446 M. Carey Lea — Notes on Silver Chlorides. 



ferricyanide and if the iron alum is in excess the whole of the 

 silver is dissolved. It has been before noticed that the reac- 

 tions between silver salts on the one hand and iron salts on the 

 other are to some extent reversible, the observations just 

 described place the matter in a somewhat clearer light. With 

 a view of ascertaining whether ferrous sulphate could' be com- 

 pletely oxidized by argentic oxide a portion of the ferrous solu- 

 tion was treated with successive portions of the oxide until the 

 latter was no longer affected by it. But when this stage was 

 reached the solution no longer contained a trace of iron, the 

 whole of it had entered into combination with the silver. 

 This combination is perfectly black and may probably have con- 

 sisted of the compound described by H. Rose, as Ag 4 0, 2FeO 

 Fe 2 O s and obtained in the same way. 



When ammonia iron alum is placed in contact with finely 

 divided silver in considerable excess in a tightly closed vial 

 the solution after a few days standing with frequent shaking 

 acquires a deep red color. This may last for a week or more. 

 The solution then decolorizes and becomes greenish but still 

 contains abundance of ferric salt. Thus it appears that although 

 silver has a powerful reducing action on ferric salts the action 

 is self-limited and ceases long before complete reduction is 

 effected for after many weeks contact in a closed vial there are 

 abundant indications of the presence of ferric salt, although 

 silver has been present in large excess. 



Art. LYI1. — Notes on Silver Chlorides ; by M. Carey Lea. 



Experiments made by J. J. Acworth* at the suggestion of 

 E. Wiedeman showed that by heating silver chloride to a tem- 

 perature of 220° C. it passes into a modification that was 

 insensitive to light. 



I think this change may be due to the complete driving off 

 of moisture. Abney showed by a well-known experiment that 

 silver chloride when exposed perfectly dry in vacuo in a glass 

 tube was totally unaffected by light, but 1 have shown that 

 fused silver chloride poured into petroleum and placed in the 

 sun-light without removing it from the liquid, was instantly 

 darkened. 



These three experiments taken together lead to the following 

 conclusions : 



1. Silver chloride dry and perfectly isolated is insensitive to 

 light. (Abney's experiment.) 



* Wied. Referate, 1890, p. 518. 



