152 DecompoHition of Iron Pyrites. 



riolesceiice was increased in pulverized pyrites, attributing it to 

 the increase of exposed surface ; also, that in nodules, this ac- 

 tion begins in the interior, where the texture is loosely granular 

 and full of clefts. So also Knop^ found, on direct experiment 

 on the pyrrhotite of Horbach, "that the vitriolescence of the 

 ore in a finely divided condition proceeds more rapidly than in 

 the form of larger fragments." In the fibrous nodules of pyrite, 

 the material appears to be also in a state of extreme lateral ten- 

 sion, which has facilitated its thorough sub-division by fissures, 

 the deep entrance of air and moisture, its rapid oxidation, and 

 ready and continual yielding to further disruption by the out- 

 ward pressure of the copperas ci-ystals formed during efflores- 

 cence. In the preceding discussion attention'has been entirely 

 given to the common varieties ot pure iron-pyrites, without re- 

 gard to the exceptional cases, only one of which is on record, in 

 which other metallic sulphides, e. g., chalcopyrite, have acted as 

 as accessory agents in producing the tendency to decomposition. 

 Other instances will be given beyond. 



II. — DETERMIlfATION OF DeGREE OF STABILITY. 



There are many practical applications of these facts, stated at 

 the close of this paper, which show the ])ressing need of arti- 

 ficial methods of experiment, by which to determine the incli- 

 nation to oxidation or degree of stability in certain specimens of 

 these ii'on sulphides. We may, in passing, here i-ofer to the ex- 

 periments of Malaguti and J. Durocher,^ who tested a series of 

 specimens of metallic sulphides with a solution containing silver 

 chloride, and then determined the amount of the silver salt thus 

 decomposed. From their genei'al results we may select the fol- 

 lowing, which, in the third column, represent the comparative 

 action of the iron sulphides on the silver salt, the amount of the 

 sulphides being taken as 100. 



' N. Jahrb. Min. Geol. Pal., (1873), 521. 

 ■^ Ann. d. Mines, (1850), 4 Ser., XVII, 294. 



