The above facts justify us in supposing the existence of combinations of organic matter with a low Oxide of Silver; and analysis indicates further that the relative proportion of each constituent in these compounds may vary. For instance, when Citrate of Silver is reduced by light, and acted on with Ammonia, a black powder remains, which was found to contain as much as 95 per cent, real Silver; but Albuminate of Silver treated in the same way yields on analysis less of metallic Silver, and more volatile and carbonaceous matter.
The use of Ammonio-Nitrate of Silver in preparing the salt tends also to increase the relative quantity of metal left in the compound after reduction and fixing. The length of time during which the light has acted, has also a modifying effect of the same kind,—the product of reduction by a powerful light being more nearly in the state of metal, and containing less both of Oxygen and organic matter.
Action of Light upon Chloride of Silver associated with organic matter.—Photographs formed on Chloride of Silver alone, would, after fixing, consist of metallic Silver, but such a process could not be carried out in practice. The addition of organic matter is absolutely necessary in order to increase the sensitiveness, and to prevent the image from being dissolved in the Bath of Hyposulphite of Soda. The blue Subchloride of Silver is decomposed by fixing, a very scanty proportion of grey metallic Silver remaining insoluble; but the red compound of Suboxide of Silver with organic matter is almost unaffected by Hyposulphite of Soda, or Ammonia.
The increase of sensitiveness and intensity produced by the use of organic matter is accompanied also by a change in the composition of the picture; the image losing the metallic character which it possesses when formed on pure Chloride of Silver, and resembling in every respect the product of the action of light upon organic Salts of Silver.
There are certain characteristic tests which may usefully be employed in distinguishing the metallic image from what may be termed the organic or non-metallic image. One of these tests is Cyanide of Potassium. An image formed upon pure Chloride of Silver, although pale and feeble, may, after fixing, be immersed ill dilute solution of Cyanide of Potassium without injury. But a photograph on Chloride of Silver supported by an organic basis, is much acted upon by Cyanide of Potassium, quickly losing its finer details.
A second test is the Hydrosulphate of Ammonia. If no organic matter be employed, the image becomes darker and more intense by treatment with a soluble Sulphuret; whilst the non-metallic image, formed on an organic surface, is quickly bleached and faded. The action of Sulphur upon the image is indeed a mode of determining the real quantity of Silver present. When existing in a very finely divided layer, Sulphuret of Silver often appears yellow; but in a thicker layer it is black. Hence the colour of the Photograph, after treatment with Sulphuretted Hydrogen, is an indication of the proportion of metal present, and the reason of the organic image becoming so perfectly faded is because it contains a minimum of Silver in relation to the intensity. We see, therefore, that the addition of organic matter to Chloride of Silver does not so much increase the actual quantity of Silver reduced by light, as it adds to its opacity by associating other elements with the Silver, and altogether modifying the composition of the image.
The employment of oxidizing agents shows also that in an ordinary Photographic process by the direct action of light, other elements besides Silver assist in forming the image: the pictures being found to be easily susceptible of oxidation, whereas the metallic image formed on pure Chloride of Silver resists oxidation.
Composition of developed images.—By exposing sensitive layers of the Iodide, the Bromide, and the Chloride of Silver to the light for a short time only, and subsequently developing with Gallic Acid, Pyrogallic Acid, and the protosalts of Iron, a variety of images may be obtained, which differ from each other materially in every important particular, and a comparison of which assists the determination of the disputed point.
The appearance and properties of the developed Photograph are found to vary with the existence of the following conditions.
1st. The surface used to sustain the sensitive layer.—There is a peculiarity in the image formed on Collodion. Collodion contains Pyroxyline, a substance which behaves towards the salts of Silver in a manner different from that of most organic bodies, exhibiting no tendency to assist their reduction by light. Hence Chloride of Silver on Collodion darkens far more slowly than the same salt upon Albumen, and the image, after fixing, is feeble and metallic. Iodide of Silver on Collodion, exposed and developed, gives usually a more metallic image, with less intensity, than Iodide of Silver upon Albumen, or on paper sized with Gelatine. By adding to the Collodion a body which has an affinity for low oxides of Silver, such for instance as Glycyrrhizine, the opacity of the developed image is increased.