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