326 Mr. M. Carev Lea 



OR 



The conclusion to be drawn as respects both the silver 

 haloids is that they undergo actual decomposition by the 

 action of moist heat, but that this effect is much more marked 

 in the case of chloride than that of bromide *. 



Chemical action. — Dilute sulphuric acid quickly changes 

 allotropic silver to normal, and therefore if the parallelism 

 which I have indicated really exists, marks made on bromide 

 paper with dilute sulphuric acid should be capable of develop- 

 ment. 



The experiment was made by drawing characters on silver 

 bromide with a glass rod dipped into sulphuric acid diluted 

 with twice its bulk of water. After allowing the acid to 

 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. 

 Feeling confident that some effect must be produced, the experi- 

 ment 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 inappre- 

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

 the agent which produced this invisible effect is capable, by 



* Light and heat act differently on silver chloride. Heat cannot 

 decompose it in the absence of moisture, but light can. This was proved 

 by an investigation made by the writer in 1889, in which it was shown 

 that fused silver chloride poured iuto p3troleum and exposed after cooling 

 to the sun's rajs was instantly blackened. 



