216 Principles of Photography, 



longer acts upon it, it will be found covered with metallic 

 silver. Papers prepared with chloride of ammonia, or chloride 

 of barium, and then nitrate of silver, give, when the spectrum 

 is thrown upon them in favourable circumstances, a range of 

 colours very closely resembling the natural ; a blue, or rich lead 

 colour, being produced between the most refrangible green and 

 the extreme edge of the violet, and beyond this a black ; but in 

 the space of the least refrangible rays, a brown, which, under 

 the red ray, passes into a red. It is supposed that, in presence 

 of moisture, chloride of silver is first changed into oxide, hy- 

 drochloric acid being formed, and that the oxide is then 

 decomposed. Iodide of silver is but little acted on by light, 

 which, however, renders it easily reducible ; it is believed to 

 produce a catalytic, rather than a photographic effect, and to be 

 the cause of the decomposition of nitrate, in presence of rer 

 ducing agents, during development. It is more sensitive 

 when in contact with nitrate; and, under the influence of 

 light, suffers the same kind of changes as the chloride. Since 

 it produces oxide, or metallic silver, the developing fluid should 

 not contain free acid, which would combine with one or both 

 before the latent image could be developed. Hence a de- 

 veloping mixture, consisting of a concentrated solution of 

 sugar of milk, which does not act on silver or its oxide, and 

 a small quantity of sulphate of iron, afford a good picture, 

 after a short exposure. When the spectrum is thrown on 

 paper containing only iodide of silver, the actinic action rarely 

 extends beyond the green; hence green or yellow has no 

 effect upon it. Bromide of silver, also, is rendered more 

 sensitive by the presence of organic matter; under the in- 

 fluence of light, it exhibits changes similar to those of the 

 chloride and iodide. The insolubility of the chloride, iodide, 

 and bromide of silver causes them to be suited for photographic 

 purposes, since it prevents them being washed away during 

 the processes which are necessary. 



The nature of the base combined with the iodine and bro- 

 mine in the collodion, exerts a marked influence ; ammonia, 

 cadmium, lithium, magnesium, potash, and zinc, are the most 

 effective. Potash labours under the disadvantage of its iodide 

 being soluble only in weak alcohol, and its bromide being 

 scarcely soluble at all. The compounds of cadmium are very 

 permanent; and hence collodion prepared with them remains 

 for a considerable time without change. Those compounds 

 which have the smallest chemical equivalents, and which leavo 

 the silver salts in the sensitive coating in the state of greatest 

 purity, seem accompanied by the most sensitiveness ; such aro 

 the compounds of ammonia, lithium, and magnesium. Those of 

 lithium and magnesium keep the plate moist; with those of 



