ON DRUG-GRINDING. 23 
stones when dry enough to be powdered. The drying is 
generally effected in a room healed to about 120°, by means 
of a stove or steam-pipe. The most common method of 
heating the drying-room appears to be by the use of a 
cockle, that is, a stove having an outer case or jacket, be- 
tween which and the fire-case there is a space for the cir- 
culation of air. A great many different forms are given to 
the cockle, which is sometimes made of iron and sometimes 
of brick ; but it should always be so placed that the fire can 
be fed from the outside of the room, so that the atmosphere 
of the room may not be contaminated with smoke or dust. 
The substances to be dried are spread out on trays, which 
are placed on open frames around the room. 
The drying process being completed, the substance is 
placed on the platform of the mill, in the path of the stone 
runners, where it is ground until sufficiently comminuted 
for the commencement of the next operation, namely, that 
of sifting it. The two operations of grinding and sifting 
are then continued simultaneously, the operator transferring 
a portion of the powder from the mill to one of the drum 
sieves, which latter, with its contents, is put into its appro- 
priate receptacle in the frame of the sifting apparatus, where 
it is subjected to the requisite succussion. The residue, 
which does notjpass through the sieve, is returned to the mill, 
and a fresh portion of powder taken out to be sifted ; and 
this mode of proceeding is continued without suspending the 
motion of any part of the machinery, until the process is 
nearly finished. 
In using the stampers, it is necessary to suspend the 
pounding or stamping while the substance under operation 
is being removed from, and introduced into, the mortar, 
which somewhat retards the process. 
The facility with which drugs are reduced to powder de- 
pends in great measure upon the extent to which they have 
been previously dried. If they be not deprived of their hy- 
grometric water to the greatest extent practicable by ex- 
