QUANTITATIVE  ESTIMATION
the soda-lime U-tubes are closed with rubber corks, through
which pass bent glass tubes. One of these glass tubes is con-
nected by rubber tubing to an oxygen gas-holder or to a
cylinder of compressed oxygen, which must be furnished with
an automatic regulating valve, and the other glass tube is
attached to a gas-holder containing air. By turning the three-
way tap, either oxygen or air may be supplied to the combustion
tube.
3. A Combustion Tube of Hard Glass.—It should be about 13
mm. inside diameter, and the walls not more than 1*5 mm.
thick. Its length should be such that it projects at least 5 cm.
(2 in.) beyond the furnace at either end. After cutting the
required length, the ends of the tube are carefully heated in the
flame until the sharp edges are just rounded. The tube is filled
as follows. Push in a loose asbestos plug about 5 cm. (2 in.) from
fig. 4.
one end. This end, to which the calcium chloride tube and potash
apparatus are subsequently attached, maybe called the/r^/ end.
Pour in coarse copper oxide at the opposite end and shake it down
to the plug until there is a layer about two-thirds the length of the
tube. Keep the oxide in position by another plug of asbestos ;
see that the plugs are not rammed too tight. Make a roll of
copper gauze about 13 cm. (5 in.) long to slide easily into the
back end of the combustion tube. This is done by rolling- the
gauze .tightly round a stout copper wire until the requisite thick-
ness is obtained. The projecting ends of the wire are then
bent over into hooks as shown in Fig. 4. This roll, or spiral,
as it is usually called, is subsequently oxidised. It is pushed
into the tube or withdrawn as occasion requires by a piece of
hooked wire. The combustion tube is placed on a layer of
asbestos in the iron trough of the furnace. The arrangement of
the tube with boat and spiral is-shown in Fig. 5. *_ ;«,
4. A Straight Calcium Chloride Tube.—It is inserteatriroidgh a
rubber cork and fixed in the front end of the combustion tube
when the latter is not in use, as copper oxide is very hygro-
scopic, and it is necessary to protect it from the moisture in
the air.