Numerous speculations having been ventured as to the peculiar chemical changes which light produces on the iodidated silver tablets, I shall make no apology for introducing a few remarks on this very interesting subject.

Numerous experiments on plated copper, pure silver plates, and on silvered glass and paper, have convinced me that the first operations of polishing with nitric acid, etc., are essential to the production of the most sensitive surface. All who will take the trouble to examine the subject will soon be convinced that the acid softens the silver, bringing it to a state in which it is extremely susceptible of being either oxidized or iodized, according as the circumstance may occur, of its exposure to the atmosphere or to iodine. The process, adopted, I believe, first in America, of producing a deposit of chemically pure silver on the plated metal, by means of the voltaic battery, which certainly gives rise to some peculiar conditions, appears to prove that the soft surface of silver is of advantage.

The sensitive surface is a combination of iodine, or of iodine and bromine, with the silver. When exposed to radiant influences in the camera, a molecular change is effected, and there is much doubt if any iodine or bromine is removed from the surface. Some have thought that the superficial film being decomposed, the iodine and bromine attack a lower surface of the plate ; but experiments are still wanting.

I have discovered that all the rays of the prismatic spectrum act on the Daguerreotype plate, except the yellow, and a circle of light of a peculiar and mysterious character, which surrounds the visible spectrum. The light acting on a prepared tablet, appears to decompose the film of iodide of silver to different depths, according to the order of refrangibility of the rays : the violet ray and extra-spectral rays effecting the . deepest decomposition, whilst the red acts to a depth inappreciably slight. Thus it is that the spectrum impressed on a Daguerreotype plate reflects natural tints of the same kind as Sir Isaac Newton's thin films ; the thickness of each film of reduced silver on the plate being in exact proportion to the chemical agency of the coloured ray by which it was decomposed.

On photographic papers, the decomposed argentine salt exists, in all probability, in a state of oxide, mixed with revived silver; but on the silver tablet the iodide is changed over all the parts on which the light acts, and pure silver in a state of extreme division results. The depth to which the decomposition has been effected being in exact relation to the intensity and colour of the light radiated from the object which we desire to copy, the mercurial vapour unites with different proportions of silver, aids in encreasing the decomposition of the silver salt, and thus are formed the lights and middle tints of the picture. The shadows are produced by the unchanged silver from which the iodine is removed by the hyposulphite of soda.

Daguerre himself laid much stress upon the necessity of exposing the plate to receive the vapour of mercury at an angle of 45°. This, perhaps, is the most convenient position, as it enables the operator to view the plate distinctly, and watch the development of the design : but beyond this, I am satisfied there exists no real necessity for the angular position. Both horizontally and vertically, I have often produced equally effective daguerreotypes. Looking at a daguerreotype picture in such a position that the light is incident and reflected at a large angle, the drawing appears of the negative character ; the silver in such a position appearing white, and the amalgam of mercury and silver a pale grey. View the plate in any position which admits of but a small angle of reflection, and we then see the design in all its exquisite beauty, correct in the arrangement of its lights and shades,—the silver appearing black, while the amalgam, by contrast in part, and partly in reality, appears nearly white.

The cause leading to the uniform deposition of the mercurial vapour is difficult of solution. It does not appear to me that any one of the hypotheses put forth, satisfies all the conditions of this peculiar phenomenon.

Few papers have been published which so completely investigate the phenomena of the chemical change in the daguerreotype, as that of Mr. George Shaw. As giving a large amount of valuable information, I transfer it from the Philosophical Magazine.

" It is well known that the impression produced by light on a plate of silver rendered sensitive by M. Daguerre's process, is wholly destroyed by a momentary exposure of the plate to the vapour of either iodine or bromine. Although this fact has long been known, the nature of the action by which so extraordinary an effect is produced has not yet been satisfactorily explained. In the hope of elucidating this subject, a series of experiments was instituted, the results of which are recorded in the following remarks.

" A silver plate prepared by exposure to iodine or its compounds with bromine, may be exposed to the vapour of mercury without being in any way affected by the exposure. If, however, the prepared plate be previously exposed to light, or made to receive the luminous image formed in the camera obscura, the mercurial vapour attacks it; forming, in the former case, a white film, and in the latter, a picture corresponding to the luminous image which had been allowed to fall on it.

" If a prepared plate, after receiving a vertical impression by light, be exposed to the vapour of iodine or bromine, it is found that the vapour of mercury no longer attacks it ; or, in other words, the impression produced by light is destroyed.

" The first experiments made for the purpose of arriving at the cause of this phenomenon had reference to the relation between the time of the exposure to light and the time of exposure to the vapour of iodine or bromine necessary to destroy the effect produced by light. Prepared plates were exposed in the camera obscura for a length of time, which previous experiment had determined to be sufficient for a full development of the picture; some of those plates were exposed during two seconds to an atmosphere feebly charged with the vapour of bromine, while others were carefully preserved from contact with the vapours of iodine or bromine. The atmosphere of bromine employed, was produced by adding thirty drops of a saturated solution of bromine in water to an ounce of water : the solution was poured into a glass vessel, and the plate was exposed to the vapour in the vessel during the time specified. The plates were then introduced into the mercury box, and by volatilizing the metal, pictures were developed on all those which had not been exposed to the vapour of bromine, while those which had been exposed to it exhibited no trace of a picture under the action of mercury. " The same experiments were repeated with iodine, with exactly similar results.