transactions op the pharmaceutical society. 363 
tll A° v! 11 ^ L instrument > than by mere maceration in the fluid in the usual 
by the old d phn C °But ^MM B^ if* ^^ thau bad been obtained 
V I, 1 ' as A . MM - Bouilay afterwards showed,* the improved re¬ 
sults depended on the continuous displacement of the saturated infusion or tine 
—ite ndependent „r eria,S ^ "Kf the menSu”,*^ 
were quite independent of pressure. The use of the long pressure-tube wm eon - 
percolidor d 1SC0 n bm 11 ed > and the resulting vessel was the “ Boullay Filter ” or 
But even the Real filter-press of 1816 was not the first in which the ores- 
^Cadet h ?n r on ta ! 1C f C ? lu ^ n was attempted to be used as an aid to filtration. 
Pi' ‘ •’ , a ^ lc ? descriptive of Real’s instrument in the ‘Journal de 
Phaimacie of April of that year, says, “The English have applied this min 
Tever and which'Sm-d f® 1 By 1 meanS of an a PP aratus called a hydraulic 
lever, ana winch is formed of a brass box surmounted by a long tube they force 
M® 0 Eob 0 ;son V iriS9S Y ° f ., charCOa1 ’ Whiel * serves t0 and’purify it.” 
' 1 b n 8 ? 8 ; wa ® do ^btl e ss unaware of this practice of his country- 
e ”’ w .^ b M. Cadet evidently knew of before 1816. y 
e influence of a column of water helow the filtering medium has recently 
been proposed by Mr. Sch a cht.t The nature of the action in such an a3 
•, ..-d b s P r P e what different to that in which the column is above the filter & It 
is still mainly a hydrodynamic action, but the flow is also partly set up by atmo- 
tLcTZ r X r i V bUt ’ u° Pr0Ve thiS St wil1 b * necessary, JQot™o 
trace the effect from its sources. Suppose, then, a long glass tube is securely 
wRhwate 1 ? Tif extremity *> y adat S lass P^te, and the tube filled to the brim 
with water. The pressure of water on the plate will then depend on the length 
ion x , columiJ as already explained, and may be increased up to 50, 60, or a 
Moreover°tl D ° VeSS -!i C ? uld 5® f ° Und strong enou g h to resist the force. 
oieo\ei, the pressure will depend only on the column of water,— it wall be 
independent oi the air it will be hydrostatic, not aerostatic. But, suppose the 
tube to be turned upside down, the liquid being prevented from escaping by the 
open end being plunged beneath the surface of water in a basin, tub, etc, Jr by 
f t ! ° *?!?? bemg curved round after the manner of barometer 
S , . mflueuce of the column of water on the inner surface of the plate 
will now be m exactly the opposite degree to what it was before. When the 
tube was upright, an increase in the length of the column produced an increase 
m the pressure on the inner surface of the plate at the bottom. Eow it is re¬ 
versed : an increase m the length of the column produces a decrease in the amount 
pressure on ip inner suiface of the plate. Before the increase of pressure 
iWfy?°f ntln ^ ed f Up i t0 llumber of feet > now the decrease ceases at about 
thirty-four met, when the water leaves the inner surface altogether, and, there- 
rore, at that distance ceases to exert any pressure at all. The cause of this diffe¬ 
rence m effect m the two cases is plain. When the tube is open end upwards, the 
gravitation of the water is opposed by the rigid sides and bottom of the vessel • 
when ^ is reversed, the gravitation is opposed by the atmosphere on the liquid 
at the open end or the tube. If, when the open end is downwards, there be no 
-wamr m the tube, this atmospheric pressure on the inner surface of the plate is 
about, fifteen pounds on every square inch ; if there be thirty-four feet of water 
m the tube, the pressure is nothing ; if the column of water is one foot long the 
pressure on the inner surface of the plate is one thirty-fourth of fifteen pounds 
mss than fifteen pounds ; if four feet long, four thirty-fourths; if seventeen feet 
mng, seventeen thirty-fourths of fifteen pounds less than fifteen pounds ; or half 
1833, ‘Journal de Pliarmacie,’ tome xix. p. 281. 
nt'pivrniwhi*"^' S « a Pf r d as at die m eeting of the British Pharmaceutical Conference 
folk Jin” mmuh m feeptembcr ’ 186o > ‘™d published in the ‘ Pharmaceutical Journal ’ of the 
9 p 9 
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