364 
TRANSACTIONS OF THE PHARMACEUTICAL SOCIETY. 
as much as at first, and so on. While, therefore, any pressure which a column 
of water exerts on the bottom of a tube when the open end of the tube is up¬ 
right, is a pressure correctly termed hydrostatic, which, in fact, can only be 
termed hydrostatic,—the pressure exerted at the same point when the tube is re¬ 
versed, can only be properly estimated and appreciated under the term aerostatic, 
for it is a pressure whose origin is the natural pressure of the atmosphere on all 
things at the surface of our earth ; that pressure, it is true, is conveyed by the 
water, but the water exerts no pressure itself; indeed, on the contrary,'the pre¬ 
sence of the water so reduces the amount of aerostatic pressure as to very soon 
(when the column is thirty-four feet long) obliterate it altogether. 
All that has just been stated concerning a simple tube closed at one end, is, 
of course, applicable in the case of the closed end being so enlarged out as to 
more nearly resemble a barrel, into one head of which a long tube is fixed. . As 
before stated, such an instrument is, when filled with liquid and the tube point¬ 
ing upwards, a pure hydrostatic arrangement. But when the whole is reversed 
and the tube points downwards, it is no longer a hydrostatic apparatus; for so 
far from an increase in the length of the column causing an increase in the pres¬ 
sure inside the barrel until the barrel bursts, it positively causes a decrease in 
the pressure, and the pressure ceases altogether when the column is thirty-four 
feet long, as in the case of the simple tube. 
In short, such apparatus as the inverted barrel and tube just described, or the 
inverted simple tube mentioned in the previous paragraph, though identical in 
construction with hydrostatic arrangements, are purely aerostatic apparatus, and 
their chief use is to illustrate aerostatic laws.* 
Mr. Schacht’s filter is just a vessel as I have been describing in the previous 
two or three paragraphs, only that the upper head 
of the barrel is replaced by a filtering medium, which 
can be securely fixed to the inner sides of the barrel, 
and so as to leave some space in which to retain the 
filtering mixture above the medium. (Its material 
and mounting is also, of course, more useful and ele- 
FosiUon of gant than this description would indicate. See the 
hole g accompanying figure.) If then the filter that I have 
previously alluded to as the so-called u hydrostatic 
filter ” is wrongly so termed, clearly the filter that 
I have just described as Schacht’s filter is not a 
hydrostatic filter, even though it closely resemble in 
construction the apparatus used in the demonstra¬ 
tion of hydrostatic laws. The most that could be 
claimed for it is that it is an aerostatic filter,—the 
increased length of the column of liquid below the 
filtering medium admitting of the increased mani¬ 
festation of atmospheric pressure on the surface of 
B 
a 
B FosiUon 
C 
'l ft) 
to 
the 
filtering 
mixture; but it is not even an aero¬ 
static filter, at least not purely or even mainly so. 
If it were, the velocity of filtration would increase 
with the simple length of the tube, instead of at a 
rate proportionate to the square root of that length; whereas the rate in practical 
* Thus when either is empty, the atmospheric pressure on both sides of the bottom of the 
barrel, or on the inside and outside of the glass plate forming the bottom of the tube, is 
fifteen pounds per square inch. When either is filled with water to the length of a dozen 
yards, and then inverted, so that the open end of the tube dips into a basin or tub of water, 
the pressure on the inside of the glass plate, or on the inside of the upper head of the tube, 
is reduced to nothing; the water, in fact, leaves those .surfaces altogether, and sinks to a 
