LABORATORY. 
cylinders, the inner of wUicli is closed at the 
top and at the bottom. The space only 
«f about half an inch is left between 
the two cylinders as shown by the dotted 
lines. In this space the vessel, c, may 
move freely up and down. The interval is 
filled with water as high as the. top of the 
inner cylinder. The cup or rim on the 
top of the outer vessel, is to prevent the 
W'ater from overflowing when the vessel, c, 
is forcibly pressed down, in which situation 
it is placed whenever gas is about to be 
collected. The gas enters from t!ie vessel 
in which it is produced ; by the communi- 
cating opening b, and passes along the per- 
pendicular pipe marked by dotted tines in 
tile centre, into the cavity of the vessel, c, 
which continues rising till it is full. 
To transfer the gas, or to apply it to any 
purpose, the cock, A, is to be shut, and an 
empty bladder, or bottle of elastic gum, 
furnished with a stop cock, is to be screwed 
on a. When the vessel, c, is pressed down 
by tlie hand, the gas passes down the cen- 
tral pipe, which it had before ascended, 
and its escape at b being prevented, it finds 
its way up a pipe which is fixed on the 
outer surface of the vessel, and which is 
terminated by the cock a. By means of 
an ivory mouth-piece screwed on this cock, 
the gas included in the instrument may be 
respired ; the nostrils being closed by the 
fingers. When it is required to transfer the 
gas into glass jars standing in water, a 
crooked tube may be employed, one end 
of which is screwed upon the cock, b, while 
the other aperture is brought under the 
inverted funnel, fixed into the shelf of the 
pneumatic trough. 
Several alterations have been made in 
the form of this apparatus, but they are 
principally such as add merely to its neat- 
ness and beauty, and not to its utility ; and 
they render it less easy of explanation. The 
counterpoises, e e, are now generally con- 
cealed in the framing, and the vessel e is fre- 
quently made of glass. 
When large quantities of gas are required 
(as at a public lectiire) the gas holder, 
fig. 9, will be found extremely useful. It 
is made of tinned iron plate, japanned 
both within and without. Two short 
pipes c, and c, terminated by cocks, pro- 
ceed from its sides, and another, b, passes 
through tlie middle of the top or cover, to 
which it is soldered, and reaches within 
half an inch of the bottom. 
It will be found convenient also to have 
an air cock r.th a very wide bore fixed to 
the funnel at b. When gas is to be trans- 
ferred into this vessel from the gazometer, 
the vessel is first completely filled with wa- 
ter through the funnel, tlie cock a being left 
open and c shut. By means of an horizon- 
’ tal pipe, the aperture a is connected with a 
of the gazometer. The cock b being shut, 
a and c are opened, and the vessel c of the 
gazometer, fig. 8, gently pressed down- 
wards with the hand. The gas tiien descends 
from the gazometer till the air-holder is full, 
which may be known by the water ceasing 
to escape through the cock c. All the 
cocks are then to be shut, and the vessels 
disunited. To apply this gas to any pur- 
pose, an empty bladder may be screwed on 
a j and water being poured through the 
funnel b, a corresponding quantity of gas is 
forced into the bladder. By lengthening 
the pipe, b, the pressure of a column of 
water may be added ; and the gas being 
forced through a, with considerable veloci- 
ty, may be applied to the purpose of a blow- 
pipe, &c. &c. 
The gazometer already described, is 
fitted only for the reception of gases that 
are confinable by water, because quick- 
silver would act on the tinning and solder 
of the vessel, and would not only be 
spoiled itself, but would destroy the appa- 
ratus. Yet an instrument of this kind, in 
which mercury can be employed, is pecu- 
liarly desirable, on account of the great 
weight of that fluid ; and two varieties of 
the mercurial gazometer have therefore been 
invented. The one of glass, is the con- 
trivance of Mr. Clayfield, and may be seen 
represented in the plate prefixed to Mr. 
Davy’s researches. In the other, invented 
by Mr. Pepys the cistern for the mercury 
is of cast iron. A drawing and representa- 
tion of it may be found in the fifth volume 
of the Philosophical Magazine ; but as 
neither of these instruments are essential 
to the chemical student, and as they are 
required only in experiments of research, we 
refer to the nfinute descriptions of their 
respective inventors. 
Very complete sketches of chemical in- 
stniments" and furnaces may be seen in 
Henry’s chemistry. 
After the general description we have 
here given of the arrangement and appara- 
tus for chemical experiments, we shall con- 
clude with a short account of the blow-pipe. 
It is a tube which terminates in a perfo- 
ration not exceeding the hole which might 
be made by a small pin. There is no dif- 
ficulty, in case of emergency, in making one 
