TRANSACTIONS OF THE PHARMACEUTICAL SOCIETY. 
371 
sure at all within the instrument, when we get a rate of flow which is apparently 
as well as actually dynamic. It is for these considerations chiefly that I think we 
should regard filtration in a dynamic aspect. A less strong, though more obvious 
reason, is that useful filtration—that is, rapidity of flow—is in proportion to the 
extent to which apparently as well as actually dynamic laws obtain in filters. 
Secondly, we have been told that pressure-filters have not hitherto proved of 
tne service m Pharmacy that was expected of them; that where they are most 
^eeaecl, namely, for the separation of solid matter in a very minute state of 
division or m a flocculent condition, there they fail, and that a turbid in- 
stead of a clear and bright filtered liquid results. Now, so long as we consider 
pressure-filters to be static instruments, this result must be inexplicable. But 
once realize their dynamic character, and the explanation of the fact would seem 
L0 be ? ls e : 7T a or Particle of solid matter finds itself at the mouth of a 
pore oi a filter ; if that particle were the object of static laws only (aerostatic or 
hydrostatic), there it would remain, resting, so to speak, on the edges of the 
pore ; and tnere it would remain, we will suppose, if the pore were the pore of 
a common filter m a common funnel, the pressure that is above the particle 
being in this case only slightly greater than that below the particle: but now 
greatly increase the pressure on that particle from above, either directly by 
adding pressure, or indirectly by taking pressure from below, then the particle 
is at once shot through the pore, it being compressed if it be a flock, or it itself 
enlarging the fibrous interior of the pore, if the particle being incompressible, 
and if the pore be in paper, cotton, wool, etc. In other words, the force which 
increases the gravitating motion of fluid particles through the pores of a filtering 
medium, increases the gravitating tendency of any solid particles which may be 
resting within or on the edges of those pores. This explanation (and the being 
able to give explanations of facts is a matter of practical value) follows, I think 
from the consideration of our subject. In Mr. Schacht’s pressure-filter, this 
stated objection to the old pressure-filters may possibly not obtain, because the 
pressure can be increased so gradually that the consolidation of the particles of 
solid matter, which are constantly increasing the resistance of the filtering me- 
dium, goes on pari passu with the pressure itself ; in other words, the closeness 
ot tne filtering medium m his instrument increases regularly with the uressure 
instead of spasmodically as in other older instruments. Whether this be so or 
not, can only be determined by experience in the use of his filter. 
Thirdly, apply hand-pressure, lever- or screw-pressure, and hydraulic pres¬ 
sure, curectly to as small a portion of the surface of a filter-bag as possible. The 
flow from a pressed bag, contaiuiug a mixture to be filtered, is less when the 
hands of two persons press it, than when only ©ae is so engaged, if the second 
man places his hands on any exposed part of the bag ; and so on, until the 
whole of the exterior surface of the bag is covered by pressing hands, when the 
flow will cease altogether. Let a second operator place his hands on the hands 
of the first, and so on, and then the additional power will be exerted to the 
most useful extent. So with a “lemon-squeezer ” or “ tincture-press,” etc., let 
as much of the pressed bag be exposed as possible. In a word, let the pressure 
which, exerted on a given area outside, is conveyed to every similar area inside 
the bag be opposed as little as possible by pressure on the corresponding areas 
of the exterior. Of course, the pipe and orifice or other means of outflow°of any 
filtering-liquid must be capable of conveying away a larger quantity of liquid 
than is ever likely to pass through it. 
Fourthly, if, in filtration, pharmaceutists, engineers, and others desire to have 
t ic full benefit which the use of <x long column of liquid, below their filter gives 
them, that column must be perfectly continuous,—there must be no break in it 
caused by the introduction of air from without the instrument, or by the accu¬ 
mulation of air coming out of solution in the water, as we all know it will do 
