130 MOHR AND REDWOOD'S PRACTICAL PHARMACY. 
of the current will be thus broken, and the operator enabled 
to pour in the whole quantity of spirit required, without 
affecting the position of the solid ingredients. The cover 
is then to be put on, and a little water poured into the 
groove, so as to render the joint air-tight. The spirit will 
percolate slowly through the different strata of dry ingre- 
dients until it reaches the bottom, then, passing under the 
tube C, it will ultimately drop down, through the tube D, 
into the receiver, while, at the same time, an equal volume 
of air passes up through the tubes to supply its place in the 
upper vessel, and equalize the pressure. Thus arranged, 
the apparatus is perfectly air-tight, so that no loss from eva- 
poration can occur, however long the process may be in 
operation. The form and size of the apparatus, also, are 
such as to admit of the solid ingredients being properly 
packed in the vessel A, upon the skilful performance of 
which the success of the process principally depends. In 
most cases it is desirable to use these ingredients in fine 
powder, and, in some instances, they may be advantageously 
mixed with coarse sand, which, by separating the particles, 
will facilitate the percolation of the liquid. If the ingre- 
dients be properly packed, the spirit, when first added, will 
be slowly absorbed by the powder, and will descend 
equally on every side, forming a line round the circum- 
ference of the vessel, which will gradually pass downwards 
until the tincture begins to flow through the tube D. Some 
experience and judgment are required in regulating the de- 
gree of pressure to be applied in packing the powders, or, if 
necessary, mixing them with sand, so as to insure a regular 
and continued percolation ; and, as differences in the nature 
of the substances operated upon, the degree of comminution 
to which they have been subjected, and the form and size 
of the apparatus employed, so far influence the results as 
to occasion the necessity for some modifications in the 
method of operating, no precise instructions can be given 
that would be applicable to all cases. 
