4 32 
SECOND REPORT — 1832 . 
slowly, till part of the mass is consolidated, and the remaining 
fluid poured off, crystals are obtained in the form of an oblique 
rhombic prism. These two forms are incompatible, that is, 
are not derivable from one common form. Pure carbon occurs 
in nature in two states, constituting diamond and graphite. The 
former crystallizes in regular octohaedrons ; the latter in six- 
sided plates striated parallel to some of their sides. These forms 
are also incompatible. Sulphur and carbon therefore possess 
two forms, or they are dimorphous (hi$ twice , popQr} shape). 
There are also compound bodies which are capable of as¬ 
suming two forms. Carbonate of lime, in calc-spar and in arra- 
gonite, is a w r ell-known example. Carbonate of lead forms also 
prismatic crystals, but it replaces carbonate of lime in the primi¬ 
tive rhomboid of plumbo-calcite. These two carbonates there¬ 
fore, and probably also their bases, are dimorphous. 
Iron pyrites (bisulphuret of iron) is met with in two incom¬ 
patible forms, constituting the common or cubical, and the 
white or prismatic pyrites of mineralogists. The biphosphate 
of soda likewise crystallizes in two forms, the more common 
being right rhombic prisms, the more unusual rectangular octo¬ 
haedrons, the form of the binarseniate. Both forms have the 
same composition. The sulphate of nickel, the sulphate of mag¬ 
nesia, and the sulphate and seleniate of zinc, assume also two 
forms, but they appear to contain unlike quantities of water *. 
In the mineral kingdom also compounds are met with of like 
composition, but having incompatible forms. Thus the garnet 
and idocrase are represented by the same chemical formula, 
but their forms are irreconcileable ; and there are many in¬ 
stances of minerals occurring in what are called pseudomor- 
phous or parasitic forms, supposed to be mere casts of the 
forms of other minerals, some of which may yet prove to be 
really capable of assuming two forms, or to be dimorphous. 
List of Dimorphous Bodies. 
I. Simple Substances. 
Sulphur. 
Carbon. 
II. Compounds of two Elements. 
Bisulphuret of iron. 
III. Compounds of three Elements. 
Carbonate of lime. 
-—-lead. 
Biphosphate of soda. 
IV. Compounds of four Elements. 
Garnet, or idocrase. 
Cause of Dimorphism .—It would lead us into too great 
length to enter into a detailed examination of the different 
* Mitscherlich, PoggendorfWwwafew, vol. xi. p.323; Ann. de Chim. 1828. 
