Effect of Temperature, Acidity, etc. 421 



ing first and changing to the stable form with time, since it is 

 specifically dependent on the acid concentration. In Table 

 XI, it is shown that at 300° even after 48 hours wurtzite is 

 entirely unchanged provided the acid concentration is kept 

 sufficiently high. The evidence given above forces the 

 conclusion that the crystal form of zinc sulphide, under the 

 conditions of our experiments, depends on the final con- 

 centration of acid with which it is heated, and not on the con- 

 centration out of which it is precipitated. For this reason the 

 final acid concentration has been used in plotting the results. 



The above conclusion is valid only when the concentration 

 of the acid falls during an experiment, and this always 

 occurred when 2 per cent zinc sulphate was used. This con- 

 centration of zinc sulphate yielded the best developed crystals 

 and was always used except in the series where the influence 

 of the zinc concentration was studied. If the acid increases 

 during an experiment and the initial acid concentration is low 

 enough for the formation of sphalerite, this sphalerite will natu- 

 rally persist, since it is the stable form, after the acid has 

 increased to the concentration from which wurtzite will form. 

 In such a case the crystalline form depends on the acidity at the 

 time of formation. In the concentrated zinc solutions, Table 

 YII, the acid increased during the experiments, and this 

 accounts for the fact that more sphalerite was found there in 

 some cases than in products from dilute zinc solutions (2 per 

 cent) with the same final acid concentration. 



G. The influence of acidity in the formation of tourtzite. — 

 The evidence accumulated by a more complete study of this 

 subject entirely corroborated the earlier indications. It has 

 been shown that, in all probability, the changing of other con- 

 ditions, such as the pressure of the hydrogen sulphide, the 

 zinc concentration, and the addition of sodium sulphate has no 

 appreciable effect on the crystalline form. The influence of acid, 

 however, is in some ways even more striking than it was in the 

 case of marcasite. At 300° and 325° we were able to obtain 

 crystalline products varying in composition from pure sphaler- 

 ite, through mixtures of the two forms, to pure wurtzite. In 

 the iron disulphide mixtures, we were not able to obtain from 

 acid solutions a product which contained more than about 50 

 per cent of the stable form (pyrite). 



In the investigation of the two sulphides of zinc, however, 

 many difficulties were encountered which were not met with 

 in the case of the disulphides of iron. In the first place the 

 range of temperature was very restricted. As has been said, 

 only amorphous zinc sulphide was obtained below 250°, and, 

 as is shown in Table XIII, only sphalerite was obtained at 

 350° or higher, though the acid concentration was increased to 

 the point at which no precipitation took place. 



