410 Prof. J. P. Cooke on the Variation of Constitution in a 
such a discussion must be based on purely hypothetical grounds, 
we could not expect to arrive at any definite conclusion. The 
facts will be viewed differently according to the theory which 
may be adopted in regard to that long-controverted subject, the 
essential constitution of matter. Leaving, however, all theore- 
tical considerations aside, there are certain practical bearings of 
the observed facts on the science of mineralogy which are of 
immediate application. 
Here are two beautifully crystallized products, as well crystal- 
lized as any that occur in nature, and yet the different specimens 
of the crystals differ from each other so widely in composition 
that any single analysis might lead to an entirely erroneous con- 
clusion in regard to the general formula of the substance. Werea 
chemist to analyse accidentally solely the crystals obtained from 
an alloy containing 58°6 per cent. of zinc, he would at once de- 
termine that the formula of the compound was Sb Zn*; and by 
a like accident he might be led to any other formula between 
this and Sb Zn®: in fact, by an analysis of a number of spe- 
cimens of needle-shaped crystals obtained from alloys of copper 
and tin, Rieffel was led to several just such improbable for- 
mule; and in my own investigations it was not until I had 
analysed a whole series of crystals, that the real nature of the 
phenomena became apparent, and the true constitution of the 
compounds determined. If, then, such great variations in com- 
position are compatible with a definite crystalline form in these 
furnace products, may not similar variations occur in the ery- 
stalline minerals formed in nature ? 
It is not necessary to make an extended investigation in order 
to answer this question ; for the materials at our hands are suf- 
ficient to give us a satisfactory reply. 
There is a compound of antimony and silver called Diserasite, 
which occurs in many localities crystallized in trimetric prisms 
homceomorphous with Sb Zn°. The formula of the mineral is 
therefore probably Sb Ag’, which would require 71°5 per cent. 
of silver; but the per cent. as given by analysis varies between ° 
75°25 and 78 per cent., and one analysis gives the per cent. as 
high as 85. Further analyses of this mineral are required in 
order to determine its constitution, but there can be no doubt 
that it varies in composition like Sb Zn, 
Silver-glance is another highly crystalline mineral. Theore- 
tically it should contain 87:1 per cent. of silver and 12-9 per 
cent. of sulphur ; but in a specimen analysed by Klaproth, the 
proportions were 85 and 15. 
Again, the analyses of pyrrhotine (magnetic pyrites) give 
results varying between 38°78 per cent. sulphur, 60°52 per cent. 
iron (variety from Bodenmais), and 43°63 sulphur, 56:37 iron. 
