REPORT ON MINERALOGY. 
345 
tourmaline as containing potassa and soda along with lime and 
magnesia; these alkaline substances taking each other’s places, 
so that there may be less of the one in proportion as there is 
more of the other. And he observes that Fuchs calls such 
elements “ vicarious” with respect to each other,—a term since 
used, and with propriety, to designate the relations of isomor- 
phous ingredients. 
I do not know whether all our English chemists fully ac¬ 
knowledge the reality of the isomorphous groups of minerals; 
but those who do, will probably agree, that one of the most im¬ 
portant objects which the chemist or the mineralogist can at 
present propose to himself, is to extend such grouping to as many 
minerals as possible. We have at present a mere mob of spe¬ 
cies ; by brigading them under a system of isomorphism, they 
may become a well-ordered army. 
It cannot be denied that there is something formidable in the 
prospect of the labour which is thus found to be incumbent 
upon those who would learn the constitution and relations of 
mineral species. The exact analysis of one or two specimens 
of each species has been considered, and justly, as a business 
requiring no small skill and sagacity, and great care and sacri¬ 
fice of time. Even the most patient and most industrious of che¬ 
mists, Berzelius and the Germans, complain of the employment 
on this ground. But it appears from the isomorphous doctrine, 
that we cannot hope to understand the chemical constitution of 
any mineralogical species or group, without subjecting to careful 
analysis, not one or two specimens only, but many, from differ¬ 
ent localities and forming different varieties. It is only thus 
that we can obtain the character, common to the whole group, 
which may be taken as its type or formula. 
In expressing the constitution of bodies, many chemists 
have found it necessary to call in the aid of notation; and the 
algebraical system introduced by Berzelius is now pretty gene¬ 
rally diffused, though modified in parts, by some of his follow¬ 
ers, as, for instance, Beudant. Such a notation is convenient, 
I conceive, in other parts of chemistry ; but it is indispensable 
in mineralogy, where the composition of bodies is often much too 
complex to be intelligibly expressed by the resources of the 
language of modern chemistry. The doctrine of isomorphism 
gives us an additional reason for the employment of such a nota¬ 
tion ; for the constitution of an isomorphous group can be most 
conveniently expressed by means of a formula in which one of 
the letters is subject to be replaced by others indicating the 
vicarious ingredients. 
I say nothing here of the merits or defects of different sy¬ 
stems of chemical notation ; for though I cannot but think it 
