mineral species independent of the Phenomena of Isomorphism. 415. 
have agreed that any essential change in the general formula was 
inconsistent with the idea of the same species. The result, how- 
ever, of my investigation is to show that the general formula of 
a mineral species may vary also, or, as I should rather say, the 
general formula is not necessarily the actual formula of each given 
specimen, but only the typical formula of the species towards 
which the mineral tends, and which it would unquestionably 
reach if it could be several times recrystallized. 
According to this view, the general formula represents not the 
actual constitution of the mineral, but only a certain typical com- 
position, which perhaps is never realized with any actual spe- 
cimen. The fact that the composition of a mineral species 
may be modified by the substitution of isomorphous elements, 
was first established by Mitscherlich, and has long been an ad- 
mitted principle in mineralogy. We must now, as I think, still 
further expand our idea of a mineral species, and admit that its 
composition may be modified by an actual variation in the pro- 
portions of its constituents. Thus it is that in mineralogy, as 
in other sciences, we are led to admit the truth of that maxim 
which every advance in true knowledge seems to verify, “ Natura 
non facit saltus.” : 
While the results of my investigations thus serve to render 
the idea of a mineral species less definite than before, I cannot 
but hope that they will tend ultimately to simplify the whole 
subject of mineralogy ; for not only may we expect to reduce the 
number of mineral species, but also, by simplifying the general 
formule of those which remain, to classify the whole with a- 
greater precision than is now possible. To do this, however, 
implies a careful revision of the whole subject-matter of mine- 
ralogy on the principles above given,—a labour of which few can 
appreciate the extent, except those who are familiar with the 
methods of physical research. ‘The work cannot be done by any 
one person; and it is the chief object of the present paper to call 
the attention of mineralogists to the importance of the subject. 
I have not thought it necessary to dwell in this paper on the 
obvious distinction between the phenomena here in consideration, 
and those of isomorphism. It was shown in my previous me- 
moir, that the variation in the composition of the crystals of 
Sb Zn® and Sb Zn? could not be explained by this principle ; and 
the distinction between the two classes of phenomena has been 
still further illustrated by a recent investigation on the crystals 
formed in alloys of copper and zine, made in my laboratory by 
Mr. F. H. Storer. These crystals, which are undoubtedly mix- 
tures of isomorphous elements, give no indications whatever of 
points of typical composition,—thus illustrating not only the 
characters of an isomorphous mixture, but also the distinction 
