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The theory of fermentation has occupied the attention of chemists ever since the manufacture of ale began to be attended to by men of science, but it is only of late that much light has been thrown ugon the subject. Lavoisier was the first person who attempted to give any thing like a theory of this intricate process. He attempted to determine the composition of common sugar, a substance which may be fermented just as well as the soluble part of malt, and which yields similar products. He endeavoured, likewise, to determine the constituents of alcohol, the substance formed by fermentation. With these data, and with a knowledge of the composition of water and carbonic acid, he formed a plausible theory, which was valuable as a first approximation, though there can be little doubt that it was erroneous in every particular. Since that time, several experiments on the subject have been made by Thenard. Gay-Lus-sac and Thenard, and Berzelius, have determined the constituents of sugar with much care; and Theodore de Saussure has made very elaborate, and we believe accurate, experiments on the composition of alcohol. These facts will enable us to form a conception of what takes place during fermentation. We shall first state the general theory, as resulting from experiments on common sugar, and then give some experiments which we ourselves have made on the saccharine matter of malt.

* If a weak solution of sugar in water be kept in a warm place, it will ferment of itself, and be converted into a spiritous liquor. This we have tried more than once, and always successfully, provided the weather was warm. A solution of sugar of grapes in water ferments still more speedily. This is said likewise to be the case with sugar of starch, and, of course, with the saccharine matter of malt. In our general view of fermentation, then, we may leave out of view the small quantity of yeast; because it is not absolutely necessary, but seems merely to render the effect more rapid, and, consequently, prevent the change of the liquid into acidity, which almost always takes place when the fermentation is slow.

When the fermentation is complete, the sugar disappears altogether, and two new substances are found in its place, namely, carbonic acid and alcohol. All that happens, then, is the resolution of sugar into the two new substances, carbonic acid and alcohol. It is requisite to know how much of each of these substances is formed from a given weight of sugar.

According to Lavoisier's experiments, 100 parts of sugar yielded, when fermented,

' Alcohol, 5770

Carbonic acid, 35*34

He does not give us the specific gravity of his alcohol, but it could scarcely be less than 0*825 ; for when his experiments were made, alcohol of greater strength was scarcely known. Now, such alcohol contains at least 11 per cent, of water, for that quantity has been actually extracted from it. From Saus-sure's experiments, it is probable that the real quantity of water contained in alcohol of the specific gravity 0-825, is 18-387 per cent., or almost a fifth. On this supposition sugar, according to Lavoisier's experiments, yields

Alcohol, 471 Carbonic acid 35*34

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