20 ON PYRITE AND MARCASITE. [bull. 186. 
FURTHER APPLICATIONS OF THE METHOD. 
Iii general, when a mineral is composed of two or more elements, 
each capable of oxidation, but at different rates, there will be certain 
conditions under which one will be oxidized to a greater extent than 
the other, and the amount of this difference may often be determined 
by analysis of the solution. If the compound is dimorphous, one form 
will be more soluble than the other, and this will show a greater dif- 
ference in the degree of oxidation of the two constituents than the 
less soluble form. The above method therefore makes it possible to 
distinguish not only between dimorphous sulphides but other dimor- 
phous compounds, such as arsenides, and to determine their relative 
amounts in mixtures. It should also be possible to determine whether 
certain complex minerals are mixtures of two distinct kinds of mole- 
cules or Avhether all the components are combined in one molecule. 
FeAsS, for example, should show a behavior different from that of a 
mixture FeS 2 and FeAs 2 in equimolecular proportions. When the 
metal is more slowly oxidized than the nonmetallic constituent a sep- 
aration of a portion of the former may be expected. Gold sulphide, 
for example, might give sulphuric acid and free gold. As pointed 
out below, it is possible to ascertain in certain cases the form of com- 
bination of constituents which exist only as impurities; in a 3 per 
cent cupriferous pyrite, chalcopyrite can be distinguished from chal- 
cocite and bornite, a determination which could not be made by 
analysis. 
III. DETAILS OF THE METHOD. 
NECESSARY PRECAUTIONS. 
In the following the precautions necessary to obtain strictly con- 
cordant results are given in detail, as well as a statement of the 
abbreviations which are permissible when only approximate results 
are desired. 
To obtain accurate results it is necessary — 
1. To employ sulphide absolutely free from oxidation products. 
2. To avoid all access of free oxygen during the experiment. 
3. To prevent the precipitation of basic ferric sulphate. 
4. To prevent change of concentration due to loss by evaporation. 
5. To make the titration with the greatest possible accuracy. 
PREPARATION OF THE MATERIAL. 
The mineral is somewhat finely ground, an operation which is greatly 
facilitated by adding water. After drying, it should be kept in a vac- 
uum or in carbon dioxide until used. As both pyrite and marcasite 
oxidize rapidly when powdered, the oxidation products must be com- 
pletely removed immediately before the experiment. About one 
gram of the dry powder is washed by decantation two or three times 
