Chapter VIII

ACTINISM—CHEMICAL RADIATIONS.

The Sun-ray and its Powers—Darkening of Horn Silver—Niepce’s Discovery—Prismatic Spectrum—Refrangibility of Light, Heat, and Actinism—Daguerre’s Discovery—Photography—Chemical Effects produced by Solar Radiations—Absorption of Actinism—Phenomena of the Daguerreotype—Chemical Change produced upon all Bodies—Power of Matter to restore its Condition—Light protects from Chemical Change—Photographs taken in Darkness—Chemical Effects of Light on organized Forms—Chemical Effects of Solar Heat—Influence of Actinism on Electricity—Radiations in Darkness—Moser’s Discoveries, &c.

Heat and light are derived from the sun, and we have attempted to show, not only that the phenomena of these two principles are different, but that they can scarcely, in the present condition of our knowledge, be regarded as modified manifestations of one superior power. Associated with these two remarkable elements, others may exist in the solar rays. Electrical phenomena are certainly developed by both heat and light, and peculiar electric changes are produced by exposure to sunshine. Electricity may be merely excited by the solar rays, or it may flow like light from the sun. Chemical action may be only due to the disturbance of some diffused principle; or it may be directly owing to some agency which is radiated at once from the sun.

A sun ray is a magical thing: we connect it in our fancy with the most ethereal of possible creations. Yet in its action on matter it produces colour; it separates the particles of solid masses farther from each other, and it breaks up some of the strongest forces of chemical affinity. To modern science is entirely due the knowledge we have gained of the marvellous powers of the sunbeam; and it has rendered us familiar with phenomena, to which the incantation scenes of the Cornelius Agrippas of the dark ages were but ill-contrived delusions, and their magic mirrors poor instruments. The silver tablets of the photographic artist receiving fixed impressions of the objects represented in the dark chamber by a lens, are far superior as examples of natural magic.

In the dark ages, or rather as the earliest gleams of the bright morning of inductive research were dispelling the mists of that phantom-peopled period, it was observed, for the first time, that the sun’s rays turned a white compound black. Man must have witnessed, long before, that change which is constantly taking place in all vegetable colours: some darkening by exposure to sunlight, while others were bleached by its influence. Yet those phenomena excited no attention, and the world knew nothing of the mighty changes which were constantly taking place around them. The alchemists—sublime pictures of credulous humanity—toiling in the smoke of their secret laboratories, waiting and watching for every change which could be produced by fire, or by their “royal waters,” caught the first faint ray of an opening truth; and their wild fancy, that light could change silver into gold, if they but succeeded in getting its subtile beams to interpenetrate the metal, was the clue afforded to the empirical philosopher to guide him through a more than Cretan labyrinth.[117]

The first fact recorded upon this, point was, that horn silver blackened when exposed to the light. Without doubt many anxious thoughts were given by these alchemists to that fact. Here was, as it appeared, a mixing up of light and matter, and behold the striking change! It was a step towards the realization of their dreams. Alas! poor visionaries! in pursuing an ideality they lost the reality which was within their grasp.

Truths come slowly upon man, and long it is before these angel visits are acknowledged by humanity. The world clings to its errors, and avoids the truth, lest its light should betray their miserable follies.

At length a man of genius announced that “No substance can be exposed to the sun’s rays without undergoing a chemical change;” but his words fell idly upon the ear. His friends looked upon his light-produced pictures as singular; they preserved them in their cabinets of curiosities; but the truths which he enunciated were soon forgotten. Howbeit his words were recorded, and it is due to the solitary experimentalist of Châlons on the Saône, to couple the name of Niepce with the discovery of a fact which is scarcely second to the development of the great law of universal gravitation.[118] But an examination awaits us, which, for its novelty, has more charms than most branches of science, and which, for the extensive views it opens to the inquirer, has an interest in nowise inferior to any other physical investigation.

The prismatic spectrum affords us the means of examining the conditions of the solar rays with great facility. In bending the ray of white light out of its path, by means of a triangular piece of glass, we divide it in a remarkable manner. We learn that heat is less refracted by the glass than the other powers; we find the maximum point of the calorific rays but slightly thrown out of the right line, which the solar pencil would have taken, had it not been interrupted by the prism; and the thermic action is found to diminish with much regularity on either side of this line. We discover that the luminous power is subject to greater refraction, and that its maximum lies considerably above that of heat; and that, in like manner, on each side the light diminishes, producing orange, red, and crimson colours below the maximum point, and green, blue, and violet above it. Again, we find that the radiations which produce chemical change are more refrangible than either of the others, and the maximum of this power is found at the point where light rapidly diminishes, and where scarcely any heat can be detected: it extends in full activity, above its maximum, to a considerable distance, where no trace of light under ordinary conditions exists, and below that point, until light, appearing to act as an interfering agent, quenches its peculiar properties. These are strong evidences that light and actinism—as this principle has been named—are not identical: and we may separate them most easily and effectually from each other. Certain glasses, stained dark blue, with oxide of cobalt, admit scarcely any light; but they offer no interruption to the passage of actinism or the chemical rays; on the contrary, a pure yellow glass, or a yellow fluid, which does not sensibly reduce the intensity of any one colour of the chromatic band of luminous rays, completely cuts off this chemical principle, whatever it may be. In addition to these, there are other results which we shall have to describe, which prove that, although associated in the solar beam, light and actinism are in constant antagonism.

When Daguerre first published his great discovery, the European public regarded his metal tablets with feelings of wonder: we have grown accustomed to the beautiful phenomena of this art, and we have become acquainted with a number of no less beautiful processes on paper, all of which, if studied aright, must convince the most superficial thinker, that a world of wonder lies a little beyond our knowledge, but within the reach of industrious and patient research. Photography is the name by which the art of sun-painting will be for ever known. We regard this as unfortunate, conveying as it does a false idea,—the pictures not being light-drawn. Could we adopt the name given by Niepce to the process, the difficulty would be avoided, since Heliography involves no hypothesis, and strictly tells the undeniable truth, that our pictures are sun-drawn. That pictures can be produced by the rays from artificial sources, presents no objection to this; these rays were still originally derived from the sun.

By whatever name we determine to convey our ideas of these phenomena, it is certain that they involve a series of effects which are of the highest interest to every lover of nature, and of the utmost importance to the artist and the amateur. By easy manipulation we are now enabled to give permanence to the charming pictures which are produced by means of that pleasing invention of Baptista Porta, the Camera Obscura. Any image, which being refracted by the lens of this instrument falls upon the table in its dark chamber, may be secured with its most delicate gradations of shadows, upon either a metallic or a paper tablet.

But let us proceed to the examination of a few of the more striking phenomena of these chemical changes. To commence with some of the more simple but no less important results.

Chlorine and hydrogen will not unite in darkness, nor will chlorine and carbonic oxide; but, if either of those gaseous mixtures is exposed to sunshine, they combine rapidly, and often with explosion. A solution of the sulphate of iron in ordinary water may be preserved for a long time in the dark without undergoing any change; expose it to the sunshine, and a precipitation of oxide of iron is very rapidly produced. The mineral chameleon, the manganesiate of potash in solution, is almost instantly decomposed in daylight; but it is a long time before it undergoes any change in darkness. The same thing occurs with a combination of platinum and lime: indeed, it appears that precipitation is at all times, and under all circumstances, accelerated by the solar rays. As these precipitations are in exact agreement with the quantity of actinic radiation to which the solutions have been exposed, we may actually weigh off the relative quantities, representing in grains the equivalent numbers to the amount of actinism which has influenced the chemical compound.[119]

We have evidence which appears to prove that this chemical agent may be absorbed by simple bodies, and that by this absorption an actual change of condition, is produced, in many respects analogous to those allotropic changes which we have previously considered. Chlorine, in its ordinary state, does not combine with hydrogen in the dark. If we employ the yellow medium of chlorine gas, for the purpose of analyzing the sun’s rays previously to their falling upon some chemical compound which is sensitive to actinic power, we shall find that the chlorine obstructs all this actinism, and, however unstable the compound, it remains unchanged. But the chlorine gas which has interrupted this wonderful agent, appears to have absorbed it, and it is so far altered in its constitution that it will unite with hydrogen in the dark.[120] In like manner, if, of two portions of the same solution of sulphate of iron, one is kept in the dark and the other exposed to the sunshine, it will be found that the solution which has been exposed will precipitate gold and silver from their combinations much more speedily than that which has been preserved in darkness—the temperature and every other condition being the same.

The phenomena of the Daguerreotype involve many strange conditions. A plate of silver, on which a slight chemical action has been established by the use of iodine, is exposed to the lenticular image in the camera obscura. If allowed to remain under the influence of these radiations for a sufficient length of time, a faithful picture of the illuminated objects is delineated on the plate, as shown by the visible decomposition and darkening of the iodized surface. The plate is not, however, in practice allowed to assume this condition; after an exposure of a few seconds the radiant influence is cut off, and the eye cannot detect any evidence of change upon the yellow plate. It is now exposed to the vapour of mercury, and that metal in a state of exceedingly fine division is condensed upon the plate; but the condensation is not uniformly spread upon its face. The deposit of mercurial vapour is in exact proportion to the amount of chemical action produced. Is the change, by which this peculiar power of condensation is effected, a chemical, calorific, electrical, or merely a molecular one? The evidences, at present, are not sufficient to determine the question. It has lately been suggested, that the mercury acts chemically only, and effects the full decomposition of the iodide of silver; and that the picture is due to this, and not to the deposition actually of the mercury vapour. In all probability we have the involved action of several forces. We have some experiments which show, clearly enough, that mercury is deposited in proportions which correspond with the intensity of solar action. A chemically prepared surface is not necessary to exhibit this result. A polished plate of metal, of glass, of marble, or a piece of painted wood, being partially exposed, will, when breathed upon, or presented to the action of mercurial vapour, show that a disturbance has been produced upon the portions which were illuminated, whereas no change can be detected upon the parts which were kept in the dark. It was thought, until lately, that a few chemical compounds, such as the iodide of silver, the material employed in the Daguerreotype and Calotype,—chloride of silver, the ordinary photographic agent,—a few salts of gold, and one or two of lead and iron, were the only materials upon which these very remarkable changes were produced. We now know that it is impossible to expose any body, simple or compound, to the sun’s rays, without its being influenced by this chemical and molecular disturbing power. To take our examples from inorganic nature, the granite rock which presents its uplifted head in firmness to the driving storm, the stones which genius has framed into forms of architectural beauty, or the metal which is intended to commemorate the great acts of man, and which in the human form proclaims the hero’s deeds and the artist’s talent, are all alike destructively acted upon during the hours of sunshine, and, but for provisions of nature no less wonderful, would soon perish under the delicate touch of the most subtile of the agencies of the universe.

Niepce was the first to show that all bodies which underwent this change during daylight possessed the power of restoring themselves to their original conditions during the hours of night, when this excitement was no longer influencing them. Resins, the Daguerreotype plate, the unprepared metal tablet, and numerous photographic preparations, prove this in a remarkable manner.[121]

The picture which we receive to-day, unless we adopt some method of securing its permanency, fades away before the morrow, and we try to restore it in vain. With some of our chemical preparations this is very remarkably shown, but by none in so striking a manner as by paper prepared with the iodide of platinum, which, being impressed with an image by heliographic power, which is represented by dark brown tints, restores itself in the dark, in a few minutes, to its former state of a yellow colour, and recovers its sensibility to sunshine.[122] The inference we alone can draw from all the evidences which the study of actino-chemistry affords, is, that the hours of darkness are as necessary to the inorganic creation as we know night and sleep to be to the organic kingdom. But we must not forget that there does exist in the solar rays a balance of forces which materially modifies the amount of disturbing influence exerted by them on matter. Not only do we find that the chemical action is not extended over the whole length of the prismatic spectrum, but we discover that over spaces, which correspond with the maximum points of light and heat, a protective action is exerted. That is, that highly sensitive photographic agents, which blacken rapidly under exposure to diffused daylight, are entirely protected from change in full sunshine, if at the same time as a strong light is thrown upon them by reflection, the yellow and extra red rays are brought to bear upon their surface. Not only so, but by employing media which will cut off all the chemical rays of the spectrum, admitting freely at the same time the luminous and calorific rays, we find that a protected band, the length of the spectrum, remains white, whilst every other portion has blackened.[123]

Among the many curious instances of natural magic, none are more remarkable than an experiment not long since proposed, by which Daguerreotype pictures may be taken in absolute darkness to the human eye. This is effected in the following manner:—A large prismatic spectrum is thrown upon a lens fitted into one side of a dark chamber; and as we know that the actinic power resides in great activity beyond the violet ray, where there is no light, the only rays which we allow to pass the lens into the chamber are those which are extra-spectral and non-luminous. These are directed upon, any white object, and from that object radiated upon a highly sensitive plate in a camera obscura. Thus a copy of the subject will be obtained by the agency of radiations which produce no sensible effect upon the optic nerve. This experiment is the converse of those which show us that we may illuminate any object with the strongest sunlight which has passed through yellow glass, the yellow solution of sulphuret of calcium, or of the bichromate of potash—these being non-transparent to the chemical rays—and yet fail to secure any Daguerreotype copy of it, even upon the most exquisitely sensitive plate. Indeed, the image of the sun itself, when setting through an atmosphere which reduces its light to a red or rich yellow colour, not only produces no chemical change, but protects an iodized plate from it; and whilst every other part of the tablet gives a picture of surrounding objects in the ordinary character, the bright sun itself is represented by a spot upon which no change has taken place.[124] In tropical climes, where a brilliant sun is giving the utmost degree of illumination to all surrounding objects, all photographic preparations are acted upon relatively more slowly than in the climate of England, where the light is less intense. As a remarkable instance of this fact, a circumstance may be mentioned, which is curiously illustrative of the power of light to interfere with actinism:—

A gentleman, well acquainted with the Daguerreotype process, obtained in the city of Mexico all the necessary apparatus and chemicals, expecting, under the bright light and cloudless skies of that climate, to produce pictures of superior excellence. Failure upon failure was the result; and although every care was used, and every precaution adopted, it was not until the rainy season set in that he could secure a good Daguerreotype of any of the buildings of that southern city.

The first attempts, which were made at the instigation of M. Arago, by order of the French Government, to copy the Egyptian tombs and temples, and the remains of the Aztecs in Central America, were failures. Although the photographers employed succeeded to admiration in Paris, in producing pictures in a few minutes, they found often that an exposure of an hour was insufficient under the bright and glowing illumination of a southern sky.

Experiments with the spectrum have been made in different latitudes, and it is found, that, as we proceed towards the equator, a band which is always left unchanged, corresponding exactly with the rays of greatest illuminating power, regularly enlarges in size, thus proving the increase of light over actinism—and the interfering power of the former.

By increasing the sensibility of the photographic preparation, this difficulty is overcome, and particularly when any organic compound enters into the preparation. So that we are now enabled to copy nature in all her varying moods, whether we employ our photographic tablets in temperate Europe, or in tropical Africa.

The degree of sensibility which has been attained is remarkable. Mr. Fox Talbot, by uniting a process devised by Dr. Woods, of Parsonstown, and another which was first introduced by the author of this volume, and combining them with an ether, obtains a most unstable compound, which he thus employs. A glass plate is covered with albumen united with the above solution, and then with nitrate of silver: this forms the sensitive surface. The plate being placed in the dark, in a camera, it is so adjusted that the image of a printed bill fixed upon a wheel may fall upon it when uncovered, and the wheel illuminated. The wheel is made to revolve with the utmost rapidity, in a perfectly dark room, and the sensitive plate uncovered. Then the whirling bill is illuminated for an inappreciably short space of time by the discharge of a Leyden jar. Notwithstanding the rapid rate at which the pointed paper is moving, and the instantaneous nature of the illumination—a miniature flash of lightning—the bill is found to be copied with unfailing fidelity upon the photographic plate. It unfortunately happens, that the preparation by which this extraordinary degree of sensibility is obtained, is very uncertain in its action—and hence it is not generally useful; but here we have the evidence to show that at a speed as rapid as that of a rifle-ball an impression may be made upon a photographic plate. There are, however, some new processes which promise eventually to rival the above for sensibility, and to be by no means of difficult manipulation. Of this character is the collodion process. The gun cotton dissolved in ether possesses some very great accelerating properties, and in combination with the silver salts, and one of the vegetable acids, it forms a sensitive surface upon which pictures may be obtained in less than a second of time.

Colour, natural colour too, has been very decidedly secured. The sun has been solicited to display his palette, and the answer has been a picture in which colour for colour in all their fidelity have been impressed. The plate upon which this result has been obtained is of a dark brown colour, and the chromatic variety is, as it were, eaten out by the solar rays. These colours have not yet been permanently fixed upon the plate employed, but from the temporary degree of fixedness which has been obtained, we may fairly hope that in a short time colour may be rendered as permanent on the productions of the photographer as on those of the painter. It is a curious and striking fact, that in the preparation of these plates, salts are used which give colours to flame; and according to the colour which is produced by them when burning, so, on the photographic plate, is that colour impressed with greater intensity than the others. To what is this leading us? Mysteries surround our advances on the domain of truth. We dare not speculate upon them: the time of their full development will arrive.

By the aid of this beautiful art, we are enabled to preserve the lineaments of those who have benefited their race by their intellect, or their heroism. We can hand down to future ages portraits of our own Wellington, and the illustrious Arago, unerring in their truthfulness. How great would be the joy of all, could we now obtain a daguerreotype portrait of a Greek poet, or of a Roman philosopher, of a Sophocles, or of a Seneca! How much discussion would be prevented did we possess a calotype portrait of the Bard of Avon, or of the Philosopher of Grantham!

By the agency of those very rays which give life and brilliancy to the laughing eye and the roseate cheek, we can at once correctly trace the outline of the features we admire, with all those shadowy details which give a reality to the “presentment.” The objects of our love may be for ever present with us in these self-painted pictures. The vicious, whom we would avoid, may be made known to us by this unerring painter. The process which nature employs is perfect; the imperfections are those of man, and these being few, he may soon learn to remedy.

To the traveller, how valuable are the processes of photography! He secures representations of those remains of temples which were in their glory when Moses wrote. He copies by one operation a tomb at Karnac, covered with myriads of hieroglyphics, or an inscribed stone in Arabia, which it would occupy him days to trace. These he can carry to his home and read at his leisure. The relics of hoar antiquity speaking to the present of the past, and recording the histories of races which have fleeted away like shadows, are thus preserved to tell their wondrous tales.

The admirer of nature may copy her arrangements with the utmost fidelity. Every modulation of the landscape, each projecting rock or beetling tor—the sinuous river in its rapid flow—the meandering stream, “gliding like happiness away;” and the spreading plains over which are scattered the homes of honest industry and domestic peace, intermingled with the towers of those humble temples in which simple-hearted piety delights to “bow the head and bend their knee;” these, all of these, may, by the sunbeam which illuminates the whole, be faithfully pencilled upon our chemical preparations.

Our art enables us to do more even than this; we have but to present our sensitive tablet to the moon, and she, by her own light, prints her mountains and her valleys, and indicates with all truth the physical conditions of her surface.

Any reference to the chemical agency of light—the luminous rays as distinguished from the chemical and calorific rays—has been avoided until we came to the consideration of this particular question of chemical change.

Upon organic compounds, as, for instance, upon the colouring matter of leaves and flowers, light does exert a chemical power: and it is found that vegetable colours are bleached, not by rays of their own colour, but by those which are complementary to them. A red dye fades under the influence of a green ray, and a yellow under that of a violet one, much more speedily than when exposed to rays of any other colour; and this, it must be remembered, is due to the coloured ray itself, and not to any actinic power masked, as it were, behind the colour, as is generally believed.[125] It was long a question whether the decomposition of carbonic acid by plants was due to the luminous or the chemical rays. It is now clearly established that the luminous rays are the most active in producing this effect; which they do indirectly, by exciting the vital powers of the organized structures. Therefore we would refer this phenomenon of gaseous decomposition to a vital power quickened by luminous excitement.[126]

We have already noticed some chemical phenomena due to heat, particularly those experiments of Count Rumford’s, which appeared to him to prove that the chemical agency of the sun’s rays was due to its calorific power. Certain chemical phenomena, we know, may be produced by thermic action; but the only variety of thermo-chemical action which connects itself immediately with the solar radiations, belongs to a class of rays to which the name of Parathermic has been given, and to which the scorching, as it is called, of plants, the browning of the autumnal leaves, and the ripening of fruits, appear to be due.[127] When we come to the consideration of those physical phenomena which belong to the growth of plants, all these peculiarities of solar action must be attended to in detail.

The manner in which we find the actinic power influencing electrical action, also shows us that the equilibrium of forces is continued through all the great principles of nature. If a galvanic arrangement is made, by which small quantities of metals may be slowly precipitated at one of the poles in the dark, and a similar arrangement be exposed to sunshine, it will be found that no metal is deposited: the sun’s rays have interfered with the decomposing power of the electrical current. At the same time we learn, that by throwing a beam of light upon a plate of copper which forms one of a galvanic pair, whilst it is under the influence of an acidulated solution, an additional excitation takes place, and the galvanometer will indicate the passage of an increased current of electricity. These two dissimilar actions appear enigmatical; but they may, there is no doubt, receive some solution from the influence of different rays on the contrary poles of the battery. One thing is quite evident,—electricity suffers a disturbance of one order, by light; and an excitement of another by its associated principles in the sunbeam. If a yellow glass is interposed between the galvanic arrangement and the sun, the electro-chemical precipitation goes on in the same manner as it would in perfect darkness, and no extra excitement is produced upon the plates of the battery. From this it would appear that actinism and not light is to be regarded as the disturbing power.[128] It has already been shown that yellow media possess the power of stopping back the chemical agent.

We have already, detailed many of the peculiarities of the different varieties of Phosphori, which would seem to be the result of light. Phosphorescence is probably excited by those rays which produce no direct effect upon the eye. If we spread sulphuret of calcium upon paper, and expose it to the action of the solar spectrum, it is found to glow (in the dark) only over those spaces occupied by the violet rays and the ordinarily dark rays beyond them; proving that the excitation necessary to the development of the phenomena of phosphorescence is due to a class of rays distinct from the true light-giving principle, and more nearly allied to that principle or power which sets up chemical decomposition. Whether the fluorescent rays, before mentioned, which are found so abundantly over the space which produces the greatest phosphorescent effect, are active in producing the phenomena, is as yet an unsolved problem.

Vision and colour, calorific action, chemical change, molecular disturbance, electrical phenomena, and phosphorescent excitation, all, each one with a strange duality, are connected with the sunbeam.

We find, when we receive solar spectra upon iodized plates, or on several kinds of photographic paper, that a line, over which no action takes place, is preserved at the top and bottom of the impressed image, and in many cases along the sides also. The only way in which this can be accounted for, as the spectrum represents the sun in a distorted form, is by supposing that rays come from the edges of the sun of a different character from those which proceed from the centre of that orb.[129]

Light from the centre of the solar disc is under different conditions from that which comes from the edge of the sun: this is due to the varying angle, which is presented to us by a circular body: calorific action seems to be more strongly manifested when the envelope of light, extending like an atmosphere to the sun, is thrown into great agitation, and waves, and great hollows—solar spots—are produced. There is some indication of the existence of a third condition on the sun’s surface, to which probably belongs the mighty chemical power which we call actinism. Electricity may be, as some have speculated, the exciting agent; a constant and violent Aurora Borealis may exist on the sun, and under the excitation of this force the others named may be quickened into full activity.

That actinism is one of the great powers of creation we have abundant proof. Nearly all the phenomena of chemical change which have been referred to light, are now proved to be dependent upon actinic power; and beyond the influence which has been ascertained to be exerted by it upon all inorganic bodies, we shall have occasion to show still further the dependence of the vegetable and animal worlds upon its agency. The influence of the solar beams on vegetation is proved by common experience; the closer examination of its action on vegetable life is reserved for the chapter devoted to its phenomena. Of its influence on animals nothing is very correctly known; but some early experiments prove that they, like other organised bodies, are subject to all the radiant forces, as indeed, independent of experiment, every observation must teach. Certain it is, that organisation can take place only where the sun’s rays can penetrate: where there is unchanging darkness, there we find all the silence of death. Prometheus stole fire from heaven, and gave the sacred gift to man, as the most useful to him of all things in his necessities: by the aid of it he could temper the severities of climate, render his food more digestible and agreeable, and illuminate the hours of darkness. So says the beautiful fiction of the Grecian mind,—which appears as the poetic dream or prophetic glance of a gifted race, who felt the mysterious truth they were yet unable to describe. Pheaton and Apollo are only other foreshadowings of the creative energies which dwell in the glorious centre of our universe. The poetry of the Hellenic people ascended above the littlenesses of merely human action, and sought to interpret the great truths of creation. Reflective, they could not but see that some mysterious powers were at work around them; imaginative, they gave to fine idealisations the government of those inexplicable phenomena. Modern science has shown what vastly important offices the solar rays execute, and that the principles discovered in a sunbeam are indeed the exciters of organic life, and the disposers of inorganic form.

It must not be forgotten that we have already alluded to a speculation which supposes this actinic influence to be diffused through all nature, to be indeed the element to which chemical force in all its forms is to be referred, and that it is merely excited by the solar rays. This hypothesis receives some support from the very peculiar manner in which chemical action once set up is carried on, independent of all extraneous excitement, after the first disturbance has been produced. If any of the salts of gold are exposed in connection with organic matter, as on paper, to sunshine for a moment, an action is begun, which goes on unceasingly in the dark, until the gold is reduced to its most simple state.[130] The same thing occurs with chromate of silver, some of the salts of mercury, argentine preparations combined with protosulphate of iron or gallic acid, and some other chemical combinations. These progressive influences point to some law not yet discovered, which seems to link this radiant actinism with the chemical agent existing in all matter.

This problem also connects itself with another class of facts which, although due, in all probability, to a great extent, to calorific radiations, and hence known under the general term of Thermography, appear to involve both chemical and electrical excitation. From the investigations of Moser and of others, we learn the very extraordinary fact, that even inanimate masses act and react upon each other by the influence of some dark radiations, and seem to exchange some of the peculiarities which they possess. This appears generally in the curious experiments which have been referred to, as confined merely to form or structure. Thus an engraved plate will give to a polished surface of metal or glass placed near it, after a very little time, a neat distinct image of itself; that is, produce such a structural disturbance as will occasion the plate to receive vapour differently over those spaces opposite to the parts in cameo or in intaglio, from what it does over the opposite. If a piece of wood is used instead of a metal, there will, by similar treatment, be produced a true picture of the wood, even to the representation of its fibres.[131]

It is also probable that chemical decomposition is produced by the mere juxtaposition of different bodies. Iodide of gold or silver, perfectly pure, has been placed upon a plate of glass, and a plate of copper covered with mercury suspended over it: a gradual decomposition of those salts is said to have been observed, iodide of mercury to be formed, and the gold or silver salts reduced to a finely divided metallic state.[132]

A body whose powers of radiating heat are low, being brought near another whose radiating powers are more extensive, will, in the course of a short time, undergo such an amount of molecular disturbance as will effect a complete change in the arrangement of its surface, and an impression of the body having the highest radiating powers will be made upon the other. This impression is dormant, but may be developed under the influence of vapour, or of oxidation.[133] A body, such as charcoal, of low conducting power, being placed near another, such as copper, which is a good conductor, will, in a very short time, produce, in like manner, an impression of itself upon the metal plate. Thus any two bodies, whose conducting or radiating powers are dissimilar, being brought near each other, will occasion a molecular disturbance, or impress the one with the image of the other. However small the difference may be, an effect is perceived, and that of the most extraordinary kind, giving rise to the production of actual images upon each surface exposed. It is thus that a print on paper may be copied on metal, by merely suspending it near a well-polished plate of silver or copper for a few days. The white and black lines radiate very differently; consequently an effect is produced on the bright metal in the parts corresponding to the black lines, dissimilar to that which takes place opposite to the white portions of the paper; and, on the application of vapour, a true image of the one is found impressed upon the other.[134]

Bodies which are in different electrical states act upon each other in an analogous manner. Thus arsenic, which is highly electro-negative, will, when placed near a piece of electro-positive copper, readily impart to its surface an impression of itself, and so in like manner will other bodies if in unlike conditions. Every substance physically different (it signifies not whether as it regards colour, chemical composition, mechanical structure, calorific condition, or electrical state,) has a power of radiation by which a sensible change can be produced in a body differently constituted.

Fable has told us that the magicians of the East possessed mirrors in which they could at will produce images of the absent. Science now shows us that representations quite sufficient to deceive the credulous can be produced on the surface of polished metals without difficulty. A highly polished plate of steel may be impressed with images of any kind, which would remain invisible, the polished surface not being in the least degree affected, as it regards its reflecting powers; but by breathing over it, the dormant images would develope themselves, and fade away again as the condensed moisture evaporated from the surface.[135]

These, which are but a few selected from a series of results of an equally striking character, serve to convince us that nature is unceasingly at work, that every atom is possessed of properties by which it influences every other atom in the universe, and that a most important class of natural phenomena appear to connect themselves directly with the radiant forces.

The alchemists observed that a change took place in chloride of silver exposed to sunshine. Wedgwood first took advantage of that discovery to copy pictures. Niepce pursued a physical investigation of the curious change, and found that all bodies were influenced by this principle radiated from the sun. Daguerre produced effects from the solar pencil which no artist could approach to; and Talbot and others extended the application. Herschel took up the inquiry; and he, with his usual power of inductive search and of philosophical deduction, presented the world with a class of discoveries which showed how vast a field of investigation was opening for the younger races of mankind,—a field in which a true spirit may reap the highest reward in the discovery of new facts, and to which we must look for a further development of those great powers with which we have already some slight acquaintance, and for the discovery of higher influences which are not yet dreamed of in our philosophy.

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