If alcohol be diluted with water, and the specific ferment mixed with it and exposed to the air at 22° C., it is rapidly converted into vinegar. The change is accompanied by the absorption of oxygen, one atom of which combines with two of hydrogen to form water, and a substance remains called _aldehyde_, further oxidation of which produces the acetic acid. We may express it chemically thus: Alcohol. Aldehyde. Water. C_{2}H_{6}O (+ oxygen and the ferment) = C_{2}H_{4}O + H_{2}O. The aldehyde becomes further oxidised: C_{2}H_{4}O + O = C_{2}H_{4}O_{2} (acetic acid). Now this method of simply oxidising alcohol to obtain acetic acid may be carried out chemically without any ferment. If slightly diluted alcohol be dropped upon _platinum black_, the oxygen condensed in that substance acts with energy upon the spirit, and union readily occurring, acetic acid results. Here the whole business of the platinum sponge is to persuade the oxygen of the air and the hydrogen of the alcohol to unite. In the ordinary manufacture this is accomplished by the vegetable cells of _Mycoderma aceti_. There are two chief methods adopted in the commercial manufacture of vinegar, both of which depend upon the presence of the _Mycoderma_. The method in vogue at Orleans when Pasteur (about 1862) commenced his studies of the vinegar organism was to fill vats nearly to the brim with a weak mixture of vinegar and wine. Where the process is proceeding the surface is covered with a fragile pellicle, "the mother of vinegar," which is produced by and consists of certain micro-organisms whose function is to convey the oxygen of the air to the liquor in the vats, thus oxidising the alcohol into vinegar. This oxidation may be carried on even beyond the stage of acetic acid (when no more alcohol remains to be oxidised), resulting in carbonic acid gas, which escapes into the air. But as in the alcoholic, so in the acetic, fermentation, there comes a time when the presence of an excess of the acid inhibits the further growth of the organism. This point is approximately when the acetic acid has reached a percentage as high as 14. But if the acid be removed, and fresh alcohol added, the process recommences.