ON DIMORPHOUS BODIES. 165 
6. It appears, therefore, that the crystalline form of a body 
is not only not a specific character, but that it is not even a 
constant character. It might also appear at first sight that 
this new result of observation would materially weaken the evi- 
dence in favour of isomorphism; that though two bodies (A 
and B) do assume the same form, or replace each other in cer- 
tain circumstances, yet since one of them (A) is capable of 
assuming two incompatible forms, they may not in all cir- 
cumstances either assume the same form or be capable of mutual 
replacement. 
7. A further observation, however, though it does not obviate 
entirely, as we shall afterwards have occasion to remark, the neces- 
sity of attending to this argument, yet establishes a beautiful con- 
nection between dimorphous and isomorphous bodies, and points 
to some more general law, probably of molecular arrangement, 
by which both classes of phenomena are regulated and linked 
together. Certain groups of isomorphous bodies have been met 
with, each individual of which groups is dimorphous or capable of 
assuming two incompatible forms (A and B), yet in their second 
form (B), as in their first (A), they are stillisomorphous. Thus 
carbonate of lime and nitrate of potash are both dimorphous, 
and one of the forms of nitre is isomorphous with calc spar, the 
other with arragonite, which are the two forms of carbonate of 
lime. Such groups have been distinguished by the term ésodi- 
morphous. All the known groups of this kind will be inserted 
in a subsequent part of this report (16). 
8. The principle of dimorphism thus recognised, is one of 
great interest in the present state of chemical physics. Con- 
‘nected on the one hand with the crystalline doctrine of isomor- 
_ phism, and on the other, as we shall hereafter see, with the 
‘chemical doctrine of isomerism, it may be regarded as standing be- 
tween the two, and as likely to throw light on the cause of both. 
9. The case of dimorphism, which was earliest known to che- 
mists and mineralogists, is presented by carbonate of lime in 
the two incompatible forms of arragonite and calc spar. Stro- 
meyer attempted to account for the difference between these 
two minerals by showing that arragonite always contained car- 
bonate of strontian to the amount of from 4 to 4 per cent., and 
from 7 to 4 per cent. of water*; aud the presence of these sub- 
* Untersuchung tiber die Mischung der Mineralkérper und anderer damit 
verwandten Substanzen. Gottingen, 1821. In this work are ten analyses of 
arragonites, undertaken in confirmation of his previously published opinion, 
which had been controverted. Great credit was due to Stromeyer for his 
beautiful analyses, but there is now no reason to believe that either strontia or 
water are necessary constituents of arragonite. 
