9 
Mr Brooke on Crystallisation. 
a glass tube, bent like a syphon, and placed with the curve 
downwards, there be introduced a small portion of mercury, 
not sufficient to close the connection between the two legs ; and 
if to this a solution of the nitrate of silver be added, so as to 
rise in both limbs of the tube ; it is said, that, when the tube is 
placed in the plane of the magnetic meridian, a rapid precipita- 
tion of the arbor Dianse will take place ; but that this process 
will be slow, when the tube is placed in a plane perpendicular 
to the same meridian. I have taken two bent tubes of nearly 
the same size, and have placed nearly equal quantities of mer- 
cury in them, and have added to these nearly equal portions of 
a dilute solution of the nitrate of mercury. I have then placed 
one tube in the plane of the magnetic meridian, and the other 
in a plane perpendicular to it, but have not been able to ob- 
serve any decidedly marked difference in the deposit of silver in 
the two tubes. If the precipitation was more copious in one 
than the other, it was in that which stood in the plane perpen- 
dicular to the magnetic meridian. In this tube also, the-b ranches 
of silver were more copious towards the west, and in the other 
tube towards the south. But, upon reversing the ends of both 
tubes, the branches of silver in each became more prominent in 
the direction of those already most distinctly formed, that is, the 
arborization now increased in the directions of east and north. 
Thus, in this experiment, the precipitation of the silver does not 
appear to have been influenced by the position of the tubes. 
Some salts, of which the sulphate of copper and nickel, and 
the sulphate of copper and zinc, are examples, will deposit much 
larger crystals during the cooling of a smaller quantity of the 
solution, which has been saturated while hot, than from the 
spontaneous evaporation of a much greater volume, at the ac- 
tual temperature of the atmosphere. 
The n.ext branch of our inquiry relates to the forms of crys- 
tals. 
These may be either primary or secondary, simple or com- 
pound, regular or irregular. 
The primary forms very rarely occur among either natural 
or artificial crystals ; nor has experiment yet pointed out any 
means, by which these may, with certainty, be produced. 
Several experiments have been made by Mr Beudant, from 
