TRANSACTIONS OF THE PHARMACEUTICAL SOCIETY. 
359 
able surface. The manner of the conveyance of this great pressure to every part 
of the filter-bag (by hydrostatic means), has already been described in treating of 
hand-pressure. For this form of pressure, cause and effect being dynamical, and 
the means which link cause with effect being statical, no term could be more 
appropriate than “hydraulic.”* 
(d) Filtration in which Gravitation is aided hu Atmospheric-pressure .— 
Two filters of this kind were proposed by Mr. G. Held at the Society of Arts, 
more than fifty years ago (Nov. 1, 1814 ; Trans. Soc. Arts, vol. xxxiv. p. 87). 
His drawings represent the filtering medium as fixed to the inner sides of a large 
tub at a point somewhat nearer the bottom than the top. The filtering mix¬ 
ture being placed on the medium, a portion of the air enclosed below was re¬ 
moved by a well-made pump, when of course the natural pressure of the atmo¬ 
sphere, previously exerted below as well as above, came into operation on the 
upper surface only of the mixture; or the tub was turned upside down, the 
filtering mixture poured into the closed chamber through an opening which was 
then closed, and air then pumped into the space between the surface of the 
mixture and the bottom (now the uppermost part) of the tub. 
Steam has also been used in the place of air to exert pressure on the upper 
surface of a filtering mixture. 
A “vacuum-filter,” smaller than Mr. Field’s, and more conveniently con¬ 
structed, is described and figured in the ‘ Practical Pharmacy ’ of Drs. Mdlir 
and Redwood, p. 205. 
A new method of applying atmospheric pressure as an aid to gravitation in 
the operation of filtration has recently been proposed by Dr. Piccard.f A fil¬ 
tering medium is attached to the inside of a funnel, or other vessel, in the 
manner just described ; the neck of the funnel is fixed by a cork into one of the 
two necks of a Wolff’s bottle; from the other neck a flexible or other tube is 
carried to a water aspirator.£ A partial vacuum is in this way produced below 
the filtering medium, when at once the natural pressure of the atmosphere ma¬ 
nifests itself on the exposed surface of the filtering mixture. 
In each of the above instruments the atmospheric pressure developed aids the 
gravitating tendency of the fluid portion of the mixture, and the rapidity of 
filtration is augmented to a certain extent. 
Practically, these atmospheric-pressure filters have less value than was at first 
anticipated ; but that fact may be passed over here, as the usefulness of filters, 
* Bramah’s Press has, by a few writers, been termed a hydrostatic machine ; and, at first 
sight, this would seem to be a legitimate name, seeing that the means of enormously multi¬ 
plying force which it possesses are partly hydrostatic means. But the effect is also partly 
due to the use of levers. Besides this, whatever useful effect the machine may produce is a 
dynamical effect, and the primary cause of that effect is also dynamical. Hydraulic, there¬ 
fore, is a better term, because hydraulics, etymologically at all events, only relate to the 
properties of water in pipes (vd ccp, water, and avAos > a pipe), and include neither or both 
statical ( araros , static or standing) and dynamical (tivvafus, power or force) matters. 
Messrs. Bramah originally named their instrument a “ Hydromechanical Press, and it was 
so called for many years; I consider that name a better one lor it than even the term 
“hydraulic.” . . , 
It is to be regretted that the effects termed dynamic, are not described by a word which, 
would more powerfully convey idea of “unrest,” which that term is supposed to indicate. ^ 
+ ‘ Zeitschrift fur Analytische Chemie,’ part 1, vol. iv., and ‘ Chemical News, vol. xn. 
p. 180. . ... 
4 A water aspirator is simply a T-shaped tube, whose horizontal portion is ot greater 
diameter than the vertical part. AVater is conveyed from a cistern through the udder poition 
of the tube. As the water passes the point where the narrower tube is inserted, it sucks in 
air, or, in more precise terms, the friction of descending fluid particles against those (gaseous 
or liquid) in the side tube is sufficient to involve the latter in the current of the former. In 
this way vacua, as good as those of the barometer, have been produced. (See also Spiengcl s 
“ Researches on the Vacuum,” Journal oi the Chem. Soc., vol. xviii. p. 9.) 
