Soda-water was formerly prepared by the ordinary chemist, but since
it has become so general a beverage has been made a separate branch
of chemical manufacture. Soda-water consists of a very weak solution
of carbonate of soda, holding a large quantity of carbonic acid in
solution, for water has the property of absorbing a certain quantity
of carbonic acid, and this quantity is increased in proportion to the
pressure exerted on the water. This pressure is secured in the first
place by machinery, and afterwards maintained by the bottle being
closely corked and the cork fastened in by means of wire.
Illustration: FIG. 1. SODA-WATER APPARATUS.
In fig. 1 there is an illustration of the machinery used. It
consists of a small vessel holding sulphuric acid, attached to
another vessel containing chalk and water kept constantly stirred
by a small windlass passing through a hole in the top, and working
air-tight. When some of the sulphuric acid is allowed to run into
the vessel holding the chalk an effervescence takes place, and a
rapid extrication of carbonic acid; this is conducted by a tube to
a gas-holder as a store. A tube leads from this gas-holder into a
sort of air-pump, and a man, by turning a windlass, not only works
this and thereby forces a certain quantity of gas into another vessel
of copper (plated with silver inside), but turns this vessel itself
rapidly round. In this vessel the solution of carbonate of soda is
placed, and is agitated under pressure with the carbonic acid thus
forced into it, and which it rapidly absorbs. From this vessel it
is drawn off into bottles, which are adroitly corked before much of
the carbonic acid can escape, and then wired down, a fresh supply
of solution of soda and carbonic acid being constantly introduced.
Potash-water and Carrara water are made in the same way, using potash
in the former, and chalk in the latter, instead of soda.