various important subjects in Mineralogy. $$ 
position of a mineral as its substance, in so far at least as our 
present information goes in chemistry, of which, in fact, it can- 
not be said, that, at some future period, something may not be 
discovered to explain or modify the results. The carbonate of 
lime appears in forms belonging to the rhombohedral and pris- 
matic systems ; the sulphuret of iron in forms of the prismatic 
and tessular systems ; nay, a simple substance, sulphur , has 
been discovered in the forms of the prismatic and hemi-prisma- 
tic systems. But if, by the word substance , we mean the natu- 
ral-historical species, then this is no longer true. The rhombo- 
hedral lime-haloide (calcspar) never appears in prismatic forms, 
nor the prismatic lime-haloide (arragonite) in rhombohedral ones; 
hexahedral iron-pyrites never affects prismatic forms, nor prisma- 
tic iron-pyrites such as belong to the tessular system. Nor can the 
incompatible forms of the varieties of sulphur be considered as 
occurring in the varieties of one and the same natural-historical 
species, even although they should exactly agree in their remain- 
ing properties (which, however, is not at present known to be the 
case), for the very reason that their forms are incompatible. 
It may be asked, however, Whether the circumstance of the 
forms being incompatible is a sure criterion of the difference of 
two Species ? The demonstration of propositions like this, in 
every science that is altogether dependent upon experience, must 
necessarily go along with experience. The laws of combination 
require that every simple form belonging to one and the same 
species, not excepting the fundamental form, should be capable 
of appearing in every individual of the species, whatever kind 
and number of forms it may already possess, or that at least it 
be possible to conceive this to be the case, according to certain 
geometrical constructions. If, therefore, two incompatible forms 
were to belong to one and the same species, they should appear 
at the same time in the same individual, which, therefore, must 
then be capable of containing even two different fundamental 
forms at once ; a mode of demonstration which may be compared 
to the reductio ad absurdum in geometry. On the contrary, it 
is a matter demonstrated by general experience, that, in every 
well determined species, the simple forms belonging to its series 
of crystallization, appear together in the most diversified combi- 
nations in one and the same individual, but that in forming 
