Wines never totally remain inactive; fermentation in some degree continues, and in time the oils, by being greatly attenuated, volatilise, fly off, and permit a readier admission of the external air into the drink. In proportion as this circumstance takes place, the latent acids of the liquor shew themselves, the wine becomes sour, and in this state is termed vinegar. Its last stage or termination is, when the remaining active principles, which the vinegar possessed, being evaporated in the air, a pellicle forms itself on the surface of the liquor, and dust and seeds, which always float in the atmosphere, depositing themselves thereon, strengthen this film into a crust, on which grows moss, and many other small plants. These vegetables, together with the air, exhaust the watery parts; after which no signs of fermentable principles remain but, like the rest of created beings, all their virtues being lost, what is left is a substance resembling common earth. Upon the whole, then, it appears, that a liquor fit for fermentation must be composed of water, acids smoothed over with oils, or saccharine salts, and a certain portion of elastic air; the heat of the air the liquor is fermented in, must be in proportion to the density of its oils; and lastly, that the pores are to be expanded by slow degrees, lest the air, by being admitted too hastily, should cause an effervescence rather than a fermentation, and occasion the whole to become sour. Wines, therefore, fermented in countries where the autumn is hot, require their oils to be more pinguious, than where the season is cooler. For the same reason beers are best made, when the air is at forty degrees of heat, or below the first fermentable point, because the brewer, in this case, can put his wort to work, at a heat of his own chusing, which will not be increased by that of the air; on the contrary, when, by its internal motion, the heat becomes greater, it will again be abated and regulated by the cold of the medium. The pores of a wort are expanded in proportion to the heat it is impressed with; on which account common small beer, brewed in summer, when the air and acids more easily insinuate themselves into the liquor, ought to be enriched with oils obtained by hotter extracts, to sheath these salts; and in winter the contrary method must be pursued. From this history of fermentation, we can, with propriety, account for the many accidents and varieties that accompany this act; and a comparative review of some of them may not be unnecessary. A cold air, closing the pores of the liquor, always retards, and sometimes stops, fermentation; heat, on the contrary, constantly forwards this act; but, if carried too high, immediately prevents it. A must, loaded with oils, will ferment with more difficulty than one which abounds with acids; it likewise is longer before it becomes perfectly fine; but, when once so, will be more lasting.