_Syphons are necessary in numerous manipulations of the laboratory_,
and modern researches in chemistry have given rise to several beautiful
devices for charging them, and also for interrupting and renewing their
action. When corrosive liquids or those of high temperatures are to
be transferred by syphons, it is often inconvenient, and sometimes
dangerous to put them in operation by the lungs. Moreover cocks and
valves of metal are acted on by acids, and in some cases would affect
or destroy the properties of the fluids themselves.
Fig. 54 shows how hot or corrosive liquids may be drawn off from a wide
mouthed bottle or jar. The short leg of a syphon is inserted through
the cork, and also a small tube, through which the operator blows, and
by the pressure of his breath forces the liquid through the syphon.
Fig. 55 represents a syphon sometimes employed by chemists. When used,
the short leg is first placed in the fluid to be decanted, the flame of
a lamp or candle is then applied to the underside of the bulb; the heat
rarefies the air, and consequently drives out the greater part of it
through the discharging orifice. The finger is applied to this orifice,
and as the bulb becomes cool the atmosphere drives up the liquid into
the void and puts the instrument in operation.
Fig. 56 is a syphon charged by pouring a quantity of the fluid to be
decanted into the funnel, the bent pipe attached to which terminates
near the top of the discharging leg. The fluid in descending through
this leg bears down the air within it, on the principle of the trompe,
and the atmosphere drives up the liquid in the reservoir through the
short leg.
Fig. 57 is a glass syphon for decanting acids, &c. It is charged by
sucking, and to guard against the contents entering the mouth, a bulb
is blown on the sucking tube. The accumulation of a liquid in this bulb
being visible, the operator can always withdraw his lips in time to
prevent his tasting it.
Fig. 58 is designed to retain its contents when not in use, so that on
plunging the short leg deep into a liquid the instrument will operate.
This effect however will not follow if the end of the discharging leg
descend below the bend near it, and if its orifice be not contracted
nearly to that of a capillary tube.
Fig. 59 is a syphon by which liquids may be drawn at intervals, viz.,
by raising and lowering the end of the discharging leg according to the
surface of the liquid in the cistern.
[Illustration: FIG. 60.]
Figs. 60, 61, 62 are syphons described by Hero of Alexandria who lived
120 B. C.; the descriptions of the figures are the translation of the
original.
Let A B C D (Fig. 60) be a vessel open at the top, and through its
bottom pass a tube, either an inclosed pipe as E F G, or a bent syphon
G H K. When the vessel A B C D is filled, and the water runs over,
a discharge will begin and continue till the vessel is empty, if the
interior opening is so near the bottom of the vessel as only to leave a
passage for the water.
As before, let there be a vessel, A B (Fig. 61), containing water.
Through its bottom insert a tube, C D, soldered into the bottom and
projecting below. Let the aperture C of the syphon approach to the
mouth of the vessel A B, and let another tube, E F, inclose the tube C
D, the distance between the tubes being everywhere equal, and the mouth
of the outer tube being closed by a plate, E G, a little above the
mouth C. If we exhaust, by suction through the mouth D, the air in the
tube C D, we shall draw into it the water in the vessel A B, so that it
will flow out through the projection of the syphon until the water is
exhausted. For the air contained between the liquid and the tube E F,
being but little, can pass into the tube C D, and the water can then
be drawn after it. And the water will not cease flowing because of the
projection of the syphon below:—if, indeed, the tube E F were removed,
the discharge would cease on the surface of the water arriving at C, in
spite of the projection below; but when E F is entirely immersed no air
can enter the syphon in place of that drawn off, since the air which
enters the vessel takes the place of the water as it passes out.
[Illustration: FIG. 61.]
Let A B C (Fig. 62), be a bent syphon, or tube, of which the leg A B is
plunged into a vessel D E containing water. If the surface of the water
is in F G, the leg of the syphon, A B, will be filled with water as
high as the surface, that is, up to H, the portion H B C remaining full
of air. If, then we draw off the air by suction through the aperture
C, the liquid also will follow. And if the aperture C be level with
the surface of the water, the syphon, though full, will not discharge
the water, but will remain full: so that, although it is contrary to
nature for water to rise, it has risen so as to fill the tube A B C;
and the water will remain in equilibrium, like the beams of a balance,
the portion H B being raised on high, and the portion B C suspended.
But if the outer mouth of the syphon be lower than the surface F G,
as at K, the water flows out, for the liquid in K B, being heavier,
overpowers and draws toward it the liquid B H. The discharge, however,
continues only until the surface of the water is on a level with the
mouth K, when, for the same reason as before, the efflux ceases. But
if the outer mouth of the tube be lower than K, as at L, the discharge
continues until the surface of the water reaches the mouth A.
[Illustration: FIG. 62.]
_The Syringe_ is an instrument of very high antiquity and was probably
the first machine consisting of a cylinder and piston that was
especially designed to force liquids. In the closed end a short conical
pipe is attached whose dimensions are adapted to the particular purpose
for which the instrument is to be used. The piston is solid and covered
with a piece of soft leather, hemp, woolen listing, or any similar
substance that readily imbibes moisture, in order to prevent air or
water from passing between it and the sides of the cylinder. When the
end of the pipe is placed in a liquid and the piston drawn back, the
atmosphere drives the liquid into the cylinder; whence it is expelled
through the same orifice by pushing the piston down: in the former case
the syringe acts as a sucking pump: in the latter as a forcing one.
They are formed of silver, brass, pewter, glass, and sometimes of wood.
For some purposes the small pipe is dispensed with, the end of the
cylinder being closed by a perforated plate, as in those instruments
with which gardeners syringe their plants.