These may be combined. Cotton, sponges or pumicestone may be saturated with the solutions of soda, permanganate of potassium, or with moistened peroxide of iron; but these means of purification also need frequent renewing like those in liquid form.

The mechanical impurities, particles of marble dust, whiting, etc, we get rid of by washing and filtering the gas; but the chemical impuri-ties of the carbonic acid we must meet with chemical remedies.

Chemical Impurities And Remedies

The principal chemical impurities in carbonic acid gas and the proper remedies are the following:

Nitrogenous Gases, Sulphurous Acid

From impure sulphuric acid, and small traces of obnoxious gases from impure carbonates. They are removed by leading the gas through a washing liquid containing soda and sulphate of iron (green vitriol).

Sulphuretted Hydrogen

From sulphur combinations in the carbonate. The removal of this requires a washing liquid that contains a 10 per cent, solution of peroxide of iron or a mixed solution of 5 parts by weight of sulphate of iron and 4 parts of bicarbonate of soda in 100 parts of water, which solution will at the same time absorb atmospheric air that may be combined with the carbonic acid gas.

Bituminous And Animalic Odors

From impure carbonates, especially from impure whiting and limestone. A solution of permanganate of potassium added to the washing liquid in purifier will destroy these bad odors, or the insertion of coarse animal charcoal into one washer, through which the contaminated carbonic acid gas is to pass.

Are these bituminous or animalic odors present to a greater extent ? Then it is necessary to pass the carbonic acid gas through a special coal cylinder described on another page.

Sulphuric acid from the generator is neutralized by the marble chips mentioned, and by the addition of some solution of carbonate or bicarbonate of soda to the liquid in purifier.