FERMENTATION is a chemical process which has been already considered in the Encyclopaedia, and will be again resumed in this Supplement under the title Animal and Vegetable SUBSTANCES. In this place we mean nothing more than to give such directions, principally especially from Mr Richardson of Hull, for the proper fermentation of malt liquors as have not been fully detailed in the article BREWING (Encycl.). This author controverts, we do not think very successfully, the conclusions drawn by Mr Henry from the experiments, of which the reader will find an account in the article FERMENTATION (Encycl.); but it is not his theory with which we are at present concerned, but his practice as that of an experienced, and enlightened brewer. Having treated of Worts, and the proper method of boiling them, for which see WORT in this Supplement, and having given an historical view of the process of fermentation, of which a pretty accurate abridgement is inserted in the articles BREWING and FERMENTATION (Encycl.), he proceeds thus: "The agency of air, in the business of fermentation, is very powerful; but as all fermentable subjects have an abundant supply, we are rather to provide for the egress of their own, than to suffer the admission of the external air, by which a great number of the fine, volatile, oleaginous parts of the subject would be carried off, and a proportionate injury in flavour and spirituousness sustained. Hence such a covering should be provided for the gyle-tun as would barely allow the escape of the common air produced by the operation; whilst the gas, or fixed air, from its greater density, resting upon the surface of the beer the whole depth of the curb, prevents the action of the external air, and consequently the escape of those fine and valuable parts just mentioned. "But towards the conclusion of vinous fermentation, this aerial covering begins to lose its efficacy; which points out the necessity of then getting the beer into casks as soon as possible, that the consequences may be prevented, of exposing so large a surface, liable to so copious an evaporation. Amongst these, a loss of spirituousness is not the least; for this evaporation is more and more spirituous, as the action approaches the completion of vinous fermentation; and that once obtained, the loss becomes still more considerable, if still exposed to the air; whence it might be termed the distillation of Nature, in which she is so much superior to art, that the ethereal spirit rises pure and unmixed, whilst the highest rectification of the still produces at best but a compound of aqueous and spirituous parts. "Nor is this entirely conjecture. Experience teaches us, that we cannot produce so strong a beer in summer, ceteris paribus, as in winter; the reason is, not because the action of fermentation does not realize so much spirit in warm weather, but because the fermenting liquor, after the perfection of vinosity, continues so long in a state of rarefaction, that the spirituous parts are dissipated in a much greater degree at that time than at any other, in a similar state of progression. And this doctrine of natural distillation seems to account for that increase of strength obtainable from long preservation, in well closed casks, and, more particularly so, in glass bottles; for Nature, in her efforts to bring about her grand purpose of resolving every compound into its first principles, keeps up a perpetual internal struggle, as well as an external evaporation; and if the latter be effectually prevented, the former must be productive of additional spirituousness, so long as the action keeps within the pale of vinous fermentation. "In order to maintain a due regulation of the fermenting power, and to answer the several purposes of the operation, a scrupulous attention to the degree of heat at which the action commences, and a particular regard to the quality and quantity of the ferment employed, are indispensably necessary." The degree of heat must be ascertained by the thermometer, and regulated by experience: the quantity of yeast can be ascertained only by the intention of the artist; but of the quality of that substance we shall treat under YEAST in this Supplement.