Fermentation is, strictly speaking, a chemical process, and 'one of the most obscure phenomena in nature, which all the ingenuity of philosophers has hitherto been unable to explain. Instead, therefore, of perplexing the reader with different theories on the sub-ject, we shall briefly relate the practical part of this interesting process, together with the circumstances attending it.

Fermentation may be defined to consist in a visible internal commotion of different bodies, reduced to a fluid state ; emitting bubbles of air, and a sparkling, pungent, vapour. But, more properly speaking, it is a gradual and spontaneous change of a body, consisting of different ingredients variously mixed, and which are now decomposed and converted into a vinous liquor. Thus we obtain, according to the methods afterwards pursued, wine, ardent spirits, beer, or vine-gar. - Hence fermentation is confined to the vegetable and animal kingdoms; and is divided into three regular stages; namely, the vinous, acetous, and putrefactive. Vegetables only are susceptible of the first ; the flesh of young animals in a slight degree undergoes the second ; and all animal sub-stances are peculiarly subject, to the last stage, or putrefaction.

The most essential requisites in every process of fermentation, are : 1. That the substances be in a fluid state ; 2. That there be a proper degree of uniform warmth, that is, in general between the 70°. and 80°. of Fahrenheit's thermometer; and 3. That the atmosphere be not entirely excluded from the fermenting bodies, nor that they be exposed to a current of air.

If, in the elementary mixture, or component parts of a vegetable body, there exist a portion of inflammable air, this spirituous ingredient gedient will be disengaged at the very commencement of fermentation : hence we obtain wine, brandy, cider, beer, etc. from grapes, apples, pears, and other fruit, from every species of corn, as well as from saccharine and mealy roots. Their productions, however, so far differ from each other, that wine contains a greater proportion of spirituous, and less of mucilaginous particles, than beer ; and that distilled spirits are deprived of all earthy and viscous ingredients. But, as all fermentable bodies, beside the inflammable spirit, possess a portion of acid and saline particles, which are not disengaged during the first, or vinous stage of fermentation, another separation of constituent parts takes place, im-mediately after the former is effected, without any farther discharge of air-bubbles, or intestine commotion of the fluid ; though a Volatile elastic vapour is observed to escape: thus, the spirituous parts, unless they have been previously drawn off by distillation, are communicated to the atmosphere, and this stage is termed the acetous fermentation; because its productions are the different sorts of vinegar obtained from wine, beer, fruit, corn, etc. - Although, in most of the fermentable sub-stances, these two stages naturally succeed each other; yet, by improper treatment, the acetous fermentation sometimes appears before the vinous can possibly commence, especially where the process is mismanaged by too great a heat ; or, in those bodies which possess little or no inflammable matter in their elements. On the contrary, such vegetables as ori-ginally contain a sufficient propor-tion of aerial and fiery constituents, will easily ferment, by the simple means of warmth and water. But, if those elementary in-gredients be in a manner deprived of their activity, by too many crude and viscid particles being combined with them, it will then be necessary to make certain additions, partly natural, and partly artificial, in order to dispose them more readily to ferment. These means, or additions, are such as have either already undergone fermentation ; or are easily disposed to ferment: of the former kind are yeast and leaven ; of the latter, honey, sugar, especially in a state of molasses, and other sweet substances, which, however, but slowly promote fermentation ; nay, if they be previously diluted or dissolved in too hot water, and in that state added to the fermentable materials, they will entirely check that process. There are, besides, other means of promoting it; for instance, the dried leaves of the vine in a state of powder ; cream of tartar, especially after it has been repeatedly moistened with strong vinegar, and afterwards dried; the crumb of bread prepared in a similar manner, and reduced to powder, etc.

If fluidity, warmth, and fresh, air, forward the fermentative process, the contrary of these, namely, dryness, cold, and exclusion of air, inevitably tend to prevent it. - There are, however, cases in which it may become necessary to impede its progress; and we may then safely resort to the means above alluded to; - But a certain, degree of heat, such as we have before stated, appears to be indispensably necessary to conduct that process with success : an undue continuance, or the least increase of heat, proves detrimental, while an appropriate temperature, in a re-markable degree promotes fermentation. These different point of heat should be accurately noted and settled by the thermometer, or other certain methods ; though, for common, or all economical purposes, they may be limited to what is in general termed a tepid and a fervid heat: the former is the bane of all vinous fermentation ; the latter, or imperceptible warmth, is the great promoter of it. And if, notwithstanding a due attention to a proper temperature and all other circumstances, the liquor will not work of itself, it should then be as sisted by such substances as are called ferments, and of which we have already given some account.

In the Memoirs of the Philosophical Society at Manchester, Mr. Henry states the result of some experiments, in which he produced a fermentation both in bread and wort, and even in punch and whey. Conje6turing, therefore, yeast to be simply a quantity of fixed air detained among the mucilaginous parts of the fermenting liquor, he boiled some wheaten flour and water to. the consistence of a thin jelly which he put in the middle of Dr Nooth's machine for communi cating fixed air to water. A con-siderable portion of gas was absorbed ; and the next day the mass was in a state of fermentation. - The third day it bore so great a resemblance to yeast, that an experiment was made on some paste for bread; for which purpose it answered tolerably well, after being baked four or five hours.

Mr. Henry made another experiment with some wort only ; part of which was impregnated with air in the same manner as the four and water, and when poured into the remainder, a brisk fermen-tation ensued in 24 hours : a strong head of veast began to collect on the surface, which on the third day was fit for tunning. In the course of the experiment, good bread was made with the yeast taken off the surface.

The dispute which has arisen concerning Mr. Henry's mode of producing fermentation, may be easily decided by a comparative trial. Let two gallons of wort be put into a separate vessel, and kept in a moderate heat for a certain time : let also two other gallons be impregnated, either wholly or in part, according to Mr. Henry's method, be put into a similar vessel, and deposited in the same place. If the fermentation commence in the liquor impregnated with fixed air sooner than in the other, the air may be rationally conjectured to induce such fermentation. At all events, Mr. Hen-ry's experiments, with respect to bread, are certainly decisive, and those relative to liquors may thus be easily ascertained ; an object of the utmost importance to the public.