METHODS OF PREPARATION AND MANUFACTURE.—The anhydrous acid may be dismissed without further notice, beyond simply stating that it may be prepared by the action of acetyl chloride upon potassium or sodium acetate, or by heating one of these acetates with benzoyl chloride.
For the production of the ordinary hydrated acid, three processes are employed—(a), acetous fermentation, chiefly carried on for the production of vinegar ; (b), the dry distillation of wood, whereby the impure or pyroligneous acid is obtained, and, by subsequent processes, the acetates of lime, iron, soda, &c.; (c), the distillation of various metallic acetates with sulphuric or hydrochloric acid, yielding the pure acetic acid of commerce.
By acetone fermentation is meant the oxidation of dilute alcohol, or various liquids containing alcohol, in the presence of yeast, or almost any azotized matter liable to decay. Not that the presence of these putrescible substances or " ferments " is necessary to bring about the change, for by the simple action of the air, or by various oxidizing agents—chromic, nitric, or hypochlorous acids &c.—alcohol may be converted into acetic acid ; but the change is effected much more rapidly and effectually in the presence of a ferment, along with an abundant supply of air. The nature and action of these ferments are as yet only imperfectly understood. According to Pasteur the formation of acetic acid from alcohol depends upon the presence of a fungus—the Mycoderma aceti—which, like platinum black, has the power of absorbing oxygen within its pores and brings it into peculiarly intimate con tact with the alcohol. His experiments go to show that this fungus (yeast-plant) can be sown on the surface of the liquid by introducing a small portion of another alcoholic liquid already in a state of fermentation, that it requires nutrition, subsisting upon the albuminous matters and mineral salts which alcoholic liquors usually contain, and that if these be absent the mycoderm remains barren for lack of food, and no acetification can take place. According to Mayer's experiments (see Ann.
Chem. Pharm.' civil.. 227) the salt most necessary to the yeast-plant is the acid phosphate of potash, since if this salt be excluded the fermentative process is checked. At the same time the presence of other substances seems necessary, if not to produce fermentation, at least to maintain the myco derm in proper condition, the salts of ammonium and magnesium, or nitrogenous organic bodies similar in eonstitulion to ammonia, pepsin, or the diastase of beer. Differing from these theories an to the nature and work of the mycoderna, Licbig and other eminent chemists regard the process of fermentation as ono of the simplest alcoholic oxidation, and certainly wood shavings NAM' have been nsed for many years in the manufacture of vinegar have been examined under the microscope without finding a trace of fungus upon them.
The " souring " of wine is an everyday and natural illustration of the process of acctous fer mentation, strong wines souring more readily than weak because they contain less vegetable matter in proportion to absolute aleohol.
During acctous fermentation a substance called aldehyde—a lower compound of alcohol and oxygen—is probably always formed. Aldehyde is an exceedingly unstable body, and to prevent loss of acetic acid through its volatilization it is advisable to bring the ferment and alcohol together with as free ad mixture of air as possible, that a rapid and more perfeot oxidation may he ensured. The Germun, or " quick vinegar " process, effects this in the following manner. A vessel is prepared of the description shown in Fig. 4, varying in size from 18 ft. high and 15 ft. in diameter to 8 ft. high and 6 ft. in diameter, a large size being preferred. This "tun" (essigbilder, or vinegar " generator" or "graduator ") is care fully hooped, and set up on any convenient platform. A cover, fitting loosely on the top, keeps out dust and dirt, and about 12 in. below this is a fixed shelf, perforated with a groat number of small holes and two or three larger ones.