410 



Allen, Cre?ishav), and Menuin- 



ous acids at 200° and 300° respectively, should intersect at an 

 average acidity of about .05 per cent. For the reason given 

 above, the only meaning which attaches to this is that in a 

 furnace set for 300°, all the product would be precipitated 

 when 200° was reached in an acid of this concentration. 



The main conclusions in this paragraph are confirmed by 

 the results obtained with hydrochloric acid solutions. 



4. Influence of hydrochloric acid. — : The influence of hydro- 

 chloric acid on the crystalline form of iron disulphide was 

 regarded with much interest, not only because side reactions 

 due to the oxidizing effect of sulphuric acid could thus be 

 entirely avoided, but also because in this way any peculiar 

 effect due to sulphuric acid could be eliminated and the spe- 

 cific influence of the hydrogen ion concentration could be put 

 to test. 



The same experimental methods were followed as before, — 

 only ferrous chloride was substituted for ferrous sulphate. 

 The ferrous chloride solution was prepared by partially dis- 

 solving a quantity of powdered iron in hot hydrochloric acid, 

 pouring the mixture on to a filter, so that the excess of iron 

 would keep the solution from oxidation, and catching the 

 filtrate in a flask which was kept full of carbon dioxide by the 

 passage of a continuous stream. After the excess of acid had 

 been neutralized by sodium carbonate, the iron was deter- 

 mined and the solution then diluted with air-free water. The 

 concentration actually taken in the experiments was equivalent 

 to the ferrous sulphate solutions in previous experiments, and 

 all the work was done at a maximum temperature of 300°. 

 The products were better crystallized than any others. 



Table V. 



Showing the influence of hydrochloric acid in the formation of 

 marcasite at 300° . 



Initial acid 

 in wt. % 



Final acid 

 in wt. % 



Average acid 

 in wt. % 



{x — a) 



Percent 

 pyrite 



o-io 



0'60 



0-35 



9*63 



15" 



•10 



•82 



•46 



9-77 



13- 



•05 



•69 



•37 



9-81 



12- 



•05 



•25 



•15 



994 



10- 



•50 



•90 



•70 



10-20 



6- 



•50 



•90 



•70 



10-31 



5' 



•25 



•93 



•59 



10-34 



4- 



•50 



•86 



•68 



9-12 



28- 



•75 



1-00 



•88 



913 



28- 



