with other Metallic Chlorides. 385 



foundly affects the sensitiveness of the silver salt to light. To 

 make an accurate comparison, two solutions of silver chloride 

 were precipitated, the one from pure HC1, the other from HOI 

 mixed with Fe 2 Cl 3 ; both were shaken up with HC1, so as to 

 remove all the more loosely combined iron salt from the one, 

 and to place the two under exactly corresponding conditions, 

 the presence of the iron salt alone excepted. They were then 

 washed. When both were exposed together to light, the differ- 

 ence was extraordinary. The normal AgCl had passed to a 

 full violet, with an exposure which produced on the other 

 scarcely any visible effect. Even after an exposure to diffuse 

 light of two hours, the difference was still very striking. 



AgCl with CoGl. — When cobalt chloride is added to HC1, 

 and then solution of silver nitrate, a pinkish precipitate is ob- 

 tained, whose sensitiveness to light is less than that of normal 

 silver chloride. But the diminution of sensitiveness is far 

 from being so great as in the case of AgCl precipitated in pres- 

 ence of ferric chloride. 



AgCl with other chlorides. — Both nickel chloride and manga- 

 nous chloride attach themselves to silver chloride when the 

 latter is precipitated in their presence. Cupric chloride seems 

 to have no such tendency. Gold chloride shows a marked 

 tendency to attach itself to AgCl. When silver nitrate is 

 added to dilute HC1 with which a little auric chloride has 

 been mixed, the precipitated AgCl has a reddish shade. Con- 

 tinued washing renders this paler but does not seem to remove 

 it. After ten decantations, each with a hundred volumes of 

 water, the color imparted by the gold is still visible. What 

 influence the auric chloride has on the sensitiveness of the 

 silver salt it is not easy to say, as the gold present is quickly 

 reduced by exposure to light, so that the AgCl, instead of 

 passing to violet and chocolate as in the case of pure AgCl, 

 gradually darkens to a pure black. 



The facility with which these compounds are formed explains 

 the necessity in analytical determinations of silver as chloride, 

 for digestion for a considerable time with dilute HC1. Even 

 then it is very doubtful if all the foreign chloride is removed. 

 Ferric chloride is especially adherent. .Indeed it has been 

 said that when iron once comes into contact with silver it is 

 next to impossible to get rid of it, and the reason lies in the 

 strong affinity which the two chlorides have for each other. 

 Accordingly much silver nitrate sold as absolutely pure con- 

 tains iron, evidently carried down with the silver chloride when 

 precipitated in the manufacture. 



These reactions of AgCl are interesting in several ways. 

 They support the theory I have proposed, of the photosalts 



