may be defined a sensible internal motion of the constituent particles of a moist fluid, mixed, or compound body: by the continuance of which motion, these particles are gradually removed from their former situation or combination, and again, after some visible separation is made, joined together in a different order and arrangement, so that a new compound is formed, having qualities very sensibly different from those of the original fluid.

Fermentation, properly so called, is confined to the vegetable and animal kingdoms; for the differences between acids and alkalies, however much they may resemble the fermentation of vinous liquors, are nevertheless exceedingly different. It is divided into three kinds; or rather, there are three different stages of it, viz. the vinous, the acetic, and the putrefactive. Of the first, vegetables alone are susceptible; the flesh of young animals is in some slight degree susceptible of the second; but animal-substances are particularly susceptible of the third, which vegetable do not so easily fall into without previously undergoing the first and second. The produce of the first stage is wine, or some other vinous liquor; of the second, vinegar; and of the third, volatile alkali. See Brewing, Vinegar, &c.

Fermentation is one of the most obscure processes in nature, and no attempt has been made to solve it with any degree of probability. All that we know with regard to it is, that the liquor, however clear and transparent at first, no sooner begins to ferment, than it becomes turbid, deposits a sediment, emits a great quantity of fixed air, and throws up a foam to the top, acquiring at the same time some degree of heat. The heat of the vinous stage, however, is but moderate, seldom or never exceeding that of the human body. The heat of the aceticus is considerably greater; and that of the putrefactive is the greatest of all, insomuch that putrefying substances, when heaped together in great quantities, will sometimes break forth into actual flame.

From these phenomena, fermentation would seem to be a process ultimately tending to the entire dissolution of the fermenting substance, and depending upon the action of the internal heat, ethereal fluid, or whatever else we please to call it, which pervades, and makes an essential ingredient in, the composition of all bodies. From such experiments as have been made upon this subject, it appears, that whether fixed air is the bond of connection between the particles of terrestrial bodies or not, yet the emission of it from any substance is always attended with a dissolution of that substance. We cannot, however, in the present case, say that the emission of the fixed air is the cause of the fermentation. It is in fact otherwise. Fixed air hath no tendency to fly off from terrestrial substances with which it is united; on the contrary, it will very readily leave the atmosphere, after it hath been united with it, to join itself to such terrestrial substances as are capable of absorbing it. The emission of it, therefore, must depend upon the action of some other fluid; most probably the fire or heat, which is dispersed thro' all substances in a latent state, and in the present case begins sensibly to manifest itself. But from what cause the heat originally begins to operate in this manner, seems to be entirely unknown and inexplicable, except that it appears somehow or other to depend on the air; for, if that is totally excluded, fermentation will not go on.