It is usual to put fermented liquors in to casks before the vinous fermentation is completely ended ; and in these closed vessels, the change goes on for many months. But if the fermentative process be suffered to proceed in open vessels, more especially if the temperature be raised to ninety degrees, the acetous fer mentation comes on. In this a still great er portion of oxygen from the air is gra dually absorbed ; and this more especially as the surfaces of the liquor are oftener changed by lading it from one vessel to another. The usual method of doing this consists in exposing the, fermented liquor to the air, in casks placed so that the sun may shine on them ; which seems to be of advantage by raising the tempe rature of the liquor. By this absorption of oxygen the inflammable substance be comes converted into an acid. If the liquor be then exposed to distillation, vinegar comes over instead of alcohol or spirit.
When the spontaneous decomposition is suffered to proceed beyond the acetous process, the vinegar gradually becomes viscid and foul ; a gas is emitted with an offensive smell ; ammonia flies off, an earthy sediment is deposited, and the re maining liquid, if any, is mere watcf: This is the putrefactive process.
Though fermentation is much better understood at present, in consequence of modern researches into the nature of the gases, than it formerly was, it still re mains an interesting object of research. It is not clearly ascertained what the yeast or fermented matter performs in this operation. It seems probable that the fermentative process in considerable masses would be carried on in succes sion from the surface downwards ; and would perhaps be completed in one part of the fluid before it was perfectly begun in another part, if the yeast, which is al ready in a state of fermentation, did not occasion the process to begin in every part of the fluid at once. Experiments
yet remain to be made towards ascertain ing the arrangements and quantity of the component parts of alcohol. It appears that hydrogen in combination with car bon and water, in certain proportions, form this compound ; that a greater pro portion of oxygen converts it into vine gar ; and that in the putrefactive pro cess the hydrogen, carbon, and oxygen, are separated from each other, and fly off in the elastic state.
In the fermentation of wine, the tartar, which probably existed for the most part already formed in the juice of the grape, is separated, and exhibits the properties which are described in treating of that substance.
The fermentation of brem.1, by leaven, is thought to be of a different nature from the vinous fermentation, In this, the mucilage of the corn is not previously brought into the saccharine state. It quickly becomes sour, if the process be not stopped by baking ; in which parti cular the fermentation seems to be of the acetous kind. The developement of carbonic acid divides the dough into thin parts, which are more effectually and better baked than they could have been in the solid consistent mass. When bread is fermented by means of yeast, the process seems to be of a saccha rine or vinous nature. A very minute proportion of alum renders bread whi ter, and its pores more small and nu merous, but how it acts has not been ascertained. It does not seem, either from its quantity or quality, to be un wholesome.