3. The strong pungent volatile spirit, which exhales from a must, when under full fermentation, has been supposed to be a loss, which might be prevented; and accordingly attempts have been made to retain these flying impetuous particles, by stopping the communication between the atmosphere and the fermenting drink. That there is a dispersion of spirits is beyond doubt, and that these exhaling vapors consist of the finest oils, which the heat forces out of the must, is equally certain. But this loss seems to be abundantly repaid by the stronger oils, which the same degree of heat attenuates and substitutes, in a larger quantity, to the former. The last oils could never come under the form of a vinous liquor, but by a power, which sooner or later dissipates some of the first. Pale ales or amber not only lay, for many days, exposed to the open air, but suffer, by the periodical renewal of the action of the air, every two or four hours, a much more considerable loss of spirits, than when fermentation is carried on uniformly. Yet experience shews, that so many oils are, by this method, attenuated, that the strength acquired greatly surpasses that which is lost. 4. The practice of fermenting _by compression_, recommended to distillers, seems, on this account, less useful, than might be concluded from theory alone; the intent of the distiller, as well as of the brewer, is to extract the greatest quantity of spiritous oils. It is impossible to ferment a must _in vacuo_; air is absolutely necessary to carry on this operation, even a superabundant quantity of oils admitted into the must, by obstructing the free admission of the air, impedes fermentation, prevents the wine from reaching pellucidity, and sometimes is the occasion of its becoming putrid.