any suitable receiver. The evolution of carbonic acid by this method is
rapid, but easily regulated, and the gas may be purified by causing
it to pass through some water contained in another two-necked bottle,
similar to the generator. The chemical change involved in this
decomposition is expressed by the following equation:

CaCO_3 + 2HCl = CO_2 + H_2O + CaCl_2
Calcium Hydrochloric Carbonic Water. Calcium
Carbonate. Acid. Acid. Chloride.

By referring to the table of combining weights given in a previous
paper, it will be seen that 100 parts of calcium carbonate will yield 44
parts of carbonic acid. Instead of hydrochloric acid any other acid may
be used, and in the practical manufacture of carbonic acid for aerated
waters sulphuric acid is the one usually employed. Carbonic acid is
colorless and inodorous, but has a peculiar sharp taste; it is half as
heavy again as air, its exact specific gravity being 1529; one hundred
cubic inches weigh 47.26 grains. It is uninflammable, and does not
support combustion or animal respiration. Under a pressure of about 38
atmospheres, at a temperature of 32 deg. F., carbonic acid condenses into
a colorless liquid, which may also be frozen into a compact mass
resembling ice, or into a white powder like snow. Carbonic acid is
soluble in water, and at the ordinary pressure and temperature one
volume of water will hold in solution one volume of the gas; under
increased pressures, far larger quantities of the gas can be held in
solution, but this is rapidly evolved as soon as the excess of pressure
is removed. Upon this property the manufacture of aerated waters
depends. The presence of free carbonic acid can be easily detected by
causing the gas to pass over the surface of some clear lime-water. If
any be present a white film of carbonate of lime will at once be formed.