426 Allen, Crenshaw, and Merwin — 



in every case when the final acidity was not higher than 1*5 per 

 cent ; pure wurtzite resulted when the final acidity was 2*8 per 

 cent or higher. 



At 325° (Table XII) pure sphalerite was obtained from 

 final acids of 2 per cent concentration or less ; pure wurtzite 

 was not obtained until the final acid rose to 4*2 per cent. 



At 350° (Table XIII) sphalerite only was obtained in every 

 experiment. At this temperature the necessary acid concentra- 



Table XIII. 



Influence of acid on crystalline form of ZnS at 350° . All solutions contained 

 ZnSOi.7SHiO and are precipitated by H^S from 0'5 g. NH t CnS. 



Initial 



Final 















H 2 S0 4 



H 2 S0 4 















in wt. % 



in wt. % 



Time 





Product 









1 



o-4 : 



Lday 



Some sphalerite, 



much amorphous, 



no 



wurtzite 



5 



4-2 : 



u 



All sphalerite 











10 



5'1 ] 



u 



Probably all sphalerite 









10 



4-3 : 



u 



All sphalerite 











10 



4-2 . ] 



u 



Some sphalerite, 



much amorphous, 



no 



wurtzite 



12 



4-4 



1 " 



No precipitate 











15 



5-9 : 



a 



it a 











tion for the formation of any wurtzite at all must be higher 

 than 5*1 per cent, above which no precipitate was obtained. 



It is evident from these experiments that the higher the 

 temperature the higher the acid concentration necessary to con- 

 dition the formation of the unstable form, wurtzite, alone. The 

 same was found to hold true for marcasite. 



In fig. 5 are plotted the final acid concentrations necessary 

 for the formation of either form of zinc sulphide alone, against 

 the temperature. Conditions of temperature and acidity 

 represented by the area above the line AB give rise to pure 

 wurtzite. The area below the line CD represents the con- 

 ditions for the formation of pure sphalerite, while in the area 

 between the two lines mixtures will occur. From this plot 

 the acid concentration necessary for the formation of wurtzite 

 at any given temperature may be determined. It is unfor- 

 tunately thus far impossible to obtain crystalline zinc sulphide 

 at ordinary temperature, and so great an extrapolation of the 

 curve AB would be unconvincing, but the indications are that 

 only a very slight acidity would be necessary for the formation 

 of pure wurtzite, if the rate of formation could be so con- 

 trolled as to give a crystalline product at ordinary temperatures. 



. 



