M. G. Lea — Allotropic Silver. 265 



remain in contact for two or three minutes, the paper was 

 immersed in running water and was washed for an hour or 

 two. 



On applying the oxalate developer nothing appeared. Feel- 

 ing confident that effect must be produced, the experiment was 

 repeated several times and the results were closely examined. 

 On one specimen it was found that the characters had appeared, 

 but reversed, that is, lighter than the ground which had 

 darkened by the development being pushed. This at once 

 gave a clue ; it showed that traces of the acid adhered too 

 strongly to be removed by washing and by locally checking the 

 development, interfered with the reaction. Accordingly, next 

 time after a very short washing, the paper was immersed in 

 water to which a trace of ammonia had been added, and after 

 ten or fifteen minutes' action the ammonia was thoroughly 

 washed out. The result was striking : as soon as the developer 

 was applied the characters which had been traced with acid 

 came out strongly as brown marks on a white surface. 



Cold sulphuric acid even undiluted is generally held to have 

 no action on silver haloids, but it is well known that the hot 

 strong acid decomposes them. The foregoing experiments 

 leave no doubt that the cold dilute acid produces an initial 

 effect invisible to the eye but revealed by greater tendency to 

 give way under the action of a reducing agent. This action of 

 the acid comes therefore exactly into line with that of light 

 and heat. In all three cases an effect is produced inapprecia- 

 ble until a reducing agent is applied. But in all three cases 

 the agent which produced this invisible effect is capable by 

 continued action under favorable 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 com- 

 binations 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, 



* In some (unpublished) experiments made some years ago to test this point, 

 I found that silver chloride exposed for several days to strong sunlight under 

 water, with frequent stirring up, and subsequent washing yielded only a trace of 

 silver to strong cold nitric acid after a contact of an hour. 



