It will be observed that we printed the word _pure_ in italics in
speaking of the carbonic acid used. This is one of the great points in
the process, as in order that the sulphureted hydrogen gas obtained
shall be concentrated and pure, only pure carbonic acid can be used in
liberating it. The apparatus employed in its preparation is perhaps
the most ingenious part of the works, and well worthy of attention by
others besides alkali makers. The method is based on the fact that if
dilute impure carbonic acid is passed into a solution of carbonate of
sodium, the carbonic acid is absorbed, bicarbonate of sodium being
formed, and the diluting gases passing away.

The bicarbonate of sodium on heating gives up the extra carbonic acid,
which can be collected and stored pure, while the liquor passes back
to simple carbonate of sodium, to be used over again as an absorbent.
This is not at all new in theory, of course, nor is this the first
proposal to use it commercially; but it is claimed that this is the
first successful working of it on a large scale.

The gases from a large limekiln supply the dilute carbonic acid gas,
which contains 25 per cent. to 30 per cent. of pure gas, the principal
diluting gas being, of course, nitrogen. This kiln gas is drawn from
the kiln by a blowing engine, and is first cooled in two large
receivers. It is then forced into the solution of sodium carbonate in
the absorption tower, 65 ft. high by 6 ft. diameter, filled with the
liquor. The tower has many diaphragms and perforated "mushrooms," to
cause a proper dispersion of the gases as they ascend through the
liquor. The strength of liquor found best adapted for the work is
equal to a density of about 30° Twaddell. After saturation the mud of
bicarbonate of sodium is drawn off and passed into the "decomposer," a