What in all these and similar processes strikes the ordinary observer as something particularly characteristic is their spontaneity: sweet milk turns sour, grape juice passes into wine, wine into vinegar, vinegar into a foul insipid fluid—without the application or addition from without of any agent or reagent; but the “spontaneity” in the eyes of the chemical investigator does not go far. All chemical reactions are spontaneous; and wherever a ease occurs of two things acting upon each other, it makes no difference whether one of them be added, say, to the solution of the other from a bottle, or whether it were present in the liquid from the first. What caused chemists to group together fermentative changes as a class of phenomena different from ordinary reactions was the fact that wherever they succeeded in reducing the phenomena to a degree of simplicity sufficient for translating the respective reaction into an equation, this equation, though perfectly correct in the sense of chemical arithmetic, appeared to assert something 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 fermentation 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 ingredients, passes into lactic acid, according to the equation C 12 H 22 O 11 .H 2 O = 4C 3 H 6 O 3 .

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 j cyanic acid, (NCHO) when left to itself, soon passes into 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 solution of pure milk sugar in pure water. And so it is in all other analogous cases. But this comes to the same as saying 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, experience shows 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 induces 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 turbidity 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 the character of a violent ebullition), the yeast at the same time becoming more and more abundant; and when a sufficient quantity of “must” is operated 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 endowed with that well-known physiological action characteristic 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 inflammable 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 metamorphosis. 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 indestructible, 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 factor in the chemical reaction; it was known only as an effervescence, a phenomenon pure and simple, not as a substance. Van Helmont (born in 1577) was the first to show that the gas which rises from fermenting “must” is different from air, and identical with the gas sylvestre formed in the combustion of charcoal and in the calcination of limestone. 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 distillation, in fact, was known and practised industrially in the 8th century; and nobody could practise this art without 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 between 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 correctness, 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 experiments for supplying these data, which, in fact, could not reasonably have been attempted by anybody before him, as it is he to whom we owe our knowledge of the qualitative elementary composition of the substances concerned, and indeed of organic substances generally.

Before giving his numbers, it may be stated that he regarded acetic acid (a small quantity of which is present in most wines) as, like alcohol and carbonic acid, a constant product of vinous fermentation. According to Lavoisier, 95 , 9 parts of cane sugar in fermenting yield [table]

And according to his elementary analysis of these substances, the proportions by weight are— [table]

From these numbers Lavoisier concluded (and he was quite justified in doing so, considering the imperfections of his methods of analysis) that sugar in fermenting simply breaks up into these three substances, without any access of matter from without. But if he thus managed to arrive at what we now know to be a substantially correct conclusion, this can be credited to him (if at all) only as a happy stroke of divinatory genius, as his numbers are all of them monstrously incorrect, the errors going far beyond what even, with his necessarily imperfect method, could be tolerated as “observational errors.” Lavoisier’s numbers were subsequently corrected by Gay-Lussac according to his own analyses of sugar, alcohol, and carbonic acid. His results, which have remained unimpugned to the present day, may be stated, with substantial correctness, to have been as follows :—

In vinous fermentation very nearly one-third of the carbon goes off as carbonic acid, while the rest passes into the alcohol; and reducing to 1, 2, and 3 parts of carbon, we have [table]

The agreement being by no means satisfactory, Gay-Lussac suspected that his analyses of sugar were infected with unobserved errors, and he corrected his figures so as to make them agree with those given above opposite to “Sums.” These values, when measured by the customary units (namely C for twelve parts of carbon, H for one part of hydrogen, O for sixteen parts of oxygen), assign to sugar the very simple formula C 1 H 2 O 1 leading to an equally simple equation for the reaction, which is :—

6×(CH 2 O) = C β H la O β = 2C 2 H 6 O + 2CO 2 ; i.e., 180 of sugar gives 2 × 46 of alcohol+ 2 × 44 of carbonic acid; or 45 ,, ,, 23 ,, + 22 ,, ,,

This equation is still looked upon as substantially correct, though not in Gay-Lussac’s sense. It is so, if by sugar we understand either of the tw o kinds of “glucose” which form the bulk of the sweetening principles in fruit juices, and which are composed according to the formula C 6 H 12 O 6 . Cane sugar, as Dumas and Boullay showed, really has the composition following from Gay-Lussac’s analysis, which, as is easily seen, corresponde to the formula C 12 H 22 O 11 = 2C 6 H 12 O 6 - H 2 O, where H 2 O means the elements of 18 parts of water; and these 18 parts of water, as Dumas and Boullay showed, actually are taken up in the fermentation of C 12 H 2θ O n = 324 parts of cane sugar.

Gay-Lussac’s equation being, as we said, only substantially correct, we must now state the qualifications implied. Schmidt of Dorpat found in 1847 that vinous fermentation always results in the formation of small quantities of succinic acid. Guerin Vary showed, by quantitative experiments, that in the fermentation of glucose the alcohol and carbonic acid produced account only for about 96 , 3 per cent, of the glucose. And the present writer happens to know that a certain German apothecary made the interesting discovery that wines, beside the unfermented remnant of glucose that may be left, may contain an unfermentable sweet principle which he recognized as glycerine. These observations, however, were little heeded until Pasteur, in a now classical memoir, proved that glycerine and succinic acid are constant products of normal vinous fermentation, the correct balance sheet of the reaction, according to him, being as follows :—100 parte of cane sugar, in fermenting, pass into 105 ’ 4 parte of glucose, which then break up, yielding (approximately) of [table]

But even this is not quite an exhaustive statement, a small portion of the sugar always passing into the form of higher alcohols (“fusel-oil”) and compound ethers.

Vinous fermentation, then, is a far more complex reaction than Gay-Lussac imagined; but it still remains true that all the products formed are derived from the dissociation of the sugar. What is it that brings about this singular decomposition! We call it a singular reaction, because it is one which sugar has never been seen to undergo when subjected by itself or as an aqueous solution to the action of heat or electricity or any ordinary reagent. And we have theoretical grounds for presuming that the reaction is not likely ever to be realized by some “reagent” that has not yet been tried. According to many experiences, an arithmetically possible reaction is the more likely to be realized the greater the heat evolution which, supposing it were realized, it would involve. Now, the reaction formulated in Gay-Lussac’s equation C β H 12 O 6 = 2CO 2 + 2C 2 H 6 O, as Professor Dewar pointed out some years ago, supposing dry sugar could be made thus to split up, would yield only an insignificant amount of heat, if any. Actual fermentation does involve a liberation of heat, as we know, but the quantity of heat per unit weight of sugar destroyed, according to Dewar’s experiments, amounts only to about 83 heat-units, which can be accounted for as being produced by the hydration of the alcohol formed, and, at any rate, is too small to characterize the decomposition of sugar into carbonic acid and alcohol as being at all of itself a probable reaction. Even the somewhat higher result previously arrived at by Dubrunfaut, namely, 135 heat-units per unit of sugar, cannot affect this conclusion. Before going further let us take an exact survey, from the chemical standpoint, of the conditions which are known to favour or impede the actual process.

(1.) Pure solutions of cane sugar or glucose do not ferment under any circumstances.

(2. ) Many kinds of impure sugar solutions, such as grape juice, brewers’ wort, &c., do ferment. The range of temperatures most favourable to this process lies between about 20* and 24* C. (or 68° and 75° F. ). But even grape juice does not ferment at temperatures lying too close to the freezing-point, nor does it fermentat temperatures exceeding a certain limit, which lies at about 60° C. (140 δ F.). The most lively fermentation comes to a stop when the liquid is boiled, and, after cooling, it takes a longer or shorter time before it resumes.

(3.) Grape juice which has been strengthened by evaporation or addition of sugar from without, does not ferment, when the ratio of water to sugar falls below a certain limit-value.

(4.) Fermentation is impeded and may be entirely stopped by addition of alcohol. Hence the wines produced from the rich juices of southern grapes always contain unfermented sugar.

(5.) Fermentation may be stopped more or less completely by addition to the liquid of even small quantities of certain reagents called antiseptics. Of these corrosive sublimate (and many other heavy metallic salts), sulphuric acid, sulphurous acid, bisulphide of carbon, and carbolic acid may be mentioned as examples.

(6.) Perfectly pure grape juice does not ferment, unless the process has been started by at least temporary contact with ordinary air. This cardinal fact was discovered by Gay-Lussac in a now classical series of experiments. He caused clean grapes to ascend through the mercury of a large barometer into the Toricellian vacuum, where he crushed them by means of the mercurial column. The juice thus produced and preserved remained unchanged; but the addition to it of ever so small an air-bell (as a rule) induced fermentation, which, when once started, was always found to take care of itself.

(7.) Ordinary vinous fermentation always involves the formation of yeast. This is the most important of positive facts made out.

(8.) The rate at which a fermentation progresses is (in a limited sense) determined by the quantity of yeast present within the liquid.

(9.) Spontaneous fermentation of grape juice is always slow in beginning; addition of yeast from without starts it immediately.

From these facts it is legitimate to conclude that it is the yeast or some constituent of the yeast which somehow or other causes the sugar to break up into alcohol, carbonic acid, glycerine, and succinic acid. But what is the rationale of the action? Chemically speaking, it would appear to be vain to attempt an answer without first knowing what yeast is made of in the chemical sense. Unfortunately the present state of our knowledge on this point is very unsatisfactory. All we know is that yeast is a highly complex mixture of chemical substances which may be arranged under the four heads of—(1) fats (forming about 2 per cent, of the whole); (2) cellulose (about 18 per cent.); (3) nitrogenous substances more or less closely allied to white of egg, some of them soluble, some insoluble in water (about 60 per cent.); (4) incombustible salts, which, when the yeast is burned, remain as “ash” (about 7 per cent.). According to Mitscherlich’s analysis, yeast-ash consists mainly of phosphoric acid (54 to 59 per cent.), united with potash (28 to 40 per cent.), magnesia (6 to 8 per cent.), and lime (1 to 4 per cent ). That such a complex mixture should act, chemically, as a whole cannot reasonably be assumed; chemists, accordingly, have always been unanimous in thinking that it is some one constituent or set of constituents of the yeast which constitutes the characteristic reagent in vinous fermentation; but none of them has succeeded in isolating that reagent. The only clue in this respect which we have is an important discovery of Mitscherlich’s, who showed that an aqueous extract of yeast, although capable of converting cane-sugar into glucose, does not induce fermentation in the glucose formed, whence it at once follows that the ferment must be sought for amongst the insoluble portion of the yeast.

Their non-success in isolating the vinous ferment did not prevent chemists from speculating on its mode of action. Berzelius gave it as his opinion (which was adopted by Mitscherlich and others of the leading chemists) that the action was a purely “catalytic” one. What he meant by this is best explained by an example. Peroxide of hydrogen (a compound of the elements of water and oxygen) is perfectly stable at ordinary temperatures. Add to it a mere speck of platinum black (a peculiar form of finely divided platinum), and it at once breaks up into water and oxygen, the platinum which caused the decomposition remaining unchanged. In an exactly similar manner Berzelius thought the yeast acted upon the sugar, and caused it to break up into alcohol and carbonic acid. The merit of the idea was that it apparently reduced the explanation of a seemingly complex to that of an undoubtedly simpler phenomenon. But unfortunately neither Berzelius nor any of his followers succeeded in...