366 
TRANSACTIONS OF THE PHARMACEUTICAL SOCIETY. 
Hydrodynamic rate 
4 
77 
... 18 
77 
5 
77 
... 20 
77 
6 
77 
... 22 
77 
7 
77 
. . . 28f 
77 
8 
77 
. . .254 
77 
9 
77 
... 27“ 
77 
Aerostatic proportion. 
Actual rate 
. 36 
77 
24 1 
7 j 
. 45 
77 
• S7| 
77 
. 54 
77 
. 304 
77 
. 63 
37 
. 324 
17 
. 72 
77 
. 354 
17 
. 81 
77 
. 37}- 
77 
Mr. Schacht’s own instrument, fitted with a medium composed of two thick¬ 
nesses of unbleached and unglazed calico, gave to that gentleman and myself the 
following rate of flow with pure water in one minute :— 
Actual rate. 
At 1 foot . 
. . 34 ounces 
2 feet . 
. . 57 ,, 
3 „ . 
. . 73 ,, 
4 „ . 
• . 87 „ 
Hydrodynamic rate. Aerostatic proportion. 
. 34 ounces ... 34 ounces 
48 
77 
68 ,, 
59 
7 3 
. . .102 „ 
68 
3? 
. . .136 „ 
The extent to which the actual rates come nearer to the hydrodynamic rates than 
to the aerostatic proportions, is just the apparent extent to which the actual fil¬ 
tration through the particular media used in the experiments, is a hydrodynamic 
operation rather than due to aerostatic pressure. This is even still more evident if a 
number other than the first be taken as the unit. Thus the actual flow through the 
flannel in one minute being fifteen ounces when the orifice of outflow is two feet 
below the level of the liquid in the filtering-glass, it should be, at eight feet, 
sixty ounces if the action were aerostatic, and thirty if hydrodynamic : as it 
actually is thirty-seven and a half, it is obvious, if these numbers were rigidly 
correct, that 80 per cent, of the aid which gravitation obtains by the use of an 
eight-feet column of liquid below a certain flannel filter, is hydrodynamic aid, 
and 20 per cent, aerostatic. Similar remarks might be made concerning the 
cloth filtering medium. On looking at the rate at which water flows through 
it, we see that whatever be the amount which passes by mere gravitation, as 
when the cloth is folded into a cone and inserted in a common funnel, that 
amount is greatly increased by the use of a long column of the water below the 
cloth. The proportion of this a!d to gravitation which is apparently due to hy¬ 
drodynamic action is somewhat less, and aerostatic action somewhat more than 
in the case of the flannel, but still mostly hydrodynamic. 
Here let me correct an error which some are likely to fall into. The table 
just given would at first sight seem to indicate that if the hydrodynamic action 
of the filter were destroyed by admitting air into the chamber below the filter¬ 
ing medium, the possibly pure aerostatic action of the resulting column would 
be superior to the normal action, to the extent that an aerostatic rate of pres¬ 
sure is superior to a hydrodynamic rate. As a matter of fact, however, such 
a filter obtained by using a long column of water below, but not continuous 
with the medium, gives smaller instead of greater amounts of filtered liquid 
in a given time. . The reason of this is obvious ; the filtered liquid has now a 
circumvent before it can flow away. The maximum amount 
of filtration can clearly only be obtained when there is no plug of air or any 
ot ler stationary plug; "when, in fact, the column of water is perfectly continuous 
with the filtering medium. The latter condition attained, the maximum amount 
of aerostatic effect is then supplemented by a maximum of hydrodynamic 
action. I say by a maximum, because there is doubtless some hydrodynamic 
action even when the chamber or glass tube beneath the filtering medium contains 
air, the water flowing down round the sides of the air-plug in an attenuated but 
not broken column. Again, the actual rate being greater than the hydrody¬ 
namic rate would seem to indicate that hydrodynamic force is supplemented by 
some other foice. Hot so. The fact is, that the hydrodynamic rate given is 
