1890.] on the PhotograpJnc Image. 141 



petroleum, and carbon tetrachloride and exposed since March. [Tubes 

 shown.] In all cases the chloride has darkened. The salt darkens, 

 moreover, in a Crookesian vacuum.* By these experiments the oxy- 

 chloride theory may be scotched, but it is not yet killed ; the question 

 now presents itself, whether the composition of the photosalt may not 

 vary according to the medium in which it is generated. Analogy 

 sanctions the supposition that when the haloid darkeus under water 

 or other oxygen-containing liquid, or even in contact with moist or 

 dry air, that an oxychloride may be formed, and enter into the com- 

 position of the photosalt.- The analogy is supplied by the corre- 

 sponding salt of copper, viz. cuprous chloride, which darkens rapidly 

 on exposure. [Design printed on flat cell filled with cuprous cliloride 

 by exposure to electric light.] Wohler conjectured that the darkened 

 product was an oxychloride, and this view receives a certain amount 

 of indirect support from these tubes [shown], in which dry cuprous 

 chloride has been sealed up in benzene and carbon tetrachloride 

 since March ; and although exposed in a southern window during 

 the whole of that time, the salt is as white as when first prepared. 

 Some cuprous chloride sealed up in water, and exposed for the same 

 time, is now almost black. [Shown.] 



When silver is precipitated by reduction in a finely divided state 

 in the presence of the haloid, and the product treated with acids, the 

 excess of silver is removed and coloured products are left which are 

 somewhat analogous to the photosalts proper. These coloured 

 haloids are also termed by Carey Lea photosalts because they present 

 many analogies with the coloured products of photo-chemical change. 

 Whether they are identical in composition it is not yet possible to 

 decide, as we have no complete analyses. The first observations in 

 this direction were published more than thirty years ago in a report 

 by a British Association Committee, | in which the red and chocolate- 

 coloured chlorides are distinctly described. Carey Lea has since 

 contributed largely to our knowledge of these coloured haloids, and 

 has at least made it appear highly probable that they are related to 



* Some dry silver chloride which Mr. Crookes has been good enough to seal 

 up for me in a high vacuum, darkens on exposure quite as rapidly as the dry salt 

 in air. It soon regains its original colour when kept in the dark. It behaves, 

 in fact, just as the chloride is known to" behave when sealed up in chlorine, 

 although its colour is of course much more intense after exposure than is the case 

 v;ith tlie chloride in chlorine. The tube in which the chloride had been sealed 

 up in benzene, gave ofi" a considerable quantity of hydrogen chloride on breaking 

 the point in June. 



t These results were arrived at in three ways. In one case hydrogen was 

 passed through silver citrate suspended in hot water, and the product extracted 

 with citric acid. " The result of treating the residue with chlorhydric acid, and 

 then dissolving the silver by dilute nitric acid, was a rose-tinted cliloride of silver." 

 In another experiment the dry citrate was heated in a stream of hydrogen at 

 212^ P., and tlie product, which was partly soluble in water, gave a brown residue, 

 which furnished "a very pale red body on being transformed by chlorl)ydric and 

 nitric acids." In another experiment silver arsenite was formed, this being 

 treated with caustic soda, and the black precipitate then treated successively with 



