I’S Mr Brooke on Crystallisation. 
of surface. Particular planes of some crystals are frequently 
found to be marked by some peculiarity by which they may be 
distinguished from the others ; and this character is common- 
ly found on the corresponding planes of all other crystals on 
which they occur, belonging to the same species of mineral. 
Thus the planes which truncate the primary terminal edges 
of the crystals of carbonate of lime, are generally observed to 
be striated parallel to their edges of combination, and the ter- 
minal planes of hexagonal prisms of that substance are generally 
opaque and very dull. 
Among artificial crystals, there will frequently be a diife- 
rence in the characters of the planes, according to the cir- 
cumstances under which they have been produced. But this 
difference is not constant in all substances. Sometimes the 
most perfect planes appear on those crystals which have been 
slowly formed ; but I have also frequently observed them on 
such as have been deposited on cooling, from a solution which 
bad been evaporated at a moderate heat to the point of crystal- 
lisation. Some of the double salts are difficult to be produced 
in good crystals by any other means. I have repeatedly tried to 
obtain, by slow evaporation, crystals with bright planes, of the 
sulphate of copper and zinc, but without success ; yet they 
have been immediately produced by cooling, from a warm satu- 
rated solution of the salt ; and I have observed the planes of 
crystals of sulphate of copper produced in this manner, more 
perfect than others which resulted from slow evaporation. 
It appears from the measurements, which have been taken 
with great care by means of the reflective goniometer, that the 
primary forms of certain salts, whose chemical constitutions 
are essentially different, are not distinguishable from each other 
by the mutual inclination of their planes. 
It is not, however, at all improbable, as I shall afterwards 
more particularly shew, that the variations in the angles and 
comparative dimensions of the primary forms of different sub- 
stances, may be so small, as not to be discoverable by our best 
goniometric instruments, in the hands of our most able observers. 
Thus, sulphate of iron, and sulphate of cobalt, are scarcely if 
at all distinguishable from each other by measurement, and 
there are other salts which present similar apparent accordances 
