358 TRANSACTIONS OF THE PHARMACEUTICAL SOCIETY. 
question, firstly, for the benefit of those who, like myself, pursue natural truth 
con amore , and, secondly, in the hope that those who, like myself, have occa¬ 
sionally to use aids to filtration, might do so with the maximum of economy and 
efficiency. 
(a) Filtration in which Gravitation is aided by Hand-pressure. —When a 
closed filter-bag containing, a mixture to be filtered is pressed between the hands, 
the muscular force of the operator aids the natural gravitating tendency of the 
fluid portion of the mixture to an extent dependent on the amount of muscular 
force exerted. Moreover, that pressure, though not the full amount of it, is 
also exerted upon every similar sized portion of the whole bag. For instance, if 
the two hands cover a space of fifty square inches, and the whole bag presents a 
surface of five hundred square inches, then the total amount of muscular force 
exerted upon the whole bag will be a quantity more or less approaching ten 
times the amount of force actually exerted by the two hands. This conveyance 
and multiplication of force is in obedience to well-known laws of hydrostatics, 
namely, that “ pressure is transmitted by fluids in all directions : the transmitted 
pressure is equal in every portion of the fluid : it is proportional to the area of 
the surface pressed.” Notice, however, that the aid which gravitation receives 
in hand-pressure on a filter-bag is correctly spoken of as muscular aid, not hydro¬ 
static aid; for hydrostatics is a branch of science that relates to or embraces the 
properties which fluids possess when they are in a state of rest, whereas the very 
essence of the act of filtration is the motion of the fluid; filtration involves 
motion ; where there is no motion there is no filtration. Filtration is therefore 
a property of fluids more properly embraced by hydrodynamics than by hydro¬ 
statics. In hand-pressure the conveyance of the muscular force of the hands to 
the parts of the bag not touched by the hands, is a hydrostatical matter, not so 
the force itself or the filtration which results. The fact is that owing to the 
escape of liquid from the pores of such a pressed bag, hydrostatic laws can never 
come into full operation inside the bag. When the flow is one of drops, the 
pressure inside approaches pure hydrostatic pressure, and when the flow ceases 
altogether the pressure inside is pure hydrostatic pressure. But in proportion 
as the flow is more and more rapid, so the amount of pressure inside the pressed 
bag is less and less hydrostatic, and the extent to which the gravitation of the 
liquid is aided, becomes more and more in accordance with the rate at which the 
motion of all gravitating bodies is accelerated. 
{Jj) Filtration in which Gravitation is aided by Lever- or Screw-pressure. 
—-When a closed filter-bag containing a mixture to be filtered is pressed in an 
instrument similar to that familiar one termed a “lemon-squeezer,” or in the 
scarcely less familiar apparatus known as a “ screw press,” the force aiding 
gravitation is still muscular force. In this case, however, the force is conveyed 
and multiplied first by a lever or levers to the bag, and then to every part of 
the filtering mixture by the hydrostatic means just described. As, however, the 
aid is more powerful than in mere hand-pressure, it is conveniently, for distinc¬ 
tion, spoken of as a lever- or screw-pressure. 
(c) Filtration in which Gravitation is aided by Hydraulic-pressure .— 
When a closed filter-bag containing a mixture to be filtered is pressed in 
Bramah’s well-known hydraulic press, the force aiding gravitation is still mus¬ 
cular force. But in this case the force is conveyed and multiplied by a lever 
whose point of resistance is a column of water ; thence, through that column, to 
a column of much larger area contained in a strong cylinder having a moveable 
piston ; against the under surface of the piston the augmented force, now again 
many more times augmented by this hydrostatic arrangement, acts with such 
effect as to drive the piston forward with immense power ; and thus, more 
strongly than in any other arrangement, pressure is exerted upon any filter-bag 
which may be placed between the end of the piston and any rigid and immove- 
