Effect of Temperature, Acidity, etc. 423 



The determination of the percentage of wurtzite in the 

 products was done by microscopic estimation, and naturally 

 the accuracy of this method, especially as the wurtzite was 

 usually coated with sphalerite and frequently mixed with 

 amorphous material, can not be compared to that of the Stokes 

 reaction which was used with the disulphides of iron. When 

 the products consisted entirely of one of the two forms, the 

 microscopic estimate was, of course, entirely satisfactory. 



The results are given in Tables X to XII, and plotted in 

 fig. 4. The final acid concentrations, instead of the average 

 concentrations, are here taken as abscissas for reasons already 

 given (p. 421). 



1. The question of a linear relation between the percentage 

 of wurtzite and the final acid concentration. — Considering the 

 difficulties of the work the results show surprising regularity. 

 When the determinations were made, the microscopist was 

 without knowledge of the experimental conditions under 

 which the different products were formed and the estimates 

 could not have been biased in any way. The results are of 

 more interest, because of the similarity to those obtained in 

 the synthesis of pyrite and marcasite. The reader will bear in 

 mind that in the latter case a linear relation was found between 

 the acidity and the percentage of marcasite, and that the only 

 satisfactory explanation of this relation was that the two crys- 

 talline forms were precipitated at the same time, the propor- 

 tions varying with the acidity. There are some facts about 

 the formation of the zinc sulphides which are not in accord 

 with the supposition of the simultaneous precipitation of the 

 two. The wurtzite is very commonly coated over with 

 sphalerite, which must, of course, have formed later. There 

 are, however, often separate crystals of sphalerite which may 

 have been formed simultaneously with the wurtzite and before 

 the final coating. Simultaneous precipitation of the two forms 

 is, perhaps, in this case, not a necessary consequence of a linear 

 relation between acidity and composition, for the reason that 

 synthetic wurtzite appears to undergo change into sphalerite 

 subsequent to its formation whenever the acidity falls suf- 

 ficiently, while marcasite never suffers a similar change so far 

 as we know. Still, in view of the difficulties both in following 

 the experimental processes and in estimating the composition 

 of the products, it would be unwise to insist on the existence 

 of a linear relation here, though the parallelism between the 

 results on the synthesis of the zinc sulphides and those on the 

 iron disulphides is remarkable. The results, however, leave 

 no doubt of the specific influence of acid on the crystalline 

 form of zinc sulphide, nor of the conclusion that when wurtzite 

 and sphalerite are formed together, the products precipitated 



