92 FERMENTATION the sense of chemical arithmetic, appeared to assert some thing which ran contrary to the known chemical tendencies of the substances concerned. To explain this by an example, let us take the case of the souring of milk, a fer mentation which, when stripped of what, from the purely chemical standpoint, would appear unessential, involves only one reaction, which consists in this, that the milk sugar of the milk, by a mere rearrangement of its ultimate in gredients, passes into lactic acid, according to the equation C 12 H. 2a O n .H 2 = 4C 3 H 6 ? . Milk Sugar. Lactic Acid. Now, this in itself is nothing exceptional. A solution of cyanate of ammonia (NCOH.NH 3 ) is no sooner prepared than it passes into one of urea CO(NH 2 ) 2 ; cyanic acid, (NCHO) when left to itself, soon passes info cyamelide, just as the milk sugar of the milk passes into lactic acid. But there is this great difference, that this latter change cannot be realized, under any known set of conditions, in a solu tion of pure milk sugar in pure water. And so it is in all other analogous cases. But this comes to the same as say ing that fermentations, as a class of chemical reactions, are characteristically non-spontaneous, and consequently must be caused by reagents, although these reagents have no place in the mere balance-sheet of the reaction. In fact, experi ence show r s that no fermentable chemical species will ferment except in presence of water, and unless it be kept by means of that water in direct contact with some specific "ferment," which, although it contributes nothing to the substance of the products which figure in the equation, nevertheless in duces the reaction " by its presence," as the phrase goes. The presence alone, of course, will not do. It is simply inconceivable that a reagent should act chemically unless it were itself in a state of chemical change, although this change may be (and with some ferments probably is) a cycle of changes which always brings back the reagent to its original condition. Of all the multitude of chemical processes which fall under our heading, vinous fermentation is the one which is by far the best known and most satisfactorily explained; and it is scarcely an exaggeration to say that the present science of the whole subject has been evolved from the study of that particular case. Hence the best course that we can adopt in this article is to begin with a popular exposition of the growth of our knowledge of vinous fermentation, which may familiarize even the general reader with the main points of the whole subject, and then to append a short epitome of the facts concerning the more important of the different fermentative changes. Vinous fermentation means that peculiar change which all native sacchariferous juices are liable to undergo when left to themselves at the ordinary temperature, and which results in the formation of some kind of "wine." The general course of the phenomena being the same in all cases, we shall assume in what follows that it is grape juice we have to deal with. Such juice, as is well known, when recently prepared, forms an intensely sweet yellowish liquid, which, if it is not so by nature, may be rendered perfectly limpid and transparent by filtration through bibulous paper. Grape juice when left to itself, after having been thus clarified, may remain unchanged for an indefinite time, but when mixed with ever so little of unfiltered juice, it is sure sooner or later to undergo a change, which manifests itself in the appearance of a turbidity in the liquid. This tur bidity is owing to two causes, namely, (1) the evolution of carbonic acid, and (2) the formation within the liquid of a finely-divided solid, which, through the gas-evolution, is partly kept in suspension, partly thrown up to the surface as a scum, and which is known by the name of "yeast." The process, from an almost imperceptible beginning, gradually develops into a more and more vivid effervescence (which not unfrequently assumes tli3 character of a violent ebulli tion), the yeast at the same time becoming more and more abundant; and when a sufficient quantity of "must" is oper ated on, the temperature of the fermenting mass soon rises perceptibly beyond that of the surrounding air. Sooner or later of course the reaction reaches a climax, from which onwards it gradually loses in intensity until at last it dies out. The yeast then settles down as a slimy deposit, above which there is left a clear yellow liquid, which, instead of the originally sweet, now has a " vinous " taste, and is en dowed with that well-known physiological action character istic of " fermented liquors." Chemically the change in the nature of the liquid consists substantially in this, that the sugar has mostly or perhaps wholly disappeared, and given place to a corresponding percentage of a volatile in flammable liquid called alcohol. To any one who has a real knowledge of these facts it must necessarily suggest itself as a highly probable hypothesis that it is the destroyed sugar which has furnished the ingredients for the formation of the carbonic acid and of the alcohol, while most persons will be inclined to look upon the yeast as a bye product formed from the secondary constituents of the juices. This view, which is endorsed substantially by our present knowledge of the matter, one is inclined to think should have forced itself even at the earliest times upon the minds all who reasoned on the process as a material meta morphosis. But although now-a-days everybody looks almost instinctively upon chemical reactions as nothing more than rearrangements of the ultimate ingredients of the bodies concerned, which ingredients in themselves are, as a matter of course, assumed to be uncreatable and inde structible, we must not forget that this notion dates back only to the time of Boyle, and that is not much longer than air, and gases generally, have been recognized as species of weighable matter. Hence for many centuries the carbonic acid was not recognized even by chemists as forming a fac tor in the chemical reaction ; it was known only as an effervescence, a phenomenon pure and simple, not as a sub stance. Van Helmont (born in 1577) was the first to show that the gas which rises from fermenting "must" is dif ferent from air, and identical with the gas sylvestre formed in the combustion of charcoal and in the calcination of lime stone. Long before Van Helmont s time, the "alcohol" had been recognized as a definite kind of matter. The art of concentrating the intoxicating principle of wine by dis tillation, in fact, was known and practised industrially in the 8th century ; and nobody could practise this art with out finding out that a spirit can be strengthened by repeated distillations, with elimination of water. But it was only about the 13th century that chemists learned to remove all the water from spirits of wine, and thus to prepare "absolute," that is, pure alcohol. Ordinary cane sugar and honey were known to the ancients ; and chemists from the earliest times took it for granted that these two substances and the sweet principles in fruit juices must be closely related to one another. It is also an old experience that cane sugar or honey when added to grape juice ferments with the sugar originally present in the latter. But the idea that the differences be tween the several kinds of sugar are owing to the existence of a number of distinct chemical species is comparatively new, and it is only in the course of the present century that the problem of isolating these several species has been satisfactorily solved. But to return to our proposition ; plausible as it is as an hypothesis, to be able to test even its potential correct ness, we must know the weights of alcohol and carbonic acid produced in the fermentation of a given weight of sugar, and know also the quantitative elementary compositions of the three substances. Lavoisier was the first to make ex-
