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.