

Amount of 



^ij. Gr. 



actioji. 



4.402 



0.25 



4.601 



.50 



4.931 



.50 



4.973 



1.00 



4.771 



1.00 





1.00 



remark ; 



: '' Thus in 



Decomposition of Iron Pyrites. 153 



Yellow cubical pyrite, containing no visible 



impurities, 

 Yellow pyrite, in icosahedrons, Kougsberg, 

 Compact marcasite, (Ke, 44.6 per cent.), 

 Yellow pyrite, in ])yritohedrons. 

 Radiated and globular marcasite, "pyrite 



blanche,'' (Fe, 45.4 per cent.), - 

 Marcasite in advanced decomposition, 



In reference to these results, the authors remark: 

 tbe series of the iron pyrites, which are generally quite pure, it 

 jippears that the facility of decomposition varies with the den- 

 sities and the crystalline forms ; in a word, with the molecular 

 condition." However, but slight analogy can probably be as- 

 sumed between the results of the action of a silver solution upon 

 jiyrites and those of the weathering now under consideration. 

 The proper experimental methods for our purpose may be either 

 chemical or physical ; the one, by exposure of the specimen of 

 ])yrites to the action of an oxidizing atmosphere or liquid ; the 

 other, by determination of the specific gravity of the sjiecimen 

 under ti'ial. 



A. — Teial of Pveites by Oxidation. 



Several methods of trial were tested, some with the object of 

 quantitative estimation of the jiroducts of oxidation. Only 

 brief reference need be made to methods found unsatisfactory. 



I. Trial of poiuder in hromine-vapor. In the following table 

 are presented the I'esults of experimental trial of a series of typi- 

 cal forms of pyrite from a number of well-known local- 

 ities, arranged very nearly in the order of decreasing specific 

 gravities. The specimens were finely powdered, and passed 

 through bolting cloth, and exactly one gramme of each was 

 spread out thinly over a shallow watch-glass. These were all 

 exposed, under a large bell-glass, to an atmosphere of bromine- 

 vapor, for the same length of time, twelve hours; the iron- 

 oxide set free was then dissolved out in sulphuric acid, and its 

 amount determined volumetrically. The results obtained, in the 

 fourth column, show the correspondence of decreasing resistance 

 to oxidation, with decrease of specific gravity and more rapid 



