1 
224 SELECTED ARTICLES. 
When leaves, flowers, &c. are operated upon, after having 
dried them, so as to have become friable, they are to be rubbed 
through a wire sieve containing fifteen meshes to the inch; 
any nerves, &c. that remain are to be cut and pounded in a 
mortar or passed through a hand mill. It is difficult to lay 
down any rules as to the fineness of different powders. Mucous 
substances should be in coarser particles than others, and where 
alcohol, and more especially ether, is employed, fine powders 
are the best. 
It is equally difficult to lay down any rule as to the degree 
of firmness with which the powder is to be packed in the 
cylinder. Experience alone can teach this, and it can only 
be acquired by practice; for each substance seems to require 
some difference in its treatment, and the fineness of the pow- 
der, as well as the quantity operated upon, also exercise an in- 
fluence in this respect. Thence it results that though this 
operation is simple in theory, it becomes difficult in ac- 
tual practice, requiring much skill and habitude of manipula- 
tion. 
The fluid is to be poured over the powder so as to form a 
stratum over the whole surface; it then penetrates in an equal 
manner, chasing the atmospheric air contained in the powder 
before it; the surface of the powder is to be kept covered during 
the whole time the operation lasts. When it is perceived that 
the fluid runs through very rapidly it is a proof that the pow- 
der is not packed tight enough ; it must therefore be compressed 
by means of the upper diaphragm. On this account it should 
be of metal, rather than of paper or linen. The too rapid es- 
cape of the fluid may also be arrested, as advised by M. Dausse, 
by partially closing the stop-cock, and only permitting the li- 
quid to escape in a small stream, but a proper packing of the 
powder in the first instance is the best means of obtaining con- 
centrated infusions, &c. and of preventing the different strata 
from becoming mixed. 
MM. Boullay have advised the powders to be used in a dry 
state; M. Dausse gives the same direction for compact substan- 
ces which do not augment sensibly in bulk on the addition of 
water; a slight enlargement in this case is an advantage, as it 
