8 INTRODUCTORY.
narrow piece of glass tubing, open at the top where it rises above
the stopper, and at the bottom ending in a long capillary reaching
nearly to the bottom of the flask. During the distillation, tiny
bubbles of air enter the flask, pass through the liquid and prevent
bumping. The stream of air is regulated by means of a screw
clamp on a short piece of rubber tubing which is placed at the top
end of the open tube. The arm of the heavy suction flask which
is used as receiver should be connected with a manometer and with
a suction pump. In spite of the fact that a little air is constantly
entering the flask during the distillation, it is easy to get the
pressure reduced to 10-18 mm. by the use of a good water pump.
Very volatile substances, such as sulphur dioxide, can be con-
densed in common gas wash bottles, with ground-glass joints,
by means of a freezing mixture. When enough liquid has con-
densed in the wash bottle so that the entrance tube dips beneath
its surface, the further condensation takes place very readily. If
the preparation is to be preserved, it is distilled into a thick-walled
glass tube, of the shape of a test tube, which is drawn down atits upper end so that when filled it can be sealed by the blast
flame (Fig. 12, p. 58). Or, the substance can be poured into a
sealing flask the neck of which is of about the thickness of a lead
pencil, and can be readily melted together.
- Pulverizing.
Minerals that are to serve as the raw materials for the various
preparations should, if possible, be bought in powdered form,
since the crushing of large masses in the laboratory is extremely
laborious. If the pulverizing must be undertaken, the material
should be ground in small portions and from time to time the fine
powder should be sifted from the coarse by means of wire gauze or
a sieve. - Drying of Rinsed Vessels.
When directions call for a dry vessel, it is sometimes the practice
to rinse the vessel with water, then with alcohol and finally with
ether, assuming that the ether will evaporate quickly and leave
a dry vessel. This method is unsatisfactory. It is far better to
dry the vessel, after rinsing it with several small portions of al-
cohol, by moving it back and forth over a small flame while
applying suction through a tube reaching to the bottom of the
vessel. Instead of suction, a gentle air blast can be used.