28 CHEMISTRY. 
Other apparatus is at the command of the chemist for producing a current 
of perfectly dry air for such purposes. fig. 22 shows at A a flask partly 
filled with sulphuric acid, connected by means of conducting tubes with the 
drying vessel and the aspirator B. The latter is a cylinder of sheet-copper 
or zinc, provided below with a cock, b, and above with a’ funnel-tube, a. 
This is filled entirely with water through the funnel-tube a, and the funnel 
then closed by means of a cork. On opening the cock b, the water will 
flow out of the cylinder B, since the air entering through the tube d 
ascends through the concentrated sulphuric acid into the vessel A, and is 
thus dried; it afterwards passes over the substance placed in e¢, to fill 
the space left vacant in the aspirator B by the withdrawal of the 
water. | 
The apparatus (pl. 31, fig. 23) is used in a similar manner, differing only 
in that the vessel of sulphuric acid is replaced by a chloride of calcium 
tube, b, in which the inhaled air is dried. Finally, in fig. 21, the aspirator 
is replaced by an air-pump. The body is placed in a small matrass, 
immersed in the kettle. From this there leads a chloride of calcium tube 
for drying the air left by the air-pump after extracting the watery vapor. 
In other particulars, the operation of this apparatus is similar to that 
already described. By means of these forms of apparatus we are enabled 
to dry a substance at the boiling point of water; other contrivances are 
necessary, however, when the heat required, and possible, is greater than 
212°F. Thus sugar at 212° indeed loses all the water mechanically 
combined, but at a temperature of 329° F., it loses a definite proportion of 
chemically combined water, and the determination of this is of great 
importance with reference to the nature of this substance. For the purpose 
of exposing this and other bodies to a higher temperature, then, than that of 
boiling water, we make use of the oil bath (fig. 18), or the air bath 
(fig. 19.) 
The oil bath (fig. 18) consists of a quadrangular box with double walls, 
about an inch apart. The intervals, a,a,a,a, between these walls, are 
filled with oil introduced through the aperture, c. In the opening, 8, is set 
a thermometer, with its bulb dipping into the oil, so as to enable us to 
ascertain the temperature ; the bottom of the box is heated by the furnace 
on which it is to be set. The substance placed in a capsule,is introduced 
into the space within the box, and the door A shut. By means of the 
thermometer, the heat may be so regulated, as not to exceed a certain 
temperature. Place, for example, a weighed vessel of sugar within this 
apparatus, and weigh it from time to time; we shall find that it decreases 
in weight for a while, and that after a time this weight remains stationary, 
the loss being not an arbitrary amount, but exactly proportioned to a given 
quantity of sugar. , 
The air bath (fig. 19), intended to answer the same purpose as the last, 
is still more convenient. A, is a cylinder of sheet copper, within which a 
ring, c,c, is laid. The upper part of this cylinder is closed by a copper 
cover provided with two openings. In one opening, a, is set a thermometer, 
and through the other the neck of a glass matrass of known weight is 
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