Determination of Pyrite and Marcasite, 



379 



difference due to the time of grinding, though marcasite was 

 repeatedly tested after grinding from 2 to 5-J h., pyrite from 1 

 to 2 h., and synthetic products from 1 to 2 h. It seems, there- 

 fore, to be true that after a sufficient excess of the sulphide is 

 reached the results are unchanged by increasing the surface 

 further, though more work would have to be done before this 

 statement could be made unqualifiedly. 



Table II. 



Shoiving the relation between the composition of the sulphide mixture,* the 

 iron dissolved, and the sulphur oxidized. 



Per cent 



(x — a) — iron dissolved 



X 



p = per cent 



of sulphur 



oxidized 



pyrite 



0-75 g. 

 sulphide 



10 g. 



sulphide 



1-5 g. 

 sulphide 



100 



4'40 



4-46 



4-59 



41-17 



51-9 



85 



5-35 











42-06 



40 5 



75 



6*08 









42-79 



33'6 



60 







7*04 



„ . 



43-75 



26-8 



50 



7-67 











44-38 



23*2 



45 





7*94 







44-65 



21-9 



35 



. 



8-48 





45-19 



19-5 



25 



9-06 



9-07 







45-77 



17'1 



17 





9-54 







4631 



15-5 



•10 







9-95 



_ 



46-66 



14-1 



5 



9-40 



10-27 







46-98 



13-1 







10*30 



10*58 



10-57 



47-28 



12-2 



For all our work on either natural or synthetic products we 

 have taken 1-05 to 1*1 g. and ground with water for 2 h. The 

 sulphide is then removed from the mortar with a " policeman," 

 filtered in a porcelain Gooch crucible on a hardened filter disc 

 and kept in a vacuum desiccator till the next morning, when 

 1 g. of it is weighed out, boiled with dilute hydrochloric acid, 

 poured on to a new hardened filter and finally washed, first 

 with dilute hydrochloric acid and then with boiled water 

 cooled in carbon dioxide. The washing is done in a special 

 apparatus in which an atmosphere of carbon dioxide free from 

 oxygen is maintained, for the details of which a former paper 

 should be consulted. f In these operations the total loss oj 

 material should he only a few centigrams. If it is considerable 



* The pyrite used was from Eoxbury, Conn., and the marcasite was from 

 Joplin, Mo. 



f Allen, Crenshaw and Johnston, this Journal, (4), xxxiii, 109, 1912; Zs. 

 anorg. Chein., lxvi, 201, 1912. 



