534 
PHARMACEUTICAL MEETING. 
filter-bed is to be formed of uniform structure throughout, capable of absorb¬ 
ing and holding the liquid menstruum by, capillary attraction, and permitting 
of slow but uninterrupted percolation equally through every part of the mass, 
under the influence of hydrostatic pressure. 
Although in all cases it is desirable and important, if possible, that the solid 
substances should be finely and equally divided, yet a deviation from these 
conditions is practically necessary in many instances, and the extent to which 
such deviation is required varies not only according to the nature of the sub¬ 
stances under operation, but also according to the form of the percolator, and 
the quantity of material operated upon at once. It is therefore impossible to 
lay down any fixed rules, or to give such verbal instructions as would enable 
an inexperienced operator to perform the process with uniform success. 
All we can say with reference to the required disintegration of the solid 
ingredients is that they should be as finely divided as is compatible with un¬ 
interrupted percolation under the conditions of the process in other respects. 
The degree of comminution may vary according to the nature of the liquid 
menstruum, and according to the quantity put into the percolator. Thus spirit 
will often percolate through a substance in fine powder, while water will not, 
and percolation may take place through a short column of solid materials, 
although it would be stopped by a longer column of the same materials in the 
same state. So the form of the percolator has something to do with the 
conditions that are required in other respects. There are two effects in the 
process, the results of which are especially influenced by the form and size of 
the percolator. These are, first, the swelling of the ingredients under the 
influence of the liquid, and secondly, the reduction in volume of the solid 
ingredients as they become exhausted by the solvent action of the liquid 
menstruum. If a long and perfectly cylindrical percolator be used, and the 
solid ingredients, after being introduced in the dry state, should swell on the 
addition of the liquid, they may become so closely compacted as to prevent 
percolation ; whereas, in a percolator of a conical form, the expanding mass 
rises in the cone without undergoing the same amount of compression. Then 
again, where the solid substances have been packed in the percolator so that 
they shall be sufficiently tight, but not too tight, at the commencement of the 
process, if the ingredients yield much of their substance to the solvent, as, 
for instance, in the case of opium, the mass will necessarily contract in volume, 
and the result of this, in a cylindrical percolator, will be that it will leave the 
sides like a loose cork in the mouth of a bottle, and the liquid will imss there 
instead of percolating through the mass ; whereas, in a percolator of a conical 
form, the contracting mass, under the circumstances indicated, will settle down 
in the cone, and still remain sufficiently tight to prevent the free passage of 
liquid at the sides. These circumstances have led to the adoption in most 
instances of a more or less conical form in the percolator, and the preference 
for this form has also been confirmed by the observation that as the liquid 
passes downwards it aggregates towards the middle of the column, so that near 
the bottom more liquid runs there than at the sides. These facts, however, 
have not sufficed to convince all operators of .the superiority of conical over 
cylindrical percolators, and many still continue to use the latter. The first 
objection I have referred to is sometimes met by wetting the solid ingredients 
and allowing them to swell before putting them into the percolator; but this 
again is not free from objection, for it is more difficult to pack a wet powder 
than a dry one without leaving vacant spaces in the mass, which the liquid 
may afterwards form into channels, through which it will run in preference to 
its percolating through the denser parts. Moreover, it is found that vege¬ 
table powders do not generally swell when wetted with spirit so much as they 
do when water is the menstruum used. Hence some of the most able modern 
