3. The process with Bichloride of Mercury and Ammonia.—The image is first converted into the usual white double Salt of Mercury and Silver by the application of a solution of the Corrosive Sublimate. It is then treated with Ammonia, the effect of which is to blacken it intensely. Probably the alkali acts by converting Chloride of Mercury into the black Oxide of Mercury. In place of Ammonia, a dilute solution of Hyposulphite of Soda or Cyanide of Potassium may be used, with very similar results.
[CHAPTER VIII.]
ON THE THEORY OF POSITIVE PRINTING.
The subject of Collodion Negatives having been explained in the previous Chapter, we proceed to show how they may be made to yield an indefinite number of copies with the lights and shadows correct as in nature.
Such copies are termed "Positives," or sometimes "Positive prints," to distinguish them from direct Positives upon Collodion.
There are two distinct modes of obtaining photographic prints;—first by development, or, as it is termed, by the Negative process, in which a layer of Iodide or Chloride of Silver is employed, and the invisible image developed by Gallic Acid; and second, by the direct action of light upon a surface of Chloride of Silver, no developer being used. These processes, involving chemical changes of great delicacy, require a careful explanation.
The action of light upon Chloride of Silver was described in Chapter II. It was shown that a gradual process of darkening took place, the compound being reduced to the condition of a coloured subsalt; also, that the rapidity and perfection of the change were increased by the presence of excess of Nitrate of Silver, and of organic matters, such as Gelatine, Albumen, etc.
We have now to suppose that a sensitive paper has been prepared in this way, and that a Negative having been laid in contact with it, the combination has been exposed to the agency of light for a sufficient length of time. Upon removing the glass, a Positive representation of the object will be found below, of great beauty and detail. Now if this image were in its nature fixed and permanent, or if there were means of making it so, without injury to the tint, the production of Paper Positives would certainly be a simple department of the Photographic Art; for it will be found that with almost any Negative, and with sensitive paper however prepared, the picture will look tolerably well on its first removal from the printing-frame. Immersion in the bath of Hyposulphite of Soda however, which is essentially necessary in order to fix the picture, produces an unfavourable effect upon the tint; decomposing the violet-coloured Subchloride of Silver, and leaving behind a red substance which appears to be united to the fibre of the paper, and, when tested, reacts in the manner of a Suboxide of Silver.