Effect of Temperature, Acidity, etc. 



403 



pared with Joplin marcasite — never more than 100 per cent as 

 would be expected if ferrous salt was occluded. 



Thus, while it would be difficult, if not impossible, to prove 

 that the synthetic products contained no inclusions, everything 

 indicates that they do contain pyrite, and that the quantity of 

 it is regularly influenced by increasing acid concentration and 

 by temperature, as we shall see farther on. 



4. Analogous behavior of zinc salts. — Further light is 

 thrown on the nature of the synthetic iron disulphides by the 



Fig. 1. 



60 



50 



£40 



30 



20 



10 



_S -X !f Ji-<>_ 



_\- — & °- 



o\ x 



o 



0.5 1.0 1.5 2.0 2.5 3.0 



Concentration of sulphurvc acid in weight percent 



3.5 



Fig. 1. The effect of acid concentration on the formation of iron disul- 

 phide. 



o=200° ; x= 300° 



similar behavior of the zinc sulphides. We shall find in the 

 next chapter that acid exerts the same specific influence on the 

 sulphide of zinc that it does on the disulphide of iron, i. e., it 

 causes that substance to crystallize in an unstable crystalline 

 form — wurtzite. But there also the unstable form is com- 

 monly mixed with the stable form sphalerite. Now the 

 method employed to identify the zinc sulphides is entirely 

 different from that used for the iron sulphides. Because of 



Am. Jour. Sci. — Fourth Series, Vol. XXXVIII, No. 227. — November. 1914. 



28 



