stokes.] ACTION OF CUPRTO SALTS. 41 
true nature of which is shown by careful inspection. Wohler 1 
attempted, without success, to convert marcasite into pyrite and 
pyrite into marcasite by heating for four hours at about 400° — 
experiments which are well worth repeating, with an extension of 
the time and with the aid of the oxidation method. Julien's assump- 
tion of extensive paramorphism is scarcely longer tenable in view of 
the results described above, although the inclosure of considerable 
portions of pyrite in some specimens of marcasite lends some support 
to it. While theoretically possible, the attractive hypothesis that 
marcasite gradually changes to pyrite without re-solution, and that 
the porosity of certain pyrites can be thus explained, is thus far 
without a sound basis, either of experiment or of observation. It is 
intended to carry out experiments -on this point in the immediate 
future. 
XII. CONSTITUTION OF PYRITE AND MARCASITE; ACTION OF 
CUPRIC SALTS. 
The importance' to chemical geology of an experimental study of 
1 he action of copper salts on pyrite and marcasite is self-evident. The 
following experiments were made, however, with the object ol* ascer- 
taining whether the method of A. P. Brown 2 — namely, heating with 
neutral copper sulphate solution at 200° — could be used to distinguish 
these minerals, and incidentally, of testing his hypothesis that the 
iron in marcasite is wholly ferrous, while that of pyrite is four-fifths 
ferric. I have therefore confined myself to the conditions of temper- 
ature and concentration observed b} 1 " him. 
Brown, in his published experiments, has limiicd himself to titrat- 
ing the ferrous iron in the resulting solution, while no determination 
was made of any ferric iron which might be present either in the 
solution or in the precipitate. As neutral ferric sulphate is practi- 
cally completely precipitated as basic salt on heating its solution, and 
as any ferric salt remaining in solution would inevitably be reduced 
by the sulphides present, it seems reasonably clear that a single experi- 
ment: with pyrite, in which only the ferrous salt is determined, in 
which no account is taken of the ferric salt supposed to be formed, 
and in which no clear evidence of complete decomposition of the min- 
eral is given, can not have great weight in establishing an important 
hypothesis. While not, questioning the correctness of Brown's single 
observation so far as it goes, I have been wholly unable to obtain the 
same results. 
In each experiment cited below the material was finely ground, care- 
fully freed from oxidation products in the manner employed in my 
oxidation experiments (p. '20), and about 0.2 gram heated with 50 
cm. 3 of 10 per cent iron-free CuS0 4 , 5H 2 solution which had been 
1 Liebig's Annalen, Vol. XC, 256. 
2 Proc. Am. Philos. Soc, Vol. XXXIII, 1894, p. 335. Chemical News, Vol. LXX1, 1895, p. 179 
