Allotropic Silver. 327 



continued action under favourable conditions, of bringing 

 about a visible change without the aid of a reducing agent. 



Light. — The silver haloids in their sensitiveness to light- 

 show an important relationship to that of allotropic silver. 

 When, for example, silver chloride precipitated with an excess 

 of hydrochloric acid is exposed to light, the darkened product 

 contains apparently no metallic silver* (it is probable that the 

 trace of silver given up to nitric acid may arise from the de- 

 composition of a very small quantity of subchloride). How- 

 ever this may be, the subchloride, and not metallic silver, is the 

 essential product. 



This has always seemed a very enigmatical result. Two 

 combinations of silver and chlorine exist — the one very stable, 

 capable of fusion without decomposition ; the other so unstable 

 that it can hardly exist isolated, and yet the stable compound 

 is rapidly broken up by light, even by a weak diffuse light, 

 whilst the unstable compound resists many days' exposure to 

 the strongest sunlight. 



In examining the action of light upon allotropic silver (see 

 Part I.), an equally remarkable effect was described. Although 

 all the other forms of energy applied readily and quickly con- 

 vert allotropic to ordinary silver, light (at ordinary tempera- 

 tures) fails to effect this change even by exposures lasting for 

 several months. If we conceive that the atomic form of silver 

 which exists in AgOl corresponds to the allotropic form, and 

 that the more condensed form of subchloride corresponds to 

 the " intermediate form," we shall obtain a reasonable expla- 

 nation of the action of light. 



The inability of light to carry the change which it produces 

 in allotropic silver beyond the " intermediate form " exactly 

 corresponds to its inability to carry the decomposition of silver 

 chloride further than to subchloride or rather to photochloride. 

 (It is understood that the silver chloride here spoken of is that 

 which is formed by precipitation with excess of hydrochloric 

 acid.) This explanation appears to remove a real difficulty, 

 and at the same time establishes a perfect parallelism between 

 that action and the action of light on allotropic silver. 



Although the foregoing study of the silver haloids was 

 made for the purpose of fixing the relations which exist be- 

 tween them and allotropic silver, the results nevertheless have 



* In some (unpublished) experiments made some years ago to test this 

 point, I found that silver chloride exposed for several days to strong sun- 

 light under water, with frequent stirring up, aud subsequent washing, 

 yielded only a trace of silver to strong cold nitric acid after a contact of 

 an hour. 



2A2 



