356 TRANSACTION’S OF THE PHARMACEUTICAL SOCIETY. 
workers in such centres of industry but few, perhaps, are so tied to their post as 
those who follow the calling of Pharmacy. Yet even the chemist and druggist 
may find nature’s laws as truly and beautifully governing the chemical changes 
he induces, and the physical operations he performs, as they do the springing 
blade, the ripening fruit, the lightning’s flash, or the course of the sun itself. 
He may not have much opportunity of finding “books in the running brooks,” 
yet in the flow of water from his filter he may discover laws identical with those 
which produce the cool spring in the valley or the rushing torrent of the moun¬ 
tain side. Let us, at this time, go no further than the one simple operation I 
have just alluded to; we may find but little to interest us in passing a few 
ounces of water or other liquid through a paper cone, but when we come to ope¬ 
rate on a larger scale, and, as aids to rapidity of filtration, use those various 
appliances which have been suggested from time to time during the past fifty 
years, we shall find ground for much thoughtful contemplation, and some room 
for experimental inquiry. The result will, I hope, be useful and interesting not 
only to the pharmaceutist, but to the hydraulic engineer and to physicists 
generally. 
Introduction. 
The nature of the operation of filtration, as usually conducted, is so simple 
that but little has been or need be published concerning it. The variety of cir¬ 
cumstances under which filtration is conducted, as well in social as in commercial 
life, has given rise to many contrivances for effecting the operation; filtering 
materials are numerous, the forms of the vessels designed to hold the materials 
scarcely less numerous, and the arrangements to facilitate and perpetuate filtra¬ 
tion many and ingenious; but the nature of the operation, as distinguished from 
the operation itself, is the same, or nearly so, under all ordinary circumstances. 
Indeed, its nature is identical with that of some operations which, conventionally, 
are quite distinct from filtration, and which are always spoken of by other 
names. In the process termed sifting we have the same action occurring as in 
filtration ; it might, in fact, be called “ dry-filtration.”* In the netting of fish 
we also have the same action. Again, the operation of “ straining ” even still 
more closely resembles that of filtration. The nature, then,, of the operations 
conducted with filters, strainers, sieves or nets is identical; the operations them¬ 
selves quite distinct, and, very properly, called by different names. What I 
wish to speak of now is not the operations themselves but their nature, that is, 
the laws which regulate their action, especially in respect to filters. 
Nature of the Operation of Filtration. 
In filtration, straining, and net-fishing we desire to separate certain solid 
matters from certain liquids ; and in sifting we desire to separate solid bodies of 
one size from solid bodies of another size. How do we proceed? As follows. 
\Ve place our mixed materials under the influence of gravitation, interposing in 
their path a medium (called a filter, a strainer, a net, or a sieve, according to 
circumstances) which arrests the gravitation of one portion of the materials but 
does not interfere, or only temporarily impede, the gravitation of the other por¬ 
tion. This, then, is the general nature of the operations as they are usually 
conducted, and it is exceedingly simple. 
Resistance of Filtering Media. 
„ The extent to which a medium impedes gravitation depends upon the degree 
of porosity of that medium. A fishing-net even of unusual smallness of mesh 
cannot, obviously, retard gravitation of water to any extent that can be appre¬ 
ciated. Indeed, I find that one thickness of common coarse flannel allows a 
* Similarly u filtration ” might be called “ wet-sifting.” 
