M. C. Lea — Silver Hemisulphate. 325 



the first) placing the vessel in a water bath kept at 100° C. for 

 several hours. 



The product is either dried in the air or (for analysis) at 

 100° C. It forms a bright brown substance, permanent in the 

 air, changing to violet when kept for some time at 160° C. 

 It has the peculiarity that when water is poured on it, it makes 

 a sharp hissing noise. This takes place with the air-dried sub- 

 stance as well as that dried at higher temperatures and as much 

 with the former as with the latter. 



The substance after purification has about one-half the 

 weight of the silver nitrate taken. 



These proportions and this mode of operating are those that 

 I have found to give the. best result. But the substance is 

 formed under a great variety of conditions. It seems impos- 

 sible to bring a silver salt into contact with alkaline hypophos- 

 phite acidified with sulphuric acid without producing more or 

 less of it. Its presence is often completely obscured by re- 

 duced silver. But a mass which looks perfectly black and 

 might be supposed to contain nothing but metallic silver will 

 leave, when treated with nitric acid, a bright brown residue of 

 the double sulphate. We have here, as before, an analogy 

 with the photosalts. For it will often happen that a blackish 

 mass, containing metallic silver and mixed or combined silver 

 chlorides will, when treated with nitric acid, resolve itself 

 into bright purple or rose colored photochloride. 



All the specimens of this new substance contain a little 

 phosphoric acid which cannot be removed. Reckoned as 

 phosphoric anhydride it amounts to a little over two per cent. 

 Three determinations gave respectively, 2 - 30 ; 2"09 ; 2*18, mean 

 2-19. 



It is apparently united with silver and this silver phosphate 

 is united so firmly with the double sulphate that it cannot be 

 detached. If it were not so united it would be dissolved in 

 the nitric acid with which the substance is three times treated 

 if it were normal phosphate, and if it were hemiphosphate it 

 would be converted (if in a free state) to normal phosphate 

 and dissolved. 



Another attempt to remove this phosphate was made by 

 heating the substance with sulphuric acid to 100° C. for ten 

 hours, followed by copious treatment with boiling distilled 

 water to wash out the sulphate which it was hoped would be 

 formed at the expense of the phosphate. It seems difficult to 

 believe that a silver phosphate could resist this treatment, but 

 a quantitative determination showed that the proportion of 

 phosphoric anhydride is not even diminished by it. 



Other modes of formation than those described here were 

 experimented on with the view of obtaining the substance 

 free from phosphate, but without good result. 



