Mr Brooke on CrystalUsation. IS 
ill their forms and angles. I do not, however, know, that a 
rhomboid of 90° and 1" may not exist, or even one approach- 
ing still more nearly to the cube. But I should expect to find 
such a crystal affected by the modifications of the rhomboid, and 
not by those of the cube ; and this would constitute a distinc- 
tive character which it could not derive from measurement. 
This view of the subject appears to be supported by the 
number of observed differences of measurement among crystals, 
which differ in their composition, belonging to all the classes of pri- 
mary forms, except the four regular solids ; and by the want 
of coincidences, except in the few cases of rhombic prisms of 60° 
and 120°, if these should really all measure exactly alike. 
The subject has been, however, very differently regarded by an 
able and intelligent chemist, M. Mitscherlich, who has publish- 
ed several papers upon it, which may be found in the Annates 
de Chimie. In his first memoir M. Mitscherlich supposes, that 
these apparently similar forms are really identical ; and that this 
identity is a necessary result of a similarity in their atomic con- 
stitution ; and he also supposes, that wherever this atomic simi- 
larity exists, identity of crystalline form will always be the re- 
sult. 
From the first of this ingenious author’s papers upon the sub- 
ject, it appears, that this similarity of atomic constitution relates 
to the proportions of oxygen contained in the components of 
each of the supposed isomorphous crystals. Thus he says, 
oxygen in phosphorous and arsenious acids, is to that in the 
phosphoric and arsenic acids as 3 to 5. In the biphosphate and 
binarseniate of potash, the oxygen of the potash is to that of the 
acids as 1 to 5, and to that of the water of crystallisation as 
1 to 2. 
Hence, the only difference between these salts consists in the 
radicle of the one being phosphorus, and of the other arsenic ; 
and all salts which differ only in this manner, are said to present 
identical crystalline forms. 
But the instances which Mr Mitscherlich has adduced in 
support of his theory, or we may almost say as its foundation, 
are not in accordance with it. 
He states, that the crystalline forms of the sulphates of lead, 
of harytes, and of strontian, are identical. 
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