REPORT ON CHEMTSTRY. 
433 
causes to which, under different circumstances, dimorphism 
may be traced. In the case of elementary substances, like sul¬ 
phur, if we suppose the atoms spherical, then different modes 
of grouping these atoms may produce two or more resultant 
forms. If the atoms have a definite and constant geometrical 
form, they may build up unlike figures, by uniting in the di¬ 
rection of their unlike axes. In binary compounds, as in the 
iron pyrites, sulphur, uniting in each of its two forms with an 
atom of iron, would produce two forms of the sulphuret, such 
as we find in nature ; and thus the dimorphism of the com¬ 
pound would be due to that of one of its elements. Or if 
neither atom be dimorphous, the two forms of the compound 
may arise, as in simple substances, either from a different mode 
of grouping the atoms, or from their uniting by different faces. 
By similar suppositions we may form an idea how ternary and 
more complex compounds might be capable of assuming, not 
two merely, but several forms. There is indeed no good theo¬ 
retical or physical reason why even elementary bodies may not 
be tri- or fefra££-morphous. 
Iso dimorphism .—There are certain of the dimorphous bodies 
contained in the list given above, the two forms of which are 
isomorphous each with each. These are the carbonates of 
lime and of lead. In one of their forms they give right rhom¬ 
bic prisms, having angles which approach very nearly to each 
other, as is stated in page 427; in the other they give a rhom¬ 
boid of 105° o', the primitive form of calc-spar, I have already 
mentioned the interesting minerals from the constitution of 
which we deduce this important fact. Carbonate of lime oc¬ 
curs prismatic in the arragonite, while the carbonate of lead is 
rhomboidal in the plumbo-calcite. As these compounds are at 
once dimorphous and isomorphous, I have called them isodi - 
morphous compounds ; and if from such compounds we were 
justified in drawing any conclusion regarding the forms of their 
bases, we might infer that the two bases, lime and oxide of lead, 
and, carrying the analysis still a step further, that calcium and 
lead are also isodimorphous. These bodies are members of 
the two isomorphous groups VIII. and IX., given in page 426. 
It is probable that we shall soon be able to embrace the whole 
of these two groups in one large isodimorphous group. 
It is a fact not unworthy of observation, that nearly all the 
instances of plesiomorphism we yet know, occur in certain com¬ 
pounds of the two bases now mentioned, and of two or three 
others, which are probably possessed of the same property, as 
they belong to one or other of the two isomorphous groups re¬ 
ferred to. 
2 E 
