M. O. Lea — Silver Hemisidphate. 323 



phate. But a specimen which was covered with a large excess 

 of undiluted sulphuric acid in a flask and was kept under boil- 

 ing water for ten hours was not altered thereby. Another 

 strong proof of its stability is found in its resistance to heat. 



The application of heat produces a somewhat curious suc- 

 cession of colors. The terra cotta or warm brown shade of 

 the moist substance changes by drying above 100° to pale 

 lilac, at 165°-170° it becomes grayish, at a somewhat higher 

 temperature, yellowish green. Considerably below red heat it 

 acquires a fine ruby red color. In cooling, this red darkens 

 almost to black, then becomes lighter again and when cold the 

 color is light olive-green. The changes are repeated as often 

 as the substance is heated and cooled. No sulphuric acid 

 vapors are disengaged even at a low reel heat. 



It was mentioned in a previous paper that when silver 

 nitrate is reduced by solutions of phosphorous or hypophos- 

 phorous acid or by acidified solutions of their alkaline salts, 

 transient colorations were produced that seemed to suggest the 

 presence of some form of allotropic silver. Since that paper 

 was published this reaction has been taken up for further 

 stud}^. It soon appeared that when the silver salt was treated 

 with a solution of alkaline hypophosphite, acidified with sul- 

 phuric acid, the result obtained was entirely different from 

 that which presented itself under any other circumstances. 

 It became clear that sulphuric acid did not act solely by setting 

 free the hypophosphorous acid, but also acted on the silver 

 with formation of a double sulphate. 



A remarkable though limited analogy here presents itself 

 between the substance just described and the photosalts of 

 silver. The silver hemihaloids are very unstable substances, 

 but acquire stability by uniting with the normal haloids. In 

 the same way the hemisulphate, which is not known to be 

 capable of separate existence, becomes perfectly stable by 

 union with the normal sulphate. The limitation to this 

 analogy lies in the fact that the last mentioned combination 

 occurs in definite proportions, which does not seem to be the 

 case with the halogen compounds. 



The new substance then is formed by the joint action of 

 sulphuric and hypophosphorous acid on a silver salt. Hypo- 

 phosphorous acid has but little action on silver sulphate 

 already precipitated, but it is different when the silver sulphate 

 is formed in presence of hypophosphorous acid. 



Several silver salts may be used. I have at different times 

 employed the nitrate, phosphate and carbonate. The latter is 

 perhaps the best, because the action with the nitrate is too 

 rapid, and with the phosphate, too slow, and' for other reasons. 



