2nd. The nature of the sensitive salt.—When Iodide of Silver is used to receive the latent impression, the image after development, although lacking intensity of colour by reflected light, is more nearly in the condition of metallic Silver than if Bromide or Chloride of Silver be substituted; and of the three salts, the Chloride gives the most intensity, with the least quantity of metallic Silver. This rule applies especially when organic matters, Gelatine, Glycyrrhizine, etc., are present.
3rd. The developing agent employed.—An organic developing agent like Pyrogallic Acid may be expected to produce a Collodion image more intense, but less metallic, than an inorganic developer, such as the Protosulphate of Iron.
4th. The length of time during which the light has acted.—Over-action of the light favours the production of an image which is dark by reflection and brown or red by transmission, corresponding in these particulars to what may be termed the non-metallic image containing an oxide of Silver.
5th. The stage of the development.—The red image first formed on the application of the developer to a gelatinized or albuminized surface of Iodide of Silver is less metallic, and more easily injured by destructive tests, than the black image, which is the result of prolonging the action. Developed photographs which are of a bright red colour after fixing, correspond in properties to images obtained by the direct action of light on paper prepared with Chloride of Silver, more nearly than to Collodion, or even to fully developed Talbotype Negatives.
To conclude the Paper, the following may be offered in the way of recapitulation:—An image consisting of metallic silver, as a rule, reflects white light, and shows as a positive when laid on black velvet; but a non-metallic organic image is dark, and represents the shadows of a picture. Collodion positives developed with protosalts of Iron are nearly or quite metallic. Photographs on Albumen or Gelatine less so than those on Collodion. Developed Photographs contain more Silver than others, if the development has been prolonged. The half shadows of the image in a Positive Print are especially liable to suffer under injurious conditions, since they contain the Silver in a less perfect state of reduction.[24]
[24] The Author omits, in this place, all mention of molecular conditions affecting intensity, inasmuch as at the present time nothing positive has been determined with regard to them. It is however known that in the use of the protosalts of Iron as developing agents, the appearance of the image is much influenced by the rapidity with which the reduction is effected—the particles of Silver being larger and more metallic when the development is conducted slowly. The process of electro-plating and other chemical operations of a similar kind prove that the physical properties of metals precipitated from solutions of their salts, vary greatly with the degree of fineness and arrangement of their particles.
ON THE VARIOUS AGENCIES DESTRUCTIVE TO PHOTOGRAPHIC PRINTS.
Action of Sulphuretting Compounds upon Positive Prints.—It was first noticed by Mr. T. A. Malone, that the most intense Photograph might be destroyed by acting upon it with solution of Sulphuretted Hydrogen or a soluble Sulphuret, for a sufficient length of time.
The changes produced by a sulphuretting compound acting upon the red image of a simply fixed print are these:—the colour is first darkened, and a degree of brilliancy imparted to it; this is the effect termed "toning." Then the warm tint by degrees alters to a colder shade, the intensity of the whole image is lessened, and the half-tones turn yellow. Lastly, the full shadows pass also from black to yellow, and the print fades.
Now in this peculiar reaction we notice the following points of interest. If at that particular stage at which the print has reached its maximum of blackness, it be raised partially out of the liquid and allowed to project into the air, the part so treated becomes yellow before that which remains immersed. Again, if a print toned by Sulphur be placed in a pan of water to wash, after the lapse of several hours it is apt to assume a faded appearance in the half-tones. The full shadows, in which the reduced Silver salt is thicker and more abundant, retain their black colour for a longer time, but if the action of the sulphuretting Bath be continued, every portion of the print becomes yellow.