Photographic Reproduction Processes

Photographic Reproduction Processes

The photographic processes with the salts of iron are all derived from the researches of Sir John Herschel. The investigations of that great philosopher are so valuable, so full of instructions that we are led to reprint them, together with those of Mr. C. J. Burnett, on the salts of uranium, etc., as an Introduction. It will be seen that the process by which blue prints are to-day obtained is exactly that Sir John Herschel devised in 1840. It is no longer an insulated and anomalous affection of certain salts of silver or gold, but one which, doubtless, in a greater or less degree, pervades all nature, and connects itself intimately with the mechanism by which chemical combination and decomposition is operated. The general instability of organic combinations might lead us to expect the occurrence of numerous and remarkable cases of this affection among bodies of that class, but among metallic and other elements inorganically arranged, instances enough have already appeared, and more are daily presenting themselves, to justify its extension to all cases in which chemical elements may be supposed combined with a certain degree of laxity, and so to speak in a tottering equilibrium. There can be no doubt that the process, in a great majority, if not in all cases, which have been noticed among inorganic substances, is a deoxidizing one, so far as the more refrangible rays are concerned. It is obviously so in the cases of gold and silver. In the case of the bichromate of potash it is most probable that an atom of oxygen is parted with, and so of many others. A beautiful example of such deoxidizing action on a non-argentine compound has lately occurred to me in the examination of that interesting salt, the ferrosesquicyanuret [pg 8] of potassium described by Mr. Smee in the Philosophical Magazine, No. 109, September, 1840, and he has shown how to manufacture in abundance and purity, by voltaic action on the common or yellow ferrocyanuret. In this process nascent oxygen is absorbed, hydrogen given off, and the characters of the resulting compound in respect of the oxides of iron, forming as it does Prussian blue with proto salts, indicate an excess of electro-negative energy, a disposition to part with oxygen, or which is the same thing, to absorb hydrogen (in the presence of moisture), and thereby to return to its pristine state, under circumstances of moderate solicitation, such as the affinity of protoxide of iron (for instance) for an additional dose of oxygen, etc. Paper simply washed with a solution of this salt is highly sensitive to the action of the light. Prussian blue is deposited (the base being necessarily supplied by the destruction of one portion of the acid, and the acid by the destruction of another). After half an hour or an hour's exposure to sunshine, a very beautiful negative photograph is the result, to fix which, all that is necessary is to soak it in water in which a little sulphate of soda is dissolved. While dry the impression is of a dove color or lavender blue, which has a curious and striking effect on the greenish yellow ground of the paper produced by the saline solution. After washing the ground color disappears and the photograph becomes bright blue on a white ground. If too long exposed, it gets over-sunned, and the tint has a brownish or yellowish tendency, which, however, is removed in fixing; but no increase of intensity beyond a certain point is obtained by the continuance of exposure. ......Buy Now (To Read More)

Product details

Ebook Number: 24016
Author: Duchochois, Peter C.
Release Date: Dec 24, 2007
Format: eBook
Language: English