In the definition which I have given, a mineral is described as a body possessed of a definite chemical composition and a regular physical form,—meaning by the latter particularly crystalline structure. But, while the crystalline relations of each species only vary within the narrowest limits, the chemical composition has a much wider latitude. Many individuals of the same mineral species are found, by analysis, to differ most widely in their chemical components, while the crystalline form is quite unaltered. Now, although we are as yet unable to trace the relation between the form of a mineral and its composition, there can still be little doubt that some such relation does exist, regulated by laws yet to be discovered; consequently, when we find the same mineral species differing widely in chemical proportions, we recognise a departùre from regular although unknown laws, and seek for a cause. This we find in the doctrine of Isomorphism. This, simply stated, in the capability of two or more substances, of analogous chemical constitution, to crystallize in similar forms.
