SECOND REPORT — 1882 . 
344 
the ingredients of a substance and their quantities, he would be 
unable to tell us what mineral species the substance was. 
We can hardly flatter ourselves, therefore, that we are at pre¬ 
sent close upon the discovery of the nature of the connexion be¬ 
tween chemical constitution and mineral character. In the mean 
time, the chemists have been far from idle in the only road which, 
under such circumstances, offers itself; and a number of excel¬ 
lent analyses of particular substances have been perpetually ac¬ 
cumulating. Still, such labour is naturally and inevitably pursued 
with less energy and connexion than would probably show them¬ 
selves, if the analysers were tempted or rewarded by the pros¬ 
pect of some general law to be extended or verified,—some 
anomalous cases to be included in a well-established analogy. 
Perhaps, however, such prospects are already opening. Mi- 
tscherlich’s Law of Isomorphism, published about twelve years 
ago, promises far more fairly than any previous portion of chemi¬ 
cal knowledge, to relieve chemical mineralogy from its stationary 
and helpless condition. According to this law, the ingredients 
of a given species of minerals are not absolutely fixed as to 
their kind and quantity ; but one ingredient may be replaced by 
an equivalent (not necessarily an equal) portion of some similar 
ingredient,-—generally some elementary body in the same degree 
of oxidation. Thus in amphibole, or in pyroxene, the lime may 
be in part replaced by portions of protoxide of iron, or of man¬ 
ganese, while the form of the crystal and the angle of its cleav¬ 
age planes remain the same;—or in such cases the angles may 
vary slightly, while the other properties remain so far un¬ 
changed as to establish a strong mineralogical connexion in the 
group thus related : so the different kinds of felspar vary only 
by the substitution of one alkali for another: and the carbonates 
of lime, magnesia, protoxide of iron, protoxide of manganese, 
and their mixtures, agree in many respects of form, &c., while 
the angle varies through one or two degrees only. Several 
other such groups might be mentioned ; as garnet; olivine ; the 
carbonates of baryta, strontia, lead, and lime (arragonite); the 
sulphates of the same bases; the sulphates of iron and of co¬ 
balt ;—again, the sulphates of zinc, nickel, magnesia; various 
phosphates and arseniates ; and several other compounds. 
It is obvious from the mere enunciation of such propositions 
that we have here chemical laws of a more general and scientific 
character, than any that can be founded on the analysis of a 
single specimen of each species. And as this principle is of 
the greatest importance to the mineralogist, it is interesting to 
observe that the mineralogist was the first person to perceive 
the necessity of such a principle. Thus Breithaupt, writing in 
1818 (Auswahl der Dresd. Gesellsch. vol. ii. p. 142,) considers 
