150 Decomposition of Iron Pyrites, 



manii on the variation of specific gravity, and of other physical 

 characteristics in series of artificial intermixtures, in various 

 proportions, of simultaneously crystallized salts. His observa- 

 tion, that ^^ crystalline intermixtures have for the most part a 

 different specific gravity than that which corresponds to the cal- 

 culated mixture-proportions of their components,'" was referred 

 to changes of volume, whose course, like that of the figures for 

 specific gravit}^ ran without any parallelism to that of the 

 changes in mixture-proportion. This conclusion, however, can 

 have no pertinency here, as his salt-intermixtures were all ob- 

 tained by fusion, usually accompanied by expansion. Those 

 now under consideration — the natural intermixtures of the two 

 dimorphous sulphides — are certainly deposits from solution, 

 and, however intimately blended, may be far from homogene- 

 ous and have probably never been attended with any change of 

 volume. 



There is every reason to believe, that, in the lessened sensi- 

 bility toward chemical action exhibited by the harder and 

 heavier of these two homologous minerals, Ave have a fresh illus- 

 tration of the well-known law, which, in another connection, 

 T. Sterry Hunt has thus stated : '' The hardness of these iso- 

 meric or allotropic species, and their indifference to chemical 

 rengents, increase with their condensation, or, in other words, 

 vary inversely as their empii-ical equivalent volumes.'" The 

 fact of tliC inferior hardness of marcasite, though not recognized 

 in the general treatises on systematic mineralogy, has already 

 been noted in the papers of Breithaupt, Senft and others ; I 

 have pointed out beyond that it specially characterizes certain 

 crystal lographic faces of that mineral. It may bo safest to refer 

 the mode of condensation of the molecule in the heavier min- 

 eral, pyrite, to subtle schemes of natural blending or interpene- 

 tration, of course far beyond microscopical detection, and, 

 it may be, passing all i)resent comprehension. But tak- 

 ing into consideration the physical properties which can 

 be recognized, Ave are led to suspect that the material of the 

 lighter crystals of marcasite, and especially of its granular 

 forms, may consist in miniature of an interlacing network of the 



^ Ueber die Krystallisation, Beobachtungen und Folgerungen, Leipzig, 

 (1884). 17. 



3 Chem. and Geol. Essays, 457. 



