SEOTION III., 1882. (827000 Trans. Roy. Soc. CANADA. 
Note on Molecular Contraction in Natural Sulphides. 
By E. J. CHAPMAN, Pu. D., Professor in University College, Toronto. 
(Read May 26, 1882). 
1. In mixtures of metallic or mineral bodies, the composition, it is well known, may be 
calculated from the specific gravity of the mixture, or the specific gravity from the compo- 
sition. In actual combinations, on the other hand, neither of these results, as a rule, can 
be deduced—a molecular contraction or expansion of the combined bodies usually, if not 
invariably, ensuing. 
2. In the case of certain natural sulphides, very striking and apparently anomalous 
differences are thus manifested. In cubical Iron Pyrites, for example, the average density 
or specific gravity equals 5.0, whilst the percentage composition is represented by S. 53.3, 
Fe. 46.7. In Copper Pyrites, a more or less closely related mineral, occurring constantly 
under the same geological conditions, the percentage composition equals 8. 34.9, Cu. 34.6, 
Fe. 30.5. This latter mineral, therefore (with much less of the light body, sulphur, and 
with heavier metallic base), should possess a priori the higher specific gravity : whereas its 
maximum density does not exceed 4.2 or 4.3. It is evident, consequently, that in Iron Pyrites 
amuch greater contraction has ensued, by which more matter has been brought into a 
given space ; or, that, in Copper Pyrites a greater molecular expansion has followed. 
3. If the theoretical specific gravity of these bodies be calculated from their composi- 
tion, as mixtures, it will be seen that in each case contraction has really ensued—the actual 
density being greater in both cases, but much greater in the case of Iron Pyrites than in 
Copper Pyrites. In the former (calling the actual or average density 5.06) the excess equals 
2, as shewn in the table under §6 below; whilst in the latter (putting the actual density at 
4.2) the excess is only 0.24. A cubic foot of Iron Pyrites contains, therefore, 124.64 lbs. more 
matter than it would contain if its components were merely in admixture ; whilst in a 
cubic foot of Copper Pyrites the excess is only equal to 15.58 lbs. The excess in Iron Pyrites 
is, of course, equivalent to the weight of two equal volumes of water ; and in Copper Pyrites 
to practically one-fourth of an equal volume of water. 
4. In these deductions, the specific gravity of sulphur has been taken at 2.0; that of 
iron at 7.8; and that of copper at 8.9. A cubic foot of water has been assumed to average 
62.32 lbs. The formula used in calculating the theoretical specific gravity is the well 
known equation : 
In this equation, b and 6’ equal the respective densities of the bodies B., B’; and V., V’. 
equal, of course, the volumes of the latter. 
