390 Allen and Crenshaio — Stokes Method for the 



iron dissolved when the finely ground and purified sulphide is 

 treated with a boiling standard solution of ferric alum. The 

 same pyrite or marcasite gives very constant values and the 

 influence of each in mixtures is additive, i. e., there exists a 

 linear relation between the iron dissolved and the composition 

 of the mixture. The sum of the errors usually amounts to 



mot 













Fig. 



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N— 



A- 5 6 7 8 9 10 II 



Dissolved iron in -terms of standard permanganate solution 



Fig. 6. 1, 1. Values of the Stokes' constant (x-a) for the purest natural 

 pyrite and marcasite, and their mixtures. 



2, 2. Values of the Stokes' constant for synthetic pyrite, synthetic mar- 

 casite and their mixtures. 



about 1 per cent, reaching a maximum of 2 per cent. There 

 are two important sources of error. First there must be a 

 sufficient excess of the sulphide, which is many times greater 



