TRANSACTIONS OF THE PHARMACEUTICAL SOCIETY. 357 
stream of water to run through it almost as fast as a similar stream falls in air. 
A piece of such flannel was fitted into a funnel-shaped vessel having a capacity 
of ten ounces; it allowed of the passage of half a pint of water in nearly the 
same number of seconds, as when there was no flannel. Similar experiments 
might have been performed with other filtering media (unsized paper, unsized 
calico, cotton-wool,gun-cotton,linen,tow,flannel, woollen cloth, felt, sponge, char¬ 
coal, sand, gravel, powdered glass, porous stone, biscuit-earthenware, asbestos, and 
the u filtering-powders,” such as fuller’s earth, used for the special purpose of 
decreasing the porosity and therefore increasing the resistance of other media), 
and thus, if the same area of filtering surface, and a similar quantity of water 
were used in every case, the time occupied in the transit of the water become a 
measure of the resistance offered by each medium. Such a table of the rate at 
which fluids pass through different kinds of organic and inorganic filtering ma¬ 
terials would, however, afford us but little more knowledge than experience 
gives already. It would do no more than vaguely indicate the porosity of the 
media; and even that only for the particular pieces of paper, cloth, etc., used 
during the passage of the particular liquid operated on. For the rapidity of fil¬ 
tration through any one given medium will greatly depend on the mobility or 
viscosity, etc., of the gravitating fluid. Again, even the same piece of filtering 
medium, especially such media as black woollen cloth, will, I find, retard the pas¬ 
sage of pure water to a considerably greater extent after it has been in use some 
hours than it did at first: this is doubtless due to the swelling of the material 
and consequent diminution in the size of its pores. Then the pores of a filter 
will, in practice, generally become choked to an unknown extent by the solid 
matter of the filtering mixture. Indeed, not unfrequently the filtering medium 
with which filtration is commenced is known to be insufficient to effect the ope¬ 
ration contemplated, success depending on the deposition of solid matter on the 
medium: in this case the filtrate is at first turbid, and only becomes clear on 
being passed and repassed through the medium. When this occurs the filtering- 
medium is no longer the mere paper, cloth, etc., that was first used, but com¬ 
posed of a mixture of that material with an unknown quantity of the solid 
matter deposited. Rapidity of filtration is also influenced by the form given to 
the filtering medium and to the vessel in which that medium is enclosed. 
Aids to Filtration. 
I have said that the nature of the operation of filtration is nearly identical in 
all cases. It is not quite so. In our kitchens, and, indeed, in the shops of those 
chemists and druggists (and there are too many) who make few if any of the prepa¬ 
rations they sell, filtration is never extended beyond the paper cone or the jelly- 
bag. And even in the busiest laboratory the operation is only occasionally ex¬ 
tended beyond that limit. Hence the nature of filtration is in nearly all cases 
equally simple, and scarcely needs explanatory comment or experimental inves¬ 
tigation ; but when mere gravitation is insufficient to effect the object contem¬ 
plated, or to effect it in a reasonable length of time, then additional force must 
be employed to drive the liquid portion of a mixture through the medium de¬ 
signed to receive the solid portion. The mixture is therefore put into a proper 
receptacle, and the whole subjected to ( a ) hand-pressure , (Jb) lever- or screw - 
pressure , (c) hydraulic pressure , (dd) atmospheric pressure , (e) hydrodynamic 
force , or to a combination of these forms of pressure. The elucidation of the 
nature of the operation of filtration in which gravitation is aided by one form of 
the last-mentioned of these means, was the object I originally had in view in 
undertaking some of the experiments mentioned in this paper. The result con¬ 
vinced me that the general notions concerning the nature of the action of some 
of the other means, and of other forms of that means, were erroneous. T there¬ 
fore determined to write a paper embracing the theoretical aspect of the whole 
