A Manual Of Photography

90 HISTORY OF PHOTOGRAPHY. is that which arises from a fresh exposure to the solar rays; where no change is mentioned, it is too slight to he worth notice. This phenomenon will presently occupy our attention. When papers prepared with any of the above, except the phosphates, are soaked for a little time in water, and dried in the sunshine, the picture produced,—it matters not what hydriodate is used,—is rendered peculiarly red, and does not change by re-exposure. By washing some of the papers with weak solution of ammonia, this peculiarity is produced in a very striking manner. The Solution of Silver.—Take of crystallized nitrate of silver 120 grains, distilled water 12 fluid drachms; when the salt is dissolved, add of alcohol 4 fluid drachms, which renders the solution opaque ; after a few hours, a minute quantity of a dark powder, which appears to he an oxide of silver, is ^ deposited, and must he separated by the filter. The addition of the alcohol to the solution was adopted from an observation I made of its influence in retarding the chemical action, which goes on in the shade, of the hydriodates on the salt of silver. Its use is, therefore, to make the action depend more on luminous influence, than would be the case without it. Nitric Ether.—The sweet spirits of nitre not only checks the bleaching process in the shade, but acts with the hydriodic salts to exalt the oxidation of the silver, or increase the blackness of it. In copying lace or any fine linear object, it is a very valuable agent, but it is useless for any other purposes, as all the faintly lighted parts are of the same tint. Hydrochloric Ether, used as the solvent of the silver, and applied without any saline wash, has a similar property to the nitric ether; but as it is readily acted on by faint light, it is of greater value. However, papers prepared with it must be used within twenty-four hours, as after that they quickly lose their sensitiveness, and soon become nearly useless. To fix with any degree of certainty the strength of the solution of the hydriodic salts which will in all cases produce the best effects, appears to me impossible ; every variety of light to which it has been exposed to darken, requiring a solution of different specific gravity. In the other divisions will be found some further remarks on the very peculiar physical phenomena presented by the action of the hydriodates on these darkened salts of silver, and details of yet more perfect forms of manipulation developed since these earlier processes were published.