FERMENTATION.

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ence is in the proportions of their elements. It is wellknown that when a solution of sugar ferments, it ischanged into spirit; it parts with a portion of carbonand oxygen, and the elements left constitute spirit;hence, during fermentation carbonic acidis given off (77).Again, when spirit and water is exposed to the air andmoderate warmth, it soon changes into vinegar; thischange is wholly effected by the absorption of a littleoxygen from the air.

255. Fermentation is a very singular process, and aknowledge of the effects which it produces enables us tounderstand many changes which would otherwise appearincomprehensible. Under ordinary circumstances, puresugar, dry or dissolved in water, may be kept for a longtime without its undergoing any change ; when, how-ever, it is mixed with a small quantity of certain matters,it ferments and is changed into spirit. The substanceadded does not combine with the sugar or its elements,but, whilst itself decomposing, it causes the sugar alsoto change.

256. Fermentation, then, is the spontaneous decom-position of a substance, occasioned by the presence of asmall quantity of decomposing matter. The yeast, or fer-ment of beer, possesses the power of inducing the decom-position of sugar and similar substances when mixed withthem. Common yeast, and all substances which possessthe power of causing fermentation, contain nitrogen ; theyare compounds of carbon, oxygen, hydrogen, and nitro-gen, and are accordingly very liable to decompose.