandise arrived in the course of the same summer.

“I consider it therefore as an established fact in the Levant, that commerce and intercourse with infected towns is sometimes attended with no bad consequence. The same thing may perhaps be asserted, without restriction, of all countries; but till the signs indicating a pestilential constitution be ascertained, no particular year can be declared exempt from danger. Predictions founded on planetary conjunctions have been long exploded; and signs derived from the known properties and alterations of the air, are almost equally fallacious. The seasons concomitant with plagues in England, as well as elsewhere, have been very dissimilar; and the same visible concurrence, usually deemed pestilential, has often, in the revolution of years, been observed to return, in various countries, without producing the dreaded consequences. Upon the whole, from all I have been able to collect, the pestilential constitution seems hitherto to be known only from its effects; neither its approach nor its retreat can be predicted and its nature remains wrapped up in MYSTERIOUS DARKNESS.”

Having thus seen, that, of the causes commonly assigned for epidemical diseases, not one can be accounted certain and determinate, it now remains to consider one more, and that is

Contagion. Though this has been generally accounted the same with infection, yet by some it has been reckoned otherwise; and indeed there seems to be a necessity for such a distinction; for, though we should prove, ever so clearly, that a disease once communicated to one person should from that person be communicated to another, yet the difficulty is to know from whence the first person had it. This source, if any such can be found, is what we may with the greatest propriety distinguish by the name of contagion, and is the sense in which it shall for the future be used in this treatise, the matter communicated from one person to another being always called infection. This indeed differs from what many celebrated physicians have said upon the subject; but the distinction certainly must exist. Dr. Cullen speaks rather indistinctly upon the subject. “We have supposed that miasmata are the cause of intermittents, and contagions the cause of continued fevers, strictly so named; but we cannot with propriety employ these general terms. The notion of contagion properly implies a matter arising from the body under disease, miasma, a matter arising from other substances. But, as the cause of continued fevers may arise from other substances than the human body, and may in such cases be called a miasma, and, as other miasmata also may produce contagious disorders, it will be proper to distinguish the cause of fevers by using the terms marsh, or human effluvia, rather than the general ones miasma, or contagion.”

From this it is not very easy to determine what the Doctor means when he speaks of specific contagion as the cause of the plague. Dr. Russel plainly ascribes it to human effluvia. “The plague (says he) is a contagious disease; that is, an emanation from a body diseased, passing into one which is sound, produces, in time, the same disease,” &c. There must, however, undoubtedly have been something originally distinct from the human body which gave rise at least to the first plague that was in the world; and some plagues recorded in history are said to have arisen in this way. Thus, Ammianus Marcellinus says that the plague which broke out in the Roman army in the time of Marcus Aurelius arose from a pestilential vapour confined in a golden coffer dedicated to Apollo. Upon opening this, the contagion diffused itself all around, and the infection spreading from one to another, produced an almost universal pestilence. Ammianus indeed is the only historian who relates this; another account of its origin is given, p. 14, but whether we believe the account of Ammianus or not (which indeed does not appear probable) it is sufficient to show what were the received opinions at the time. In like manner every one has heard of pestilential effluvia breaking out from the earth, from graves, &c. so that we certainly look upon this doctrine of contagion as the cause of diseases to have been pretty generally received. We are also informed by Dr. Mead, from M. Villani, who wrote the history of those times, that the great plague of 1346 began in China, where, according to the report of some Genoese sailors, it was occasioned by a great ball of fire that either burst out of the earth, or fell down from heaven. This is thought incredible by Dr. Mead, and no doubt is so, but it shows the general opinion, that the original cause or contagion which produces a plague is distinct from the infection which is afterwards communicated from one to another. In the French Encyclopedie, we have this account of the ball of fire, or fiery vapour, without any comment.

As to the opinion of pestilential vapours arising out of the earth, though we are assured that people have been suddenly killed by explosions, probably of the electrical kind, or by lightning issuing from under their feet, yet we are not furnished with any well authenticated accounts of a plague having arisen from any such cause. About 19 years ago a violent fever raged epidemically through a small district in the north of Scotland, which was said to have originated in the following manner. Some young men having heard that a certain place in their neighbourhood had, in the time of a plague been a burial ground, took into their heads to dig into it. They did so, and one of them immediately fell sick, but recovered. The father of two of the young men, exceedingly displeased at the conduct of his sons, and apprehensive of the consequences, filled up the hole they had dug in the ground, soon after which he fell sick and died, and the fever continued to rage in the neighbourhood for some time. The mother of another of the parties concerned also died, and boils broke out on various parts of the bodies of the sick. This was the account given in some of the news-papers of the time, and had the matter been thoroughly investigated and attested, would have been decisive in favour of pestilential contagion being capable of taking up its residence in the earth. As it stands at present, it can only draw our attention to what may happen in another case, should any similar one occur.84

With regard to epidemics occasioned by the action of electricity, we cannot indeed produce any instance; but we have one of a distemper more dreadful than even the plague itself; and that is of a person suddenly struck by an electric flash (generated either in his own body, or in the room where he was) and by this stroke reduced to a most deplorable condition, which soon ended in death. The account stands on the authority of Mr. Joseph Battaglia, surgeon at Ponte Bosio, who transmitted it to Florence, and is as follows.

“Don G. Maria Bertholi, a priest residing at mount Valere in the district of Livizzano, went to the fair of Filetto, on account of some business which he had to transact, and after spending the whole day in going about through the neighbouring country, in order to execute commissions, in the evening he walked towards Fenille, and stopped at the house of one of his brothers-in-law, who resided there. No sooner had he arrived, than he desired to be conducted to his apartment, where he put a handkerchief between his shoulders and his shirt, and, when every body retired, he began to repeat his breviary. A few minutes after, a loud noise was heard in Mr. Bertholi’s chamber; and his cries having alarmed the family, they hastened to the spot, where they found him extended on the floor, and surrounded by a faint flame, which retired to a greater distance in proportion as it was approached, and at length disappeared entirely. Having conveyed him to bed, such assistance as seemed necessary was given him. Next morning I was called, and after examining the patient carefully, I found that the teguments of the right arm were almost entirely detached from the flesh, and hanging loose, as well as the skin of the lower part of it. In the space contained between the shoulders and the thigh, the teguments were as much injured as those of the right arm. The first thing, therefore, to be done, was to take away those pieces of skin; and, perceiving that a mortification was begun in that part of the right hand which had received the greatest hurt, I scarified it without loss of time; but notwithstanding this precaution, I found it next day, as I had suspected the preceding evening, entirely sphacelous. On my third visit, all the other wounded parts appeared to be in the same condition. The patient complained of an ardent thirst, and was agitated with dreadful convulsions. He voided by stool bilious putrid matter, and was distressed by a continual vomiting, accompanied with a violent fever and delirium. At length the fourth day after a comatose sleep of two hours, he expired. During my last visit, while he was sunk in the lethargic sleep of which I have spoken, I observed with astonishment, that putrefaction had already made so great progress, that his body exhaled an insupportable smell. I saw the worms which issued from it crawling on the bed, and the nails of his fingers drop of themselves; so that I thought it needless to attempt any thing farther, while he was in this deplorable condition. Having taken care to get every possible information from the patient himself, respecting what had happened to him, he told me, that he had felt a stroke, as if somebody had given him a blow over the right arm, with a large club, and that at the same time, he had seen a spark of fire attach itself to his shirt, which in a moment was reduced to ashes, though the fire did not in the least injure the wrist-bands. The handkerchief which he had placed upon his shoulders, between his shirt and his skin, was perfectly entire, without the least appearance of burning, his drawers were untouched, but his night-cap was destroyed, though a single hair of his head was not hurt. That this flame under the form of elementary fire, burnt the skin, reduced the shirt to ashes, and entirely consumed the night-cap, without in the least touching the hair, is a fact which I affirm to be true: besides, every symptom that appeared on the body of the deceased, announced severe burning. The night was calm, and the circumambient air very pure: no bituminous smell could be perceived in the chamber, nor was there the least trace of fire or of smoke. A lamp, however, which had been full of oil, was found dry, and the wick almost in ashes. We cannot reasonably suppose this fatal accident to have been occasioned by any external cause; and I have no doubt that if Maffei were still alive he would take advantage of it to support an opinion which he entertained, that lightning is sometimes kindled in the human body and destroys it.”

Another account, to the same purpose, is given in Mr. Battaglia’s paper. “On the 21st of April, 1781, the first battalion of the brigade of Savoy set out from Tortona, in order to go to Arti, when the weather was excessively hot. On the 22d, having made rather a forced march, the soldiers suffered a great deal from the ardour of the sun, so that, at the village of Serre, where they halted, one of them, named Bocquet, a man of twenty-five years of age, whose skin being very hard and thick had not perspired, sent forth a loud cry, which seemed to announce some extraordinary commotion, and instantly fell down. Mr. Bianet, surgeon major to the regiment, found the patient in convulsions. When he was carried to the hospital the upper part of his body, to the thighs, appeared to be withered and black, and in a gangrenous state. Mr. Bianet employed scarifications, but without effect; it was impossible to make him swallow any thing; and it was found necessary to abandon him to his dismal fate. His body soon exhaled a putrid smell, and he died at the end of five hours. That his disorder might not be communicated to others, he was interred together with his clothes. Upon inquiry, after his death, it was found that this man was addicted to the constant use of spiritous liquors, and that he had even drank of them to excess upon the march.”

Other instances there are, still more terrible, of people actually taking fire and being consumed to ashes by some internal cause; but, as nobody was present either at the beginning or during the continuance of these extraordinary inflammations, nothing certain can be said about them. That such things, however, have happened, is certain, of which one of the most remarkable instances is that of Signora Corn. Zangari, an Italian lady. She retired to her chamber in the evening somewhat indisposed, and in the morning was found in the middle of the room reduced to ashes, all except her face, legs, skull and three fingers. The stockings and shoes she had on were not burnt in the least. The ashes were light, and on pressing them between the fingers vanished, leaving behind a gross, stinking moisture, with which the floor was smeared; the walls and furniture of the room being covered with a moist cineritious soot, which had not only stained the linen in the chests, but had penetrated into the closet, as well as into the room overhead, the walls of which were moistened with the same viscous humour. This lady had been accustomed to use a bath of camphorated spirit of wine when indisposed.

Dr. Zimmerman, from the 64th volume of the Philosophical Transactions, relates the case of a poor woman who perished in this miserable manner at Coventry in England in the year 1772. “She fell out of bed, and was found next morning burnt to death, though the fire in the grate had been small, and the furniture in the room had suffered but little. Except one thigh and leg, there were not the least remains of any skin, vessels or viscera; and the greater part of the bones were completely calcined, and covered with a whitish efflorescence.”

On these unfortunate people it has been observed that they were generally intemperate in the use of spiritous liquors. Of the poor woman at Coventry, whose case has been just now related, it is said, that she had been in the practice of drinking from half a pint to a quart of rum every day, and this she continued, notwithstanding her being affected with jaundice and other complaints. Mr. Wilmer, who communicated this case to the Royal Society, concludes it with these words: “That her solids and fluids were rendered inflammable by the immense quantity of spiritous liquors she had drank, and when she was set fire to she was probably soon reduced to ashes.”

On other cases of a similar nature it has been remarked, that the miserable sufferers were “for the most part advanced in years, remarkably fat, and had been much addicted to the use of spiritous liquors, either in their drink, or applied in friction to the body; whence it has been concluded that these people perished by their whole substance spontaneously taking fire, the principal seat of which had been the entrails, or the epigastric viscera; and that the exciting cause was naturally found in the phlogiston of the humours, called forth by that of the spiritous liquors combined with them.”85 But solutions of this kind cannot by any means be admitted. We have not the smallest reason to think that either the solid or fluid parts of the bodies of hard drinkers are more inflammable than those of other people; neither is it credible that any person could live with his body in such a state. Besides, the most inflammable bodies will not begin to burn unless fire actually be applied to them, while others much less inflammable to appearance, will yet take fire spontaneously. Thus, even spiritous liquors themselves, though they flame violently when thrown into a fire, or when a burning body is applied to them, yet there is not an instance of such liquors taking fire of themselves; nay, they cannot even be set on fire by pouring them upon a red-hot iron, while, on the other hand, heaps of wet vegetables, which we should think scarce at all inflammable, do yet very frequently take fire spontaneously. The author lately quoted, however, justly observes that M. Bartholi, the unfortunate priest above mentioned was plainly struck first by electricity from without, a spark of fire attaching itself to his shirt, and a faint flame surrounding his body; so that the fire did not seem to have been generated in his body, but in the atmosphere. There are instances of people being surrounded with these luminous appearances without being hurt; particularly of a woman at Milan, whose bed was surrounded with a light of this kind. Mr. Loammi Baldwin, of this country, was also surrounded by an electric light, while raising a kite in the time of a thunder storm, and Dr. Priestley makes mention of a gentleman, who, after having worked an electric machine for a long time in a small room, perceived, on leaving it, a luminous vapour following him. But the instances most to our present purpose are some recorded in the Philosophical Transactions, of luminous vapours coming from the sea, attaching themselves to corn-stacks, and setting fire to them. One of this kind is particularly mentioned in Lowthorp’s Abridgement of the Transactions, as having taken place in Ireland, coming repeatedly from the sea, and setting fire to corn and hay, so that the people were greatly alarmed. At last they found that it might be driven off by making a great noise, and that it would avoid any sharp-pointed iron instrument. Had such a vapour attached itself to a human body, it is possible that it might have set fire to it as well as to the stack of corn or hay. Whether these accounts render the story of the Genoese sailors concerning the ball of fire occasioning the plague of 1346 more credible, we leave the reader to judge. They certainly show, however, that the electric fluid will sometimes interfere with the human body in a very terrible manner, producing, where it does not kill instantaneously, symptoms equal to those of the very worst plague, as in the case of the priest and soldier above mentioned.

Another hypothesis concerning the origin of pestilential diseases is that of swarms of little animals invisibly existing in the atmosphere; which, being taken into the body by the breath, are supposed to corrupt or otherwise vitiate the blood and other parts of the body, as we see in the plague and other epidemic disorders. This hypothesis, so generally exploded, and so apparently improbable, seems to receive some support from a discovery of an insect made by Mr. Henry Baker, F. R. S. and published in his work entitled “The Microscope made Easy.” He called it the insect with net-like arms. “It lives (says he) only in cascades, where the water runs very swift. Some of them being kept in a vial of water, most died in two days, and the rest, having spun themselves transparent cases, which were fastened either to the sides of the glass, or to pieces of grass put into it, seemed to be changed into a kind of chrysalis; but before they assumed this form, they altered their shape (in a manner he represents by a figure.) None of them lived above three days; and, though fresh water was given them two or three times a day, yet in a few hours it would stink to a degree scarce conceivable, and that too at several yards distance, though, in proportion to the water, all the included insects were not more than as one to one million, an hundred and fifty thousand. This makes it probable that it is necessary for them to live in a rapid stream, lest they should be poisoned by the effluvia issuing from their own bodies, as no doubt they were in the vial.”

From this account it is not difficult to conceive that animals, though exceedingly small, may yet emit such poisonous effluvia as will destroy much larger ones in their neighbourhood. It will by no means be incredible that, had one or two such offensive animals been thrown into a jar containing gold-fishes,86 the whole of these beautiful inhabitants would have perished at once. Let us suppose such a thing to have actually happened; that a malicious person had put them in over night, and in the morning the proprietor of the fishes finds them all dead, and the water offensive to the last degree. He sends for a neighbouring philosopher, who, happening to be ignorant of the existence of such animals, endeavours to account for the phenomenon upon some of the received principles of philosophy. How much theory would here be wasted, and what endless disputes might ensue without even a possibility of arriving at the truth! Just so it is with epidemic diseases. The cause is invisible, and, until it becomes discoverable by our senses, it can never be known; for, as has already been observed, a cause never can be known merely by its effects, unless we have seen it, or somebody who has seen it gives us information. And this will certainly be found to hold good in every instance, even from the Supreme Cause himself to the diminutive insect just mentioned.