Carbonic acid gas is produced in various ways, namely -

1. By respiration or breathing in men and animals.

2. Carbonic acid is abundantly produced in the process of fermentation, and is the cause of the briskness in bottled beer, champagne, and other fermented liquids.

3. By burning lime in a limekiln, or heating carbonate of lime to a red heat in any way. This process is followed in making liquified carbonic acid.

By the operation of subterranean heat in volcanic districts, upon limestone beneath the surface, large volumes are produced and are continually finding their way to the atmosphere. The springs in such districts are also frequently highly charged with it, and the gas escapes with effervescence. (This is the cause of the effervescence of genuine natural mineral waters, as those of Selters, Vichy, etc., etc.).

4. In water of rivers, etc., from the gradual oxidation of vegetable and other organic substances.

5. In coal mines, from the decomposition of the coal.

6. By burning charcoal or other forms of carbon.

7. Chalk, marble, limestone, Iceland spar, oyster shell, pearlash, carbonate of soda, etc., all yield carbonic acid gas when treated with a stronger acid, as sulphuric, muriatic, etc.

The second of these sources of carbonic acid is largely employed in London in the manufacture of aerated bread, and is found to answer admirably. (Carbonating machinery to be used in the manufacture of aerated - properly named carbonated - bread, is also introduced in the United States. The carbonic acid gas is forced through the dough at a pressure of 100 pounds. This does away with the use of yeast.) The gas, which lies in a thick layer on the surface of the vats, is pumped by a suitable pump into a large india-rubber bag, and carried to the factory. It is probable, however, that this system would not suit at all for carbonated waters, for the fumes of the fermentation, which are an advantage to the taste of the bread, would probably give an unpleasant effect in carbonated waters.

The sixth source has been employed in France in the manufacture of carbonated waters, but it involves great outlay in plant for the purification of the gas, and is not recommended.

The means of production last named is that which is universally used in various forms for making carbonated waters.

Even in the simple form of seidlitz powders (which we must recognize as producing carbonated waters), the effervescence results from the action of the acid in the one paper, on the carbonate of soda, etc., in the other.

In seltzogenes, carbonators, etc., the gas and the pressure are produced in the same way by a similar mixture.

In the manufacture of carbonated waters on a large soale, the gas is usually produced by placing chalk, in the form of whiting, or powdered marble, or some other form of pure limestone, mixed with water, in a closed leaden or wooden vessel, and then introducing gradually sufficient sulphuric acid to disengage all the carbonic acid, and convert the residue into a neutral salt - sulphate of lime. The chemical combination which goes on in freeing the gas, produces a considerable amount of heat, and as increased heat is unfavorable to the combination of the gas with water, it is desirable that the gas should be cooled before being used.

Occasionally muriatic acid is employed, but it is not so much to be recommended, as it throws off a great amount of vapor, which may easily pass over with the carbonic acid, and be difficult to separate. The former system is also cheaper, and has no practical drawback whatever. The spent whiting, or marble dust, is a harmless compound, and can be easily removed and disposed of.