28 E. J. CHAPMAN : NOTE ON MOLECULAR 
5. Of all natural sulphides, cubical Iron Pyrites appears to present the greatest contrac- 
tion. Marcasite (Prismatic Pyrites) approaches it very closely—the excess of weight in the 
latter species being equal to that of 13 vol. of water. In Pyrrhotine (Magnetic Pyrites) it 
equals one volume. In Galena, Argentite, Copper Glance, Stibnite, Bournonite, and 
many other sulphides—quite irrespective, apparently, of atomic constitution or crystalliza- 
tion—it equals one-half the weight of an equal volume of water ; whilst in Zinc Blende it 
amounts to only one-fifth, and in the arsenide Smaltine to one-eighth of that weight. In 
Realgar and in Greenockite (Cd 8) there is apparently no contraction; nor is any revealed 
in the lead and silver tellurides, Altaite and Hessite. 
6. As these somewhat curious relations do not appear to have been referred to in mineral- 
ogical publications, I have ventured in this brief notice to bring them before the attention 
of the Society. The annexed table exhibits the relations as presented by the more com- 
monly occurring sulphides and allied compounds ; but, in some cases, owing to the difficulty 
of obtaining absolutely accurate densities, the results are necessarily approximative only. 
This, however, does not in any way invalidate the general fact that, among minerals of 
related composition and constitution, very striking differences of molecular condensation 
occur. The question naturally arises as to whether these differences result merely from 
accidental causes, or are the outcome of some definite law. Accidental they can scarcely be— 
as the amount of contraction is essentially the same in examples of the same substance 
occurring under widely different conditions, and in widely separated localities. They would 
thus appear to depend upon some general law; but the nature of this law, in the present 
state of our knowledge, is seemingly without explanation. 

