154 Decomjyosition of Iron Pyrites. 



decay by exposure to the weather. The facts in the last column 

 could only be deternriined, in some cases, from the hand-speci- 

 mens ; but the deposit of iron-ochre on some of these {e. g.^ No. 

 46), plainly did not indicate the results .of weathering, but 

 either of subterranean decomposition, (an agency distinct from 

 ^' weathering" in both character and duration), or merely of 

 the transport of iron-ochre from the upper parts of the vein or 

 gangue and its deposit upon these crystals. 



This mode of experimental trial of a pyrite can evidently be 

 used to obtain satisfactory information as to its inclination to 

 oxidation, in comparison with one of the stable varieties, such 

 as that of Elba, Piedmont, or Colorado, of which specimens are 

 easily obtainable. 



2. Trial of poivder in aqueous solution of bromine. In the 

 hope of ensuring more speedy and uniform results than those of 

 the preceding process, the method was tried of placing weighed 

 equal quantities of finely pulverized jiyrite, from different local- 

 ities, in stoppered bottles, and adding simultaneously equal vol- 

 umes of a weak solution of bromi?)e in water. However, the 

 action was very rapid, the bromine becoming exhausted and the 

 solution decolorized in a few minutes, in every case. This 

 method was tlierefoi-e abandoned. 



