14 Mr Brooke on Crystallisation. 
It is true, that the crystals of these three substances belong 
to the same class of primary forms ; but they differ too much in 
the inclination of their planes, to allow us to regard the forms as 
identical, and to ascribe the differences either to errors of mea- 
surement, or to imperfection of the measured crystals : For the 
crystals of these substances are among the most perfect and bril- 
liant that occur, and they have been measured often, and with 
corresponding results. 
Their primary forms are right rhombic prisms^ and the planes 
M incline on M' at the following angles : 
Sulphate of Lead, 
Barytes, 101 .42 
Strontian, 104 .00 
These, we may remark, are natural crystals, and they evi- 
dently do not appear to support our author’s theory. 
Nor do the artificial salts of those three substances in all cases 
better accord with it. The nitrates certainly agree in haying 
for their common form a regular octahedron^ but their acetates 
differ even more than their sulphates. The primary form of 
the acetates is a right oblique-angled prism ^ the planes M and 
T inclining to each other as follows : 
Acetate of Lead, 109®,32 
Barytes, 113 .12 
Strontian, 96 .10 
This theory is not better supported by the carbonates of lime, 
of iron, and of zinc, which are stated to be isomorphous. The 
primary forms of these substances are rhomboids, and the inclina- 
tion of P on P' has been ascertained to be as follows : 
Carbonate of Lime, 105°.5 
Iron, 107 .0 
Zinc, 107 .40 
Under these circumstances it appears, that the theory of iso- 
morphous atoms cannot be sustained ; and I am told, that it 
has, upon more mature consideration, been abandoned by its 
author, who will probably admit, that apparently similar forms 
belonging to substances which differ in composition, do really 
differ from each other in measurement, by some small quantity 
which the goniometer does not detect. 
2 
