156 Decomposition of Iron Pyrites. 



3. Trial of powder hy long exposure to the air. A number of 

 pulverized and weighed samples of pyrite from 16 localities had 

 been set aside in watch glasses upon a shelf in the laboratory, 

 and soon gave apparent evidence of gradual oxidation, by sul- 

 phurous odor, more or less decided caking of the powder, and 

 sometimes a slight change of tint ; this conclusion was con- 

 firmed by a chemical test. In the hope that a method of trial 

 might be devised, founded on even so slow a process of atmo- 

 spheric oxidation, the samples were left undisturbed for two 

 months. The amount of iron set free as oxide was then deter- 

 mined by solution in sulphuric acid and titration in the usual 

 way. The results varied from 0.5 to 2 per cent, of the quan- 

 tities taken, the latter amounting to 0.5 to 3.8 grams. On ar- 

 ranging the figures in parallel columns with those of the specific 

 gravities, no correspondence or progression of any kind was 

 found. The oxidation had evidently been almost entirely super- 

 ficial and soon stopped, even within a thin layer of the fine pow- 

 der, by its speedy caking. The method was therefore given up 

 as unsatisfactory. 



4. Trial of crystals in hromine- vapor. A series of small crys- 

 tals of pyrite of varying densities, all compact and with bright 

 lustre, was then exposed to the vapor of bromine in the same 

 way, under a large bell-glass, as in Method No. 1. Only two 

 observations wei'e recorded in each case, after exposures of 10 

 and of 50 days respectively ; the latter was unnecessarily pro- 

 longed, distinctive action having been developed by the end of a 

 fortnight. 



