Mr. Hunt 07i Actino-Chemistry^ 285 



mosphere of chlorine. Some other salts of silver exhibit this 

 curious phsenomenon in a striking manner, as will be showQ 

 in future communications. , ., „ . , r^ "^^ 



30. Pure chloride of silver was dried on a glass plate dftd 

 heated nearly to the point of fusion ; it was then placed in an 

 atmosphere of chlorine and darkened in very bright sunshine. 

 No loss of weight could be detected by the most careful ma- 

 nipulation and with a very delicate balance. There is no very 

 satisfactory way of explaining this, except we suppose that the 

 liberated chlorine is retained by cohesive force in connection 

 with the finely divided particles of what we must regard as a 

 subchloride of silver, unless we return to the old theory of 

 the compound nature of chlorine. 



31. In these experiments the presence of organic matter has 

 been carefully avoided. It now became necessary to inquire 

 into the condition of the chloride of silver darkening by the 

 solar rays on paper. Bath post paper, highly glazed, was co- 

 vered with chloride of silver in the usual way, all free nitrate 

 of silver being washed off'. The prepared paper was then ex- 

 posed to light for forty-eight hours, in which time it darkened 

 and became of a fine olive-brown. The paper was now cut in 

 pieces; some parts were immersed in very dilute nitric acid, 

 which produced no change; others in ammonia, which had 

 not the slightest effect upon them ; therefore it was evident that 

 no oxide of silver was present. On putting fragments of the 

 paper into nitric acid diluted with equal parts of water, all the 

 darkened portion was rapidly dissolved off*, and the paper was 

 left of a lilac colour. Hence we have very satisfactory proof 

 that metallic silver is eventually formed on the surface of the 

 chloridated photographic papers, and that the under sensitive 

 surface is preserved in the condition of a subchloride of silver 

 by the opacity of the superficial coat. j' 



32. From other experiments which do not properly belong 

 to this paper, I am inclined to believe that the first action of 

 the solar rays (I avoid using the term light, considering these 

 phaenomena as the result of a modified principle) is to liberate 

 one- half of the combined chlorine, which is very readily, 

 moisture being present, replaced by oxygen. By the conti- 

 nued action of the exciting cause, the other proportional of 

 the combined gaseous element is in like manner set free and 

 replaced. We have seen (3, 4, 5, &c.) that oxide of silver is 

 in a short time decomposed under Actinic power, and hence 

 we have eventually, nearly pure metallic silver in a state of 

 extremely fine division. It will be seen that organic matter 

 plays an equally important part with the chloride of silver as 

 it does with the nitrate of that metal when under the influence 



