200 Prof. J. P. Cooke on the Variation of Constitution in a 
other considerations as from the chemical analyses. 
Sufficient has been said, I think, to show that variations m 
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 
ed so tk eS 
nite crystalline form is compatible with quite a wide variation of 
composition, and has in this way pointed out an explanation of ei 
the variation observed in the mineral kingdom. But more than — 
this, the investigation has also indicated a method by which, © 
amidst all this variation, the true constitution of the mineral cam 
be determined. : 
In the compounds of zine and antimony, although the definite 
crystalline form was compatible with a wide variation in the 
proportions of the constituent elements, yet the point correspond- 
ing to the typical composition was marked by several unm 
takeable properties, which clearly enough indicated the true for- 
mulze ounds, ‘These properties are discussé at 
length in my original memoir, and need therefore only to be al- 
luded to in this connection 
acter of the crystals appears to be modified by the change of 
composition, although the crystallographic elements remal? 
same. Thus in the crystals having the theoretical compositions 
the octahedral planes are greatly developed, giving to the crys 
tals the general appearance of a truncated octahedron.* emit” 
the crystals take up an excess either of antimony OF pnslior™ 
basal planes become more and more dominant, and the ¢ty . 
are at last reduced to thin plates. In fact, so marked aret 
changes, that, after a little experience, a person could tell the ae 
proximate composition of the crystals from their general appear 
* See figure accompanying my original memoir. 
