362 
TRANSACTIONS OP THE PHARMACEUTICAL SOCIETY. 
the pressure exerted by a few ounces of liquid to the operation of filtration must 
have occurred to many minds. Eeplace the bottom of the barrel just mentioned 
by a filtering medium, tie a piece of filter-cloth, etc., over the open bottom of 
such a barrel, and there is what would most naturally be termed a hydrostatic 
filter at once. Such an arrangement was proposed for the filtration of oil, about 
the year 1828, by Mr. J. Robison, at the Society of Arts (Trans. Soc.’ Arts, 
xlvii.-xlviii. p. 162). Mr. Robison drew, or rather forced, off the oil through 
a filter (placed in the upper part of the barrel, thus supplementing the advan¬ 
tages of repose and subsidence) by a column of water in a tube reaching to the 
bottom of the barrel, and of u a height sufficient to give the requisite hydrostatic 
pressure.’'' And a pressure approaching to hydrostatic pressure Mr. Robison 
certainly had in the upper part of the barrel, when his oil filtered through the 
medium in drops or in a very thin stream ; and he would doubtless nearly double 
the rate of flow on doubling the height of the column of water, and so on, as 
already explained. But if the oil flowed in a fairly full stream from the filter 
when the column of water was at a height of, say, a foot above the orifice of out¬ 
flow, then the amount of oil obtained in a given time would not be doubled 
until the column approached four feet in length ; the effect would now be, even 
apparently, mainly due to hydrodynamic laws, the amount obtained be more 
directly in proportion to the square root of the length of the column instead of 
to the simple length. Such a filter is, therefore, obviously more correctly 
termed a hydrodynamic filter. _ To speak of it as hydrostatic conveys a wrong 
impression of its action, and induces in the mind an erroneous estimate of its 
powers.. Probably this kind of filter will now always retain the name of “ hydro¬ 
static ; ’ but the fact should be borne in mind that the legitimate claims 
it has to the name rest in the form of the instrument, and in the way the hy¬ 
drodynamic effect of the column is conveyed to every portion, of similar area, of 
the filtering medium, rather than in the effect produced. 
Twelve years before Mr. Robison published an account of the oil-filter just 
decsribed, Count Real proposed an apparatus termed a “filter press,” the figure 
of which in vol. ii. of the ‘Journal de Pharmacie,’ p. 192 (1816), forcibly re¬ 
minded me of the barrel and long vertical tube I have already alluded to. It 
v/as simply a tin box, over the open bottom of which the filtering medium was 
stretched, and into the lid of which a long tube was firmly fastened. Count 
Real considered that such materials as tea, coffee, cinchona bark, and the ingre¬ 
dients for tinctures and extracts generally, would be exhausted of their soluble 
principles more effectually if exposed in iiis apparatus to the pressure produced 
by the hydrostatic multiplication of the weight of the exhausting fluid in the 
and hence give only half the pressure, but then that pressure is exerted on every half- 
inch of surface, and so the effect on every inch will be the same as before. It is the height, 
not the thickness, of the column that effects the result. Whatever the pressure is at a 
given length of column, it is doubled at twice the length, tripled at thrice the length, 
quadrupled at tour times the length, and so on in proportions exactly parallel with the 
simple distance. 
* This was filtration per ascensum, and is in use to the present day. With regard, how¬ 
ever, to the name, given to this method of filtration, it is important to observe, that the 
ascent of the filtering-liquid is still produced by gravitation, produced by the descent of the 
liquid m the long column. The name is good enough for indicating the act performed, but 
should be per descensum, if the nature of that action were desired to be indicated. The 
modification of this filter devised by Mr. W. 11. TV amor, of Philadelphia (see Parrish’s 
‘ ? r ?' c ^ ica l ' Pharmacy,’ 3rd edit. p. 101), is stated to afford a barrel of oil daily. The cause 
of the advantage of these filters, through which liquids pass from below upwards, is, that 
s0 . matters settle.away from, rather than on, the medium, and hence the hydrodynamic 
action oi the machine is maintained. When the current is in the opposite direction, the 
medium chokes, hydrodynamic action becomes more and more feeble, the liquid passes in 
drops smw ly and more slowly, succeeding each other, hydrostatic action has now nearly full 
possession of the field, and the instrument is, to a corresponding extent, useless. 
