THE GEOLOGIST 
JUNE, 1859. 
ON ROCKS ; THEIR CHEMICAL AND MINERAL COMPOSI- 
TION, AND PHYSICAL CHARACTERISTICS. 
By H. C. Salmon, Esq. Plymouth. 
(Continued from page 59.) 
XXI. Relation between tub Physical Characters of a 
Mineral and its Chemical Composition. — 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 ciystalline 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 departure from regular although unknown laws, and seek for a 
cause. This we find in the doctrine of Isomorphism. This, simply 
stated, is the capability of two or more substances, of analogous 
chemical constitution, to crystallize in similar forms. Thus, as a few 
examples in the mineral kingdom, we find that Corundum (Al) and 
vol. II. s 
