The Poetry of Science; or, Studies of the Physical Phenomena of Nature

by Robert Hunt

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]