Some bodies, which under the influence of light, undergo chemical changes, have the power of restoring themselves to their original condition in the dark. This is more remarkably displayed in the iodide of platinum, which readily recieves a photogenic image by darkening over the exposed surfaces, but speedily loses it by bleaching in the dark. The ioduret of Daguerre's plate, and some other iodides, exhibit the same peculiarity--This leads us to the striking fact, that bodies which have undergone a change of estate under the influence of day-light have some latent power by which they can renovate themselves. Possibly the hours of night are as necessary to inanimate nature as they are to the animate. During the day, an excitement which we do not heed, unless in a state of disease, is maintained by the influence of light and the hours of repose, during which the equilibrium is restored, are absolutely necessary to the continuance of health. Instead of a few chemical compounds of gold and silver, which at first were alone supposed to be photographic, we are now aware that copper, platinum, lead, nickel, and indeed, probably all the elements, are equally liably to change under the sun's influence. This fact may be of benefit to engravers, for if steel can be made to take photographic impressions, the more laborious process of etching may be dispensed with. In fact, in the latter part of this work, a process is described for etching and taking printed impressions from Daguerreotype plates. As yet this process has produced no decided beneficial results--but future experiments may accomplish some practical discovery of intrinsic value to the art of engraving. A very simple experiment will prove how essential light is to the coloring of the various species comprising the vegetable and animal kingdoms. If we transplant any shrub from the light of day into a dark cellar, we will soon see it lose its bright green color, and become perfectly white. Another effect of light is that it appears to impart to bodies some power by which they more readily enter into chemical combination with others. We have already said that chlorine and hydrogen, if kept in the dark, will remain unaltered; but if the chlorine alone be previously exposed to the sun, the chlorine thus solarised will unite with the hydrogen in the dark. Sulphate of iron will throw down gold or silver from their solutions slowly in the dark; but if either solution be first exposed to sunshine, and the mixture be then made, in the dark, the precipitation takes place instantly. Here is again, evidence of either an absorption of some material agent from the sunbeam, or an alteration in the chemical constitution of the body. It was from understanding these principles and applying them that philosophers were enabled to produce the Calotype, Daguerreotype, &c. For the effects and action of light on the camera, see Chapter V.