384 THE PHOTOGRAPHIC IMAGE. 



chloride; it was taken up by Sahler, and has recently been revived by 

 Dr. W. E. Hodgkinson. It has been thought that this theory is dis- 

 posed of by the fact that the chloride darkens under liquids, such as 

 hydro carbons, which are free from oxygen. I have been repeating 

 some of these experiments with various liquids, using every possible 

 precaution to exclude oxygen and moisture; dry silver chloride heated 

 to incipient fusion has been sealed up in tubes in dry benzene, 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 j)resents 

 itself, whether the composition of the photo-salt may not vary according 

 to the medium in which it is generated. Analogy sanctions the sup- 

 position that when the haloid darkens 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 composition of the photo- 

 salt. The analogy is supplied by the corresponding salt of copper, viz, 

 cuprous chloride, which darkens rapidly 0:1 exposure. [Design printed 

 on flat cell filled with cuprous chloride 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 

 exi)osed 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 ex- 

 cess of silver is removed and colored products are left which are some- 

 what analogous to the photo-salts proper. These colored haloids are 

 also termed by Carey Lea photo-salts because they present many anal- 

 ogies with the colored 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 observatioas in this direction were 

 published more than 30 years ago in a report by a British Association 

 Committee,t in which the red and chocolate- colored chlorides are dis- 



* Some dry silver chloride which Mr. Crookes has beeu good enough to seal up for 

 mo iu a high vacuum darkeus ou exposure quite as rapidly as the dry salt in air. It 

 soon regains its original color when kept in the dark. It behaves, in fact, just as the 

 chloride is known to behave when sealed up in chlorine, although its color is of 

 course much more intense after exposure than is the case with the chloride in chlorine. 



+ 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 chlorohrydic acid, and then dissolving 

 the silver by dilute nitric acid, was a rose-tinted chloride of silver." Iu another ex- 

 periment the dry citrate was heated iu a stream of hydrogen at 212° F., and the pro- 

 duct, which was partly soluble in water, gave a brown residue, which furnished "a 



