Properties of some Minerals and their varieties. 267 
same cause, viz: the admixture of Cu, Ni, Ag, Au, which 
gives pyrite its metallurgical value. The + varieties of FeS, 
We simply mention this fact without wishing to establish any 
haa | 
1n- 
composition by regular gradations, but rather by abrupt or: 
and consequently remains identical within certain | 
Before establishing relations between the form an erg 
electricity, it must first be shown that the material in hand is 
identical. 
Our observations have demonstrated that in some cases the 
change in thermo-electrical character corresponds to a change he 
the chemical composition, and always to an alteration in the 
density. 
* $i fie compare note 1, p. 261. It may be men- 
tioned in width teat a change in density must also accompany the change of + 
in the case of iron at 
