112 REPORT — 1859. 



cesses on glass, in a supporting film, and generally in iodide of silver formed 

 under conditions in which nitrate of silver was in excess. There are also 

 generally present other ingredients, such as certain forms of organic matter, 

 and in some cases bromide or even chloride of silver. 



That it is not a matter of indifference whether the supporting basis, or 

 film, consist of pyroxyline, or albumen, or gelatine, or of these severally com- 

 bined with other bodies or with each other, one might readily suppose from 

 what has been already said under the head of direct processes ; and it will 

 be no difficult matter to show more than a probability that this is not due to 

 a "molecular," but to a "chemical" distinction in the action of these bodies. 



The usual sensitive surface contains, if it does not consist in, iodide of 

 silver with an excess of nitrate. But there are processes in which the plate 

 is studiously washed with water to remove the nitrate, whereby, though it is 

 impaired in sensitiveness, it retains enough of that quality for the produc- 

 tion of excellent results. Though this retention of a susceptibility to the in- 

 visible impression has been attributed to mechanical causes, such as the state 

 of division of the iodide, the porosity of the film, &c, the following facts 

 seem to favour a chemical explanation. Pure pyroxyline united with pure 

 iodide and nitrate of silver, from which the nitrate of silver has subsequently 

 been removed, and the film dried, is not susceptible of quick development 

 after exposure in the camera ; a mere trace of albumen introduced before the 

 removal of the soluble silver-salt, however, prevents its entirely losing this sus- 

 ceptibility. Gelatine, certain forms of sugar, resins, and various other bodies 

 widely differing from one another in point of chemical character, possess a 

 similar property, though the precise regulation of the processes employing 

 them can hardiy be said to be as yet mastered by the photographist. The 

 products of decomposition contained in " old collodions," and some of the 

 fresh preparations of pyroxyline, in which secondary products are not studi- 

 ously prevented from being formed, would seem to share this power with the 

 classes of bodies referred to. 



But a question of the utmost interest to the scientific inquirer is involved 

 in the chemistry of the iodide of silver; first, in respect to its power of 

 forming combinations with the nitrate of silver, and secondly, as regards the 

 probability of these combinations forming photographic compounds with the 

 albuminous and other bodies alluded to. 



That the excess of nitrate of silver which is necessary in the Jirst prepara- 

 tion of all the sensitive films does not act the same part as that excess does 

 in the case of the chloride in direct processes, will be evident at once, inas- 

 much as the iodide of silver does not undergo reduction in the manner that 

 the chloride does. In searching, therefore, for an explanation of the necessity 

 of free nitrate, the mind naturally dwells on the compounds shown by 

 Schnauss* and A. Kremer-t" to be formed by the action of strong solution 

 of nitrate of silver on the iodide. Although the production of these bodies 

 in any quantity and in a state of chemical purity needs conditions not 

 present on the photographic film, yet there seems little doubt that, as iodide 

 of silver is dissolved by the nitrate, traces of these remarkable compounds 

 can readily exist in the films containing these two ingredients. If so, the 

 highly photographic character of the compound containing 2-8 per cent, of 

 iodide of silver described by Kremer, and the fact of these bodies being 

 decomposed with the separation of iodide of silver by the action of water, are 

 facts of high interest to the photographic chemist, and seem to throw con- 

 siderable light on the hitherto obscure processes in which iodide of silver is 



* Archiv der Pharra. xcii. 260. t Journ. fiir Prakt. Chem. lxxi. 54. 



