i6o Dr. Wollaston on the primitive Crystals of 
to three substances very different in their composition, to car- 
bonate of lime, to magnesian carbonate of lime (or bitter-spar) 
and to carbonate of iron. 
It has been objected to Mons. Hauy, that according to his 
method identity of form should be accompanied by identity of 
composition, unless the form were one of the common regular 
solids. For though in that case any geometrician would readily 
admit it to be very probable, that many different substances 
might concur in assuming the same form of cube, of octa- 
hedron, or of dodecahedron, &c. there does not appear a cor- 
responding probability that any two dissimilar substances 
would assume the same form of a particular rhomboid of 105° 
and a few minutes, to which no such geometric regularity or 
peculiar simplicity can be ascribed. 
But though so accurate a correspondence, as has been 
hitherto supposed to exist in the measures of the three carbo- 
nates above-mentioned, might be justly considered as highly 
improbable, no degree of improbability whatever attaches to 
the supposition, that their angles approach each other by some 
difference, so small as hitherto to have escaped detection. 
And this in fact I find to be the case. 
Since the angles observable i n fractures of crystalline sub- 
stances are subject to vary a little at different surfaces, and 
even in different parts of the same surface (as is evident from 
the confused image seen by reflection from them), I shall not 
at present undertake to determine the angles of these bodies 
to less than five minutes of a degree. This, indeed, is the 
smallest division of the goniometer that I usually employ, as 
I purposely decline giving so much time to these inquiries, as 
would be requisite for attempting to arrive at greater precision. 
