PHARMACY. 
is kepi moist by means of the descent of 
ivater, from a vessel placed above it, through 
minute syphons or spongy worsted threads. 
But as, during the process of distillation, 
f)ermanently elastic fluids are often pro- 
duced, which would endanger the breaking 
of the vessels, these are permitted to es- 
cape either through a tubulature, or hole, in 
the side of the receiver, or rather through 
a hole made in the luting. Receivers hav- 
ing a spot issuing from their side, are used 
when we vvish to keep Separate the pro- 
ducts obtained at different periods of any 
distillation. For condensing very volatile 
vapours, a series of receivers, communicat- 
ing with each other, termed adopters, were 
formerly used ; but these are now entirely 
superseded by Woulfe's, apparatus, which 
consists of a tubulated retort, adapted to a 
tubulated receiver. With the tubulature of 
the receiver, a three-necked bottle is con- 
nected by means of a bent tube, the further 
extremity of which is immersed. One or 
more inches, in some fluid contained in the 
bottle. A ^ries of two or three similar 
bottles are connected with this first bottle 
in the same way. In the middle tubulature 
of each bottle a glass tube is fixed, having 
its lower extremity immersed about a 
quarter of an inch in the fluid. The height 
of the tube above the surface Of the flOid 
must be greater than tlie sum of the columns 
of fluid standing over the further extremi- 
ties of the connecting tubes, in all the bot- 
tles or vessels more remote from tlie retort. 
Tubes so adjusted are termed tubes of 
safety, for they prevent that reflux of fluid 
from the more remote into the nearer bot- 
tles, and into the receiver itself, which 
would otherwise inevitably happen on any 
condensation of vapour taking place in the 
retort, receiver, or nearer bottles. Dif- 
ferent contrivances for the same purpose 
■have been described by Messrs. Welter and 
Burkitt; and a very ingenious mode of con- 
necting the vessels without lute has been 
invented by Citizen Girard, but they would 
not be easily understood without plates. 
The further tubulature of the last bottle is 
commonly connected with a pneumatic ap- 
paratus, by means of a bent tube. When 
tlie whole is properly adjusted, air blown 
into the retort should pass through the 
receiver, rise in bubbles through the fluids 
contained in each of the bottles, and at last 
escape by the bent tube. In the receiver, 
those products of distillation are collected 
which are condensable by cold alone. The 
^rst bottle is commonly filled witli water, 
and the others with alkaline solutions, or 
other active fluids ; and as the permanently 
elastic fluids produced are successively sub- 
jected to the action of all these, only those 
gases will escape by the bent tube which 
are not absorbable by any of them. 
In separating permanently eiastic fluids 
or gases from the substances in which they 
are found, we are compelled tO employ a 
distinct pneumatic apparatus ; and the gas 
may then be received either into vessels ab- 
solutely empty ; or, filled with some fluid, 
on which it exerts no action. 
The first mode of collecting gases may be 
practised by means of a bladder, moistened 
sufficiently to make it perfectly pliable, and 
then compressed so as to press out every 
particle of air from its cavity. In this state 
it may be easily filled with any gas. An 
oiled silk bag will answer the ^ same pur- 
pose, and is more convenient in some res- 
pects, as it may be made of any size or 
form. Glass or metallic vessels, such as 
balloons, may also be emptied for the pur- 
pose of receiving gases, by fitting them 
with a stop-cock, and exhausting the- air 
from them by means of an air-pump. 
But the second mode of collecting gases 
is the most convenient and common. In 
which case the vessels may be filled either 
with a fluid lighter, or heavier, than the 
gas to be received into it. 
The former method is seldom employed; 
but if we conduct a stream of any gas 
heavier than atmospheric air, such as car- 
bonic acid gas, muriatic' acid gas, &c. to 
the bottom of any vessel, it will gradually 
displace the air, and fill the vessel. On the 
contrary, a gas lighter than atmospheric 
air, such as hydrogen, may be collected in 
an inverted vessel by conducting a stream 
of it to the top. But gases are most com- 
monly collected by conducting the stream 
of gas into an inverted glass-jar, or any 
other vessel filled with water or mercury. 
The gas ascends to the upper part of the” 
vessel, and displaces the fluid. In this way 
gas may be kept a very long time, provided a 
small quantity of the fluid be left in the ves- 
sels, which prevents both the escape of the 
gas, and the admission of atmospheric air. 
The vessels may be made of variousshapes • 
but those most commonly employed are 
cylindrical. They may be either open only 
at one extremity, or furnished at the other 
with a stop-cock. The manner of filling 
them with fluid, is to immerse them com- 
pletely in it, with the open extremity di- 
rected a little upwards, so that the whole 
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