414 . Prof, J. P. Cooke on the Variation of Constitution in a 
nine times as much gas as an alloy of 50 per cent., although the 
crystals of the last were fully as definite as those of the first*. 
It is evident from the above facts, that the points corresponding 
to the theoretical composition of the two compounds of zine and 
antimony, are also points of maxima and minima of various pro- 
perties. Now I have no doubt that the same truth will be found 
to hold in the mineral kingdom. In a mineral like tourmaline 
or mica, for example, the specimen having the exact theoretical 
composition may probably be discovered by examining a large 
number of specimens, and discussing their various physical pro- 
perties. All the physical properties may be of value in this 
connexion, such as lustre, hardness, specific gravity, specific 
heat, &e.; and no mechanical rules can be laid down. Much 
must depend on the discretion of the observer ; and im any cases 
such properties will be selected as are best adapted to the cir- 
cumstances of the case. In comparing different crystals of the 
same mineral, it is obviously important to select such as have 
been formed in a different matrix; for it is only with such that 
we should he led to expect great variations of composition. It 
is also evident that the phenomena would be complicated when 
there has been a substitution of isomorphous elements; and until 
the effect of such substitution on the physical properties can be 
traced, it will be necessary to select specimens of as uniform a 
constitution as possible, 
With one other consideration I will close this paper. The 
principle which has been here discussed must modify materially 
our notion of a mineral species. The idea of a mineral species 
has hitherto involved chiefly two distinct characters :—first, a de- 
finite crystalline form; second, a constant general formula; and 
any important variation in either of these characters has been 
regarded as equivalent to a change of species. Rutile and anatase 
are regarded as different species, because their crystalline forms 
are slightly different, although both minerals have identically 
the same constitution; and again, magnetite and Franklinite, 
which have the same form, are regarded as different species, 
because they have a slightly different composition. It is true 
that the actual composition of a mineral may vary very greatly 
by the substitution of isomorphous elements, and yet, if the ge- 
neral formula remains constant, the species may not be changed, 
But the extent to which such substitution can be carried without 
changing the species is not so well settled among mineralogists 
as could be desired, and the same rule is not applied to all species. 
The difference between the varieties of garnet, for example, is as 
great as that between the species magnetite and Franklinite. 
Leaving, however, this point undetermined, all mineralogists 
* See Table in the memoir before cited. 
