ON THE PHOTOGRAPHIC IMAGE. 109 



however, it is suffered to dry (of course in the dark), the stain thu9 formed, 

 instead of a grey, exhibits a dense black tone, which immediately afterwards 

 passes on into a brown. The former of these is probably suboxide. 



But if the tissue of the paper is not to be altogether excluded from the 

 list of possible cooperative agents present in these processes, there are other 

 substances of which the influence can be demonstrated in a manner quite 

 satisfactory to the photographist. Gelatine as size was long employed with- 

 out his being conscious of its importance ; and he now uses albumen as a 

 photographic glaze, and sometimes other substances, such as grape sugar, 

 Iceland moss, caseine, &c, on account of the fine tones and permanence in 

 the fixing bath which they impart to his pictures. Gelatine and albumen both 

 combine with nitrate of silver ; and the character of the combination is one 

 which chemistry has yet to explain with completeness. These compounds 

 differ from each other in many important respects : we shall select that with 

 gelatine for illustration. The characters of the compound of gelatine and 

 nitrate of silver are exhibited by the following statements. 



If a she et of transparent gelatine be floatedupon a solution of nitrate of 

 silver, the solution loses a considerable amount of the dissolved salt. When the 

 proportion of the gelatine to the bulk and strength of the solution is suffi- 

 cient, free nitrate of silver is scarcely to be detected in the bath, and what 

 silver is found there is probably in the form of a gelatine-compound, which 

 is not entirely insoluble. The gelatine mass, though but slightly soluble in 

 cold, is so to a considerable amount in hot water, and retains at once the 

 neutrality and the taste of the nitrate. The solution gives the following re- 

 actions : — 



Caustic potash throws down a bulky olive-brown precipitate, which clots 

 into a tough extensile mass. This dissolves by boiling with excess of the 

 precipitant, yielding a very dark, and when diluted, a clear yellowish-brown 

 solution. 



Strong ammonia produces no precipitate, but on boiling forms a pale 

 orange-yellow solution, on which the light produces little or no change. 



Chloride of ammonium, introduced cautiously, produces no precipitate, but 

 in excess renders the solution turbid. The clear liquid is not rendered tur- 

 bid by boiling ; but a few drops of nitric acid, if the temperature be raised 

 to the boiling point, suffice to render it milky from separation of chloride of 

 silver, which may be redissolved by ammonia, or darkened by the light. 



Iodide of potassium, unless carefully introduced, throws down a turbidity 

 of a yellow tint, in it. But if this be removed by filtration, it will be found 

 that the addition of the most dilute nitric acid and boiling throws down a 

 fresh amount of iodide of silver. 



Cold nitric acid produces no change in the gelatino-nitrate (?) of silver, 

 even when formed from the ordinary commercial gelatine ; but boiling throws 

 down sometimes a small quantity of chloride, originating in the impurity of 

 that body. 



Chlorhydric acid in minute quantity produces also no precipitate until 

 boiled, when the chloride of silver separates from the compound. 



The gelatinous mass, formed by the action of the nitrate of silver solution 

 upon the gelatine, becomes, on exposure to the sunlight, of a red colour. 

 The change is a rapid one, and is accompanied by a shrinking of the mass 

 to its original character of a thin sheet as it dries. The colour attained by 

 prolonged solar influence is by transmitted light a deep ruby, and a "bronzed " 

 green by reflected light. Sheets of the gelatino-nitrate of silver thus solarized 

 no longer swell up or dissolve in boiling water, but only after long boiling 

 become disintegrated in filmy fragments. Potash gives, on boiling, a clear 



