mineral species independent of the Phenomena of Isomorphism, 411 
(variety from Baréges), The constitution of the mineral is still 
uncertain ; but its true formula is probably FeS, which would 
require 86'4 per cent. sulphur and 63°6 per cent. iron, Lastly, 
the analyses of antimony-glance give results varying between 
Antimony 74°06, had ‘Antimony 73°5, 
Sulphur 25°94, Sulphur 26:5. 
The true formula of this mineral is undoubtedly Sb 8%, which 
would require only 72°88 per cent. of antimony. 
Similar examples might be very greatly multiplied. Those 
just cited were selected at random from the first few pages of 
Dana’s ‘ System of Mineralogy.’ They are all examples of binary 
compounds which occur almost chemically pure in nature; so 
that the phenomena in question are not complicated by those 
of isomorphism. 
When we pass to minerals of more complex constitution, the 
same phenomena cau be made evident, although not quite so 
easily, on account of the introduction of the phenomena of sub- 
stitution by isomorphous elements. It will not, however, be 
necessary for me to cite examples; for it is a fact perfectly well 
known to all mineralogists, that, after making allowances for the 
substitution of isomorphous elements, the various analyses of 
such minerals as mica, hornblende, garnet, and tourmaline differ 
very greatly from each other,—a difference, moreover, which no 
mere error of analysis will explain, and which must therefore be 
referred to an actual variation in composition. In the silicates 
this variation in composition is made evident by the variation of 
what is termed the “ oxygen ratios;” and it is well known to 
mineralogists that in many species this variation is very large. 
For example, in mica the following ratios between the oxygen in 
the base and acid have been observed in merely the Muscovite 
variety :-—13: 16, 134: 16, and 142:16; and similarly wide 
variations might be pointed out in other well-known species. It 
is in consequence of such variations as these that the general 
chemical formulz of some of the best-known mineral species, 
such as mica and tourmaline, are still uncertain; and in other 
cases, where the true formula is probably known, the constitution 
of the mineral has been determined quite as much from other 
considerations as from the chemical analyses. 
Sufficient has been said, I think, to show that variations in 
composition similar to those which I have observed in zine and 
antimony occur in many minerals; and I trust that the results 
of my investigation will serve to throw light on this whole class 
of phznomena, which have so greatly perplexed mineralogists, 
and rendered all strictly chemical classifications of mineral spe- 
cies so unsatisfactory. This investigation has shown that a defi- 
