stokes.] MISCELLANEOUS SPECIMENS. 35 
employing fine powders the rapidity of the change will be vastly 
increased, while the net results, according to the mass law, will be 
the same. 
DETECTION OF CHALCOPYRITE IN PYRITE OR MARCASITE AND IN 
ROCKS. 
Chalcopyrite mixed with pyrite or marcasite may be readily detected, 
if not in too small amount or too finely divided, by exposing the 
sample to bromine vapor for half a minute, and then to hydrogen 
sulphide gas; the chalcopyrite is blackened, while the iron sulphides 
remain bright. Particles of chalcopyrite which would otherwise be 
overlooked are thus easily detected and their outlines sharply defined. 
I have used the same procedure to deted and establish the nature of 
minute grains of chalcopyrite inclosed in rocks. 
X. MISCELLANEOUS SPECIMENS AND SPECIMENS OF DOUBTFUL 
NATURE. 
With the aid of the pyrite-marcasite curve we are now in a position 
to determine the quant it at ive composil ion of specimens of pyrite and 
marcasite in regard to which there is some uncertainty. The sup- 
posed nature of the specimens, according to the geologist, mineralo- 
gist, or dealer from whom they were obtained, is stated, as well as 
the composition established by my method. 11 is to be understood 
that the figures express the relative amount of pyrite and marcasite 
on a scale of 100, impurities being excluded. In this connection I 
wish to acknowledge the generosity of Dr. A. A. Julien, who has 
placed at my disposal a number of the identical specimens described 
by him in his valuable paper repeatedly referred to above. As the 
study of these specimens is especially important in confirming or 
refuting his hypothesis (p. 27), I have grouped them together at the 
end of this section, and have in each case quoted his original descrip- 
tion and specific gravity determination, comparing in parallel columns 
the results obtained by the specific gravity and the oxidation methods. 
