2l8 
Dr, Kidd’s observations 
out, the vapour will soon begin to crystallize round the wick 
in very distinct thin transparent laminae. 
This experiment affords one mark of distinction between 
this substance and benzoic acid, and also between it and 
camphor : for under similar circumstances, benzoic acid 
crystallizes in acicular crystals, which are often grouped in 
a stelliform manner ; and camphor crystallizes, or is rather 
congealed, in globular particles having a stalagmitic ap- 
pearance. 
The most usual crystalline form of this substance is a 
rhombic plate, of which the greater angle appears to be 
from ioo° to 105° : crystals at least of that form I have re- 
peatedly obtained from its solutions in water, in alcohol, in 
acetic acid, in the yellow oil described in the last section ; 
and lastly, by melting and very slowly cooling the substance 
itself. Sometimes several of these plates are variously 
grouped together ; sometimes a single plate intersects an- 
other plate at nearly right angles, so that in some points of 
view the compound crystal appears simply cruciform. The 
only distinct modifications which I have observed of the 
common form are a rhomboidal plate, which is very nearly 
rectangular ; and an hexagonal plate : the latter variety may 
be easily traced from the rhombic plate by the incomplete 
developement of the smaller angles of the usual rhomb. 
The following process has been found most successful in 
illustrating the crystallization of this substance. 
If 25 grains of it be dissolved by the assistance of heat 
in half a fluid ounce of alcohol, and the solution be cooled 
slowly in a glass matrass, it will begin to crystallize when 
