Decoviposition of Iron Pyrites. 133 



the air and of its moisture, into sulphate of iron ; the pyrite 

 then loses its coherence in proportion as the cement of the crys- 

 tallized particles is decomposed." 



A little later, in 1828, Kohler stated in his paper' on the py- 

 rites of Gross Allmerode in Hesse, that the ordinary nodular 

 forms, consisting of marcasite (strahlkies), decompose readily, 

 and break up, with an efflorescence of copperas, while the crys- 

 tals themselves resist decay ; in the latter, the crystalline form 

 is identical with that of pyrite, but the specific gravity is con- 

 siderably lower. His words are : '^ A distinguishing property 

 of this radiated pyrites is its mode of decomposition. The or- 

 dinarily very fibrous nodules possess the tendency, with an efflor- 

 escence of copperas, to break up entirely by degrees ; the crys- 

 tals themselves resist the disintegration. The explanation is 

 well known, which Berzelius gives of this phenomenon. 

 Through chemical analyses, however, the undecomposed masses 

 exhibit no differences of importance. Certain analytical results, 

 obtained on different varieties of the Allmerode pyrite, have 

 convinced me that it shows the same relative proportions as Bi- 

 nary Pyrite." In 1829, seven years after his first experiments, 

 Berzelius adds:" ^MVheii a portion of common pyrites was per- 

 mitted to fall asunder, I found it to be caused by the formation 

 of a small quantity of protosulphate of iron, which burst asun- 

 der the crystallized mass. When the salt was dissolved in water, 

 no trace of free sulphur was obtained, from which it appeared 

 that the efflorescing pyrites contains particles of Fe S (sul- 

 ])huret of iron), which, changing to the state of salt, tears asun- 

 der the rest which undergoes no change ... I have since ob- 

 tained a satisfactory proof of the accuracy of this explanation. 

 I healed carbonate of iron gently in a stream of sulphuretted 

 hydrogen. There were formed, first sulphuret, and afterwards 

 bi-sulphuret of iron. The experiment being stopped before all 

 the iron was changed into bi-sulphuret, a pyrite was obtained, 

 which, in a few days, fell asunder in all directions, and changed 

 into a woolly mass of vitriol of ten times its former volume. 

 Sesqui-sulphuret of iron prepared from the oxide has not this 



' Pogg. Ann., (1828), XIV, 96. 

 ^ Berzel. Arsberat, (1829), 129. 



