H. N. Stokes — Pyrite and Marcasite. 417 



oxidation rate of the iron is not readied. Marcasite dissolves 

 much more rapidly, the result being that a still greater portion 

 of the sulphur, about 82 per cent, escapes. The same expla- 

 nation naturally applies to their different behavior towards 

 nitric acid as observed by Pentield. It is not an indication of 

 the different chemical constitution, but of a different solution 

 tension conditioned by different crystalline structure. Doubt- 

 less a similar method can be applied in other cases of dimorphism 

 to determining which form is more soluble. I propose to 

 apply the method to distinguishing other dimorphic com- 

 pounds. 



For the details of the operation and. apparatus I must refer 

 to the extended article, stating here that the material must be 

 absolutely free from oxidation products, and must therefore be 

 extracted with acid and washed and dried in carbon dioxide, 

 and that air must be rigidly excluded during the operation and 

 complete condensation of the steam provided for. The stand- 

 ard solution oxidizes both pyrite and marcasite much more 

 slowly at 20°, but the per cent of sulphur oxidized is greater, 

 namely about 81 per cent for pyrite and 31 per cent for 

 marcasite. 



Doubtless the action of ferric salts plays an important part 

 in nature in the disintegration of pyrite and marcasite, the 

 iron acting as a transferer of oxygen. The conditions under 

 which the sulphur undergoes complete oxidation are as yet not 

 clear, and this subject is now under investigation. 



Jifixtures of pryite and marcasite. — It is clear that the 

 oxidation coefficient of any given mixture of pyrite and marca- 

 site cannot be deduced by any simple process from the 

 coefficients of the pure minerals. Each mineral is here decom- 

 posing in a solution containing the reaction products of the 

 other, and only an extensive knowledge of the influence of con- 

 centration, acidity and dissociation would enable us to deduce 

 theoretically a curve for such mixtures. It has therefore been 

 necessary to construct the curve from data obtained by experi- 

 ments on artificial mixtures. Since the action is a surface 

 action, the result depends, not on the relative weights, but on 

 the relative surfaces, and uniformity in this respect was 

 obtained by always grinding weighed portions of the minerals 

 together. The results, with different preparations and different 

 samples, show that perfect agreement can be obtained in this 

 way, the results for different samples of a given composition 

 not differing more than one-half unit. The following table 

 gives the mean results obtained : 



Per cent pyrite, 5 10 20 40 60 80 90 95 100 

 Value of p, 18-0 16'0 15'2 17'1 22'3 29'0 40-3 48'9 52'9 60*5 



Am. Jour. Sci. — Fourth Series, Vol. XII, No. 72.— December, 1901. 

 29 



