TRANSACTIONS OP THE PHARMACEUTICAL SOCIETY. 
361 
static power which is exerted only in producing the filtering effect. I shall 
however, refer again to this matter presently. Let us for the present consider 
the nature of the action when the filtering-liquid passes in a fair stream, as it 
nearly always does m a practical filter. The amount of filtered liquid then ob¬ 
tained approaches more nearly to an amount which is in proportion to the 
square root of the length of the column of liquid in the instrument,-a given 
amount being obtained from a one-foot column, that amount is only doubled 
when the length of the column approaches four feet. With a four-feet column 
only rather more than twice as much filtered liquid is obtained instead of 
neai y four times,, as was the case when the liquid passed through in drops. 
And this result might be expected. For, according to the discoveries of Torri¬ 
celli, liquids escaping from apertures in the bottom of any vessel obey the ordi- 
nary iaws of gravitation, and flow at a rate directly parallel with the square 
root of the instance from the aperture to the level of the liquid. At an aper¬ 
ture four feet below the surface of water in a vessel, the velocity of the outflow¬ 
ing liquid will be twice as fast as when the surface is onlv one foot above the 
aperture, three times as fast at a depth of nine feet, four"times at sixteen feet 
and so on. If now such an aperture be covered by a piece of netting, the rate 
ox outflow will still be much the same as before, because the netting presents 
very little resistance. If the netting be replaced by one thickness of coarse 
flannel, the rate of motion will still, according to my experiments, be nearly pa¬ 
rallel with the squares of the distances from the orifice to the surface of the 
water With black cloth about two and a half times as much was obtained 
from the vessel I used, when the column of liquid was four feet long, as when it 
v r as one foot long, and so on ; until, on closing the aperture with tightly- 
packed sponge, nearly four times as much was obtained from a four-feet as from 
a one-foot column. The final experiment could only be to so block up the 
aperture as that the pressure on the plug, now no longer a filtering medium, 
should be exactly four times as great with a four-feet column as with a one-foot 
column of liquid. These experiments and considerations show that any filter 
in which gravitation is aided by the use of a long column of liquid above the fil¬ 
tering medium must be considered to be a hydrodynamic filter, rather than a 
hydrostatic, one,—a filter whose useful action depends chiefly on the ordinary 
law of liquids in motion • a filter whose useful effect is in direct proportion 
to the extent to which the hydrodynamic law of acceleration of motion 
manifests itself, and in inverse proportion to the extent to which the hydrostatic 
law relating to the amount of pressure at the base of a still column of licmid 
manifests itself. 1 
The filtering arrangement which has commonly, but wrongly, been termed 
the hydrostatic filter ” has probably become to be so called because similar in 
form to the vessels used by lecturers, or figured in books, to demonstrate the 
truth of the hydrostatic laws previously referred to. A common barrel stand¬ 
ing on one of its heads, and having a long vertical tube screwed tightly into its 
other head, is one form of such vessels. It is ’well known that such a barrel, full 
of liquid, may be burst by simply pouring into the vertical tube a few pounds, 
01 even, if the tube is very narrow, a few ounces of water. The reason of this 
is obvious. If the area of the tube is, say, the fiftieth of an inch, then the weight 
of the few ounces of water in the tube is exerted on every fiftieth of an inch 
inside the barrel, making a total pressure of many thousands of pounds, which 
of course the barrel cannot bear.* To adapt this hydrostatic multiplication of 
* Ifc will not he out of place to here remind the pharmaceutist, that the thickness of 
the vertical column of liquid has nothing to do with the effect produced: a colvtmn of a 
given height and an inch in area, will produce its effect on every inch of surface, a column 
of the same height and only half an inch in area, will, of course, only wehdi half as much 
VOL. VII. ° 9 B 
