358 M. C. Lea — Darkened Silver Chloride 



metallic silver, and excluding oxygen, free or combined, thus 

 demonstrating that that element is not needed and plays no 

 necessary part in the formation of silver photochloride. 



Anhydrous cupric chloride, which I at first thought of em- 

 ploying, proved to be insoluble in naphtha. In rendering ferric 

 chloride anhydrous there is a possibility of forming oxychloride ; 

 it could therefore not be employed. As it is not in the least 

 important which halogen is used, I concluded to take iodine 

 which proved to be slightly soluble in naphtha with a beauti- 

 ful violet coloration. The mode of operation was as follows. 

 Pure silver reduced by cadmium from the chloride was heated 

 nearly to redness in a porcelain capsule, and at the instant of 

 removal from the flame, was dropped into naphtha. Some frag- 

 ments of iodine were added. Owing to the very small amount 

 of iodine soluble in naphtha, the action was slow, but contin- 

 uous and regular. As fast as the iodine dissolved, it was taken 

 up by the silver. At the end of some hours the iodine had dis- 

 appeared wholly, and the naphtha was colorless. Fresh naphtha 

 replacing it failed to dissolve any iodine. The whole of it 

 had combined with the silver to a black compound. This ex- 

 periment may be varied by using a piece of clean silver foil or 

 even a silver coin that has been boiled a few moments with 

 nitric acid, washed and heated by a blast lamp. Immersed in 

 the naphtha with iodine its surface soon becomes perfectly 

 black. 



This reaction forms the complement of the other and the 

 two show that whether we start from silver chloride and pro- 

 ceed by reduction or from metallic silver and proceed by iodi- 

 zation, in either case we can obtain a photosalt under condi- 

 tions which rigorously exclude all possibility of the presence 

 of moisture or of oxygen in any shape. 



Therefore the photosalt is not an oxysalt, but, as I endeav- 

 ored to prove two years ago, a compound of normal salt with 

 subsalt. 



The action of light upon silver chloride appears to take place 

 in the following manner. 



If any substance is present with which chlorine can combine, 

 either directly or by substitution,* the AgCl is decomposed with 



* As to the action of light on. silver chloride perfectly isolated, i. e. in a perfect 

 vacuum would appear from an interesting experiment of Abney's that no decom- 

 position takes place. It was found that AgCl in vacuo did not darken even by 

 prolonged exposure. 



This experiment does not indicate that the presence of moisture is essential for 

 decomposition, it simply proves that some substance (by no means necessarily 

 water) must be present upon which chlorine can act Accordingly, when the 

 vacuum tube contained mercury, the AgCl was decomposed by exposure to light. 

 — It may be remarked that this last mentioned fact, properly considered, would 

 have been found to be fatal to the oxychloride theory, inasmuch as darkened 

 chloride was formed in the total absence of oxygen. 



