M. Carey Lea — AUotropic Forms of Silver. 483 



To verify this conclusion by additional evidence, the sub- 

 stance was examined as to its behavior when heated. For if 

 any other element were chemically combined with the silver it 

 would only be (in view of the high percentage of silver) hydro- 

 gen or oxygen. We might have to do with a hydride, analo- 

 gous to Wurtz's hydride of copper, or possibly an oxide, but 

 not probably as Ag 4 would contain only 96*43 per cent of 

 silver. 



The presence of either hydrogen or oxygen in combination 

 with silver seems to be pretty certainly negatived by the action 

 of dilute sulphuric acid on this (and the two other substances, 

 B and C, to be described farther on). They are all converted 

 into gray metallic silver without the slightest escape of gas. 

 This seems tolerably conclusive in itself ; and the result of ex- 

 posing a great number of specimens of all the forms A, B and 

 C to the action of heat was equally so. As the object was to 

 expose the fresh and moist material to a gradually increasing 

 heat from that of boiling water to a low red heat without 

 interrupting the process, the following arrangement was found 

 convenient. 



A piece of Bohemian glass tube about six inches long was 

 sealed in the lamp at one end, the other closed with a rubber 

 cork, through which passed a small gas delivery tube and 

 another tube passing into a small test tube partly filled with 

 water and having another tube through the cork passing under 

 the surface of the water, thus preventing regurgitation. The 

 material was thus first exposed for some hours to a heat of 

 about 150° C. in a chloride of calcium bath ; this was next 

 removed and the heat continued to low redness. Only traces 

 of gas were evolved and this was found to be in all of the 

 many trials made, carbonic acid, derived from the citric acid 

 adhering. This treatment was repeated many times with all 

 the different varieties of the substance and with the same 

 result. The temperature was always raised sufficiently high to 

 ensure the complete conversion of the material into normal 

 gray silver, but in no case was oxygen or hydrogen set free. 



It could not be overlooked that in all these trials the mate- 

 rial had passed into an insoluble form before the silver deter- 

 mination was made. There remained therefore this possibility : 

 that the silver, so long as soluble, might be in combination 

 with citric acid and that its change to the insoluble condition 

 was caused by its separating from the citric acid. It seemed 

 desirable that this view should be tested. As the object was 

 to determine the condition of the silver in the substance as 

 originally formed, avoiding as far as possible to change that 

 form by attempts at purification, the only course available was 



