1902 Encyclopedia > Fermentation


FERMENTATION, a chemical term, which, in accordance with its derivation from fervere (to boil), was originally applied indiscriminately to all chemical changes involving the effervescence of a liquid, but which, in its modern accept-ation, has in itself nothing at all to do with effervescence, being used to designate a peculiar class of metamorphoses which certain complex organic materials are liable to, and of which the well-known change which grape juice under-goes when it "ferments " into wine, the souring of wine or milk, and the putrefaction of animal or vegetable matter may be cited as familiar examples.

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 case 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 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 iu-gredients, passes into lactic acid, according to the equation
C12HMOu.H20 = 4C3H603.

Milk Sugar. Lactic Acid.

Now, this in itself is nothing exceptional. A solution of cyanate of ammonia (NCOHNHs) is no sooner prepared than it passes into one of urea CO(NH2)2; 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 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 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 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 the 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 of 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 different 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 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 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 hag been satisfactorily solved.

But to return to our proposition; plausible as it ic 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 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 con-cerned, and indeed of organic substances generally.
Before giving his numbers, it may be stated that he re-garded 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

Alcohol 577
Carbonic acid 35'3
Acetic acid 2-5

And according to his elementary analysis of these sub-stances, the proportions by weight are—
Carbon. Hydrogen. Oxygen.
In 35'3 of carbonic acid .... 9'9 nil 25"4
In 57 7 of alcohol 16-7 9-6 31 -4
58-5 61-4
In 2-5 of acetic acid 0-6 0"2 17
27-2 268
In 95-9 of sugar 26 8 7'7

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 mon-strously incorrect, the errors going far beyond what even, with his necessarily imperfect method, could be tolerated as " observational errors." Lavoisier's numbers were sub-sequently 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 al-cohol; and reducing to 1, 2, and 3 parts of carbon, we have
Carbon. Hydrogen. Oxygen. Sum.
In carbonic acid 1 + nil + 2'667 = 3-667
0-5 4-0 = 7-5 0-458 3-667 = 7-125
In alcohol 2 + 0-5 + 1'333 = 3-833
Sums 3
Found in cane sugar .. 3

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 CjHgOj leading to an equally simple equation for the reaction, which is :—

6x(CHsO) = C6H1206 = 2CaHsO + 2COa; i.e., 180 of sugar gives 2 x 46 of alcohol+ 2 x 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 two kinds of "glucose" which form the bulk of the sweetening principles in fruit juices, and which are composed according to the formula C6H1908. Cane sugar, as Dumas and Boullay showed, really has the com-position following from Gay-Lussac's analysis, which, as is easily seen, corresponds to the formula C12H22On = 2C6H19O0 - H20, where H,0 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 tf C12H22On = 324 parts of cane sugar.

Gay-Lussac's equation being, as we said, only substan-tially 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 suc-cinic acid. Guerin Vary showed, by quantitative experi-ments, 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, besides 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 parts of cane sugar, in fermenting, pass into 105-4 parts of glucose, which then break up, yielding (approximately) of

Alcohol 51-1
Carbonic acid 49 '4
Succinic acid 07
Glycerine 3'2
Matter passing to the yeast 1 -0
Sum 105-4

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 de-composition Î We call it a singular reaction, because it is one which sugar has never been seen to undergo when sub-jected 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 formu-lated in Gay-Lussac's equation C6H1206 = 2C02 + 2C2H60, 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 quan-tity 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 hydra-tion 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, can-not affect this conclusion. Before going further let us take an exact survey, from the chemical standpoint, of the con-ditions 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 fermentât temperatures exceeding a certain limit, which lies at about 60° C. (140° 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 ma}' 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 pro-cess 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 grax>es to ascend through the mercury of a large barometer into the Torricellian vacuum, where he crushed them by means of the mercurial column. The juice thus producedand 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 1 Chemically speaking, it would ap-pear to be vain to attempt an answer without first knowing what yeast is made of in the chemical sense. Unfortu-nately 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 unani-mous 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 suc-ceeded 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 hydro-gen (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 proving the objective existence of the analogy by experimental evidence. Hence Berzelius's theory really amounted to no more than showing that vinous fermentation and the " catalytic" reactions of inorganic chemistry were both unexplained phenomena.

Something far more worthy of the name of a theory had been offered 200 years before by Stahl. The originator of the phlogiston theory justly divined that vinous fermenta-tion and putrefaction are phenomena of the same order, and, starting from the well-known infectious nature of the latter, explained both as disturbances in the "molecules" of the fermenting body, brought about by a pre-existing mole-cular motion. " Ein Körper der in der Faulung begriffen ist bringet in einem anderen von der Faulung annoch befreiten sehr leichtlich die Verderbung zu Wege, ja es kann ein solcher, bereits in innerer Bewegung begriffener Körper einen anderen annoch ruhigen, jedoch zu sothaner Bewe-gung geneigten sehr leicht in eine solche innere Bewegung hineinreissen."

These ideas of Stahl's, at the time of Berzelius's catalytic theory, had long been forgotten, and they remained lost to science until they were revived and brought into a more definite form by Justus Liebig, who, in a powerful and com-prehensive memoir on fermentative changes, which he pub-lished in 1848, used them as the basis of a new theory of these phenomena, which justly attracted universal attention, as it—or rather the wonderfully lucid memoir which embodied it—exhibited the subject in a clearer light than anything else that had been said or written on it before. With Liebig as with Stahl, all " fermentations" and "putrefactions" are analogous phenomena. Putrefactions are owing mainly, to the inherent instability of the albuminoid constituents of the respective substances in presence of water. So unstable are these albuminoids that even an incipient oxidation (see Gay-Lussac's experiment) may suffice to disturb their chemical equilibrium to such an extent as to cause the whole of the atoms of the mass to gradually rearrange themselves into new products of lesser complexity and consequently higher stability. The decom-position when once started, readily propagates itself through the whole mass, aided as it is by the inherent tendency of the molecule to pass into more highly stable forms, just as a stone wdiich rolls down a hill and strikes other stones on its way causes them to roll down likewise. This is so clear and plausible as almost to command assent. It is less easy to agree with Liebig when he tries to explain fermentation, when he says, for instance, that the sugar in grape juice, although not naturally gravitating towards a rearrangement as alcohol plus carbonic acid, is nevertheless caused to undergo this change by its immediate contact with the albuminoids of the juice or yeast, which are in a state of atomic commotion; and it is still less easy to see how such an atomic revolution could progress from sugar to sugar, as he says it may. That the nitrogenous matters of the juice, in all ordinary cases of vinous fermentation, assume the form of yeast, is, according to Liebig, a purely accidental phenomenon, and, if yeast is so characteristically powerful as a ferment, it is so only through its consisting largely of exceptionally unstable albuminoid substances.

Liebig's ideas, more perhaps through the brilliancy of his mode of exposition than the force of his arguments, took firm hold of the scientific mind of the time ; amongst chemists at least the general impression was, and it prevailed for a considerable time, that Liebig's theory in a satisfac-tory manner summed up the whole of the empirical know-ledge of the subject—although it totally ignored at least one most important feature in the phenomena which had been brought out and firmly established by Schwann and Cagniard-Latour.

In 1680 a Dutch philosopher, Leuwenhoek, fell upon examining yeast under the microscope, and found it to consist ot minute globular or ovoid particles. Microscopes in his time were very imperfect or he would have made a great discovery. Schwann and Cagniard-Latour, who (about 1838, and independently of each other) resumed the old Dutchman's inquiry, used the better instruments of their time, and discovered that Leuwenhoek's globules are membranous bags, which exhibit all the morphologic characteristics of vegetable cells, and, like these, when brought under the proper conditions, increase and multiply in the biologic sense. Taking this together with the long known fact that in vinous fermentation the yeast increases as the process progresses, they naturally concluded that yeast is a species of plant, and that it is the life of that plant which somehow or other causes the chemical change. It is the special merit of Schwann to have adduced powerful experi-mental evidence in favour of this view. In his case, the observations on yeast were incidental only to a more com-prehensive investigation, the original aim of which had been to solve the great question of spontaneous generation. Pro-cesses of putrefaction having long been known to be invari-ably accompanied by the formation of vibriones and other microscopic organisms endowed with voluntary motion, he prepared infusions of flesh and other putrescible matters in glass flasks, and, after having hermetically closed these, ex-posed them for a time to the heat of boiling water, so as to destroy every trace of living germs that might be present. The contents, when preserved in that condition for ever so long, showed no sign of putrefaction or of life of any kind. But when exposed to the air they did putrefy, and soon swarmed with living organisms of various kinds. Obvi-ously it was the air which caused this two-fold change. But then the air which had been shut up with the infusions did not act. This, however, might have been owing to an absorption of the oxygen by the juices. Schwann therefore, in another set of experiments, allowed the boiled (and con-sequently germless) infusions to communicate freely with the atmosphere, in such a manner, however, that no particle of air could enter the flasks without having first passed through a red-hot glass tube, and thus been freed from any germs that might float about in it. In this case the air had fair play in a chemical sense, but yet, not only did no life of any kind make its appearance, but even the chemical changes failed to set in. Exactly similar results were ob-tained by Schwann in experiments with grape juice, whether previously mixed or not with yeast. Gay-Lussac's famous experiment failed when the air-bell, before being admitted to the juice, had been heated, and thus freed from living germs. In a few of these experiments, it is true, the re-sults were contradictory to the general evidence afforded by the rest of the work. But Schwann had no doubt in his mind about the close, analogy between vinous fermentation and putrefaction; and as the putrefaction experiments had all given one and the same answer, he explained these ano-malies as having been caused by unobserved slips in the respective experiment, and did not admit them to invali-date his general proposition that both putrefaction and fer-mentation are inseparably connected with characteristic biologic phenomena;—the less so, as his experiments on the action of certain antiseptics had shown that what is an " antiseptic " to a fermentative change is r- poison to the organisms characteristic of the case. Thus, for instance, he found that white arsenic and corrosive sublimate, being poisonous to both plants and animals, stop both putrefac-tion and fermentation ; while extract of mix vomica, being destructive of animal but not of vegetable life, prevents putrefLction, but does not interfere with vinous fermenta-tion. . The mechanism of the latter process he imagined to consist probably in this, that the "sugar-fungus" (the yeast) lives at the expense of the nitrogenous matters and of the sugar of the juice, and that those of the elements of these substances which the plant does not assimilate are elimi-nated chiefly as alcohol. This special theory of Schwann's, as the reader is aware, is not quite correct, but it does not affect his general views on the phenomenon, which were fully confirmed by subsequent investigators. Amongst these we may mention Helmholtz, who showed that oxygen evolved by electrolysis from water does not, like air, induce vinous fermentation. The same observer showed that boiled grape juice, when tied up in a bladder, does not ferment, even when suspended within a tub full of fermenting juice. The evidence afforded by this experiment was considerably strengthened by Mitscherlich, who proved that even a septum of filter paper effectually stops the propagation of the reac _ tion. More striking still is an experiment which was made, many years later, by H. Hoffmann. He took a test-tube full of sugar water, and by a plug of cotton wool inserted within it divided the liquid into two parts. To the upper part he added yeast, which of course induced fermentation there; but the change did not propagate itself through the cotton wool to the lower portion. The same material had done good service some years before in the hands of Schroder and Dusch, who proved in 1854, by a most extensive series of experiments, that the something in air which enables it to start fermentative changes in boiled infusions of meat or malt, in grape juice, &c, can be effectually removed by filtration of the air through cotton wool. It is true the "&c." here does not iaclude milk, which they found to turn sour in filtered as well as in ordinary air, but this exception was subsequently explained away by Pasteur, who found that germs immersed in alkaline liquids may survive tempera-tures considerably higher than 100° C.

A number of other important researches, which/led to substantially similar results, must be passed over here, and may be, because what we have quoted has never been dis-proved, and is consequently quite sufficient to show that, in the case of vinous fermentation and putrefaction at any rate, those atomic motions, which, according to Liebig, cause the disintegration of the fermenting substances,—if the notion is to be maintained at all,—cannot be admitted to have an existence outside the living bodies of certain organisms characteristic of the respective changes. To any unpre-judiced person this would appear to be sound logic; but Liebig did not see it, and for a long time he had the majority of chemists at least on his side. No reasonable person could have denied the irresistible force of the argu-ments of Schwann and his followers; but these chemists somehow or other managed to ignore the facts, until Pasteur, by means of a most thorough and extensive experi-mental research (of which the principal portions were published from 1857tol861), simply forced the attention oi everybody to the physiological side of the subject, and, by absolutely unimpeachable evidence, proved that Schwann's views are substantially correct. Of this investigation it is impossible to speak otherwise than in terms of the highest admiration. Even the purely critical portion of Pasteur's work would be enough to immortalize his name. He did the whole of the work of Schwann and the rest of his pre-decessors over again, modifying and perfecting the experi-mental methods, so as to silence any objection or doubt that might possibly be raised, repeating and multiplying his experiments until every proposition was firmly esta-blished. But his work was synthetical as well as analyti-cal. Some of his discoveries will be noticed below; suffice it here to mention one of the general results which he arrived at. Vinous fermentation is only one of a number of fermen-tative changes to which sugar is liable. The same substance sugar, which, when placed under certain conditions, breaks up into alcohol and carbonic acid, under certain other sets of conditions ferments into lactic acid, or through lactic into butyric acid, or into gum plus mannite. This has long been known. What Pasteur showed is that each of these changes is the exclusive function of a certain species (or at least genus) of organism. What the yeast plant is for vinous a certain other organism is for lactic, a third for mannitic, a fourth for butyric fermentation. No two of these species, even if they belong to the same genus, will ever pass into each other. Pasteur arrived at this great generalization by means of his invention of an ingenious method for cultivating pure growths of the several species, so that each of them could be examined separately for its chemi-cal functions. To explain the method, let us suppose we wanted an unmixed growth of the species of yeast-plant called Saccharomyces cerevisiae. The first step of course is to procure a specimen of yeast which, when examined under the microscope, proves a fair approximation to what is wanted. This being done, we place a quantity of brewers' wort in a flask provided with two necks, one long and very narrow and bent like a gas-evolution tube, the other short and wide like the tubulus of a retort. We boil the wort in the flask to kill the germs, and during the boiling close first the tubulus by means of a (germless) glass stopper, then the narrow tube by means of a previously ignited plug of asbestos. We now allow the flask to cool down slowly, and by exposing it for several days to the proper temperature make sure that there is no potential life in our medium. We then, by means of a thin platinum wire, introduce a speck of the yeast through the tubulus, which, of course, is stopped up again without delay. We then place the flask in a chamber kept at the particular tempera-ture which is most favourable to the development of " saccharomyces." The saccharomyces-cells, being in the majority and enjoying a position of advantage, will multiply at a greater rate than the foreign cells, of which many in fact will go to the wall, so that what we ultimately obtain is a closer approximation to pure saccharomyces than the speck of yeast which we used as seed, and which, in general, was already a little better (in all probability) than the bulk of the yeast it came from. Of the relatively pure yeast we now again take a speck and sow it in a fresh supply of germless wort, and so on until the foreign cells can be assumed to be " Darwinized" out of existence. This method of Pasteur's, apart from its scientific value, is of the greatest practical importance ; supposing it to be carried out on a large scale—which, in fact, has been done with some measure of success—it will enable the brewer to grow and use pure saccharomyces, and thus to avoid all the many " diseases " which beer, as brewed with ordinary yeast, is liable to.

After Pasteur's researches it became impossible not to admit that those fermentative changes which he investi-gates are, at least in a practical sense, physiological and not purely chemical phenomena ; but there appears to be no necessity for assuming, as many do, that it is the life of those minute organisms, qua life, which directly causes the phenomena. It seems far more rational and philosophic to adopt the view taken by Berthelot and Hoppe-Seyler, who go no further than to admit that those living organisms are the only known sources for the ferments proper, which in themselves are chemical substances pure and simple.

Proceeding now to give a short account of the different fermentative changes, we begin with those that are proved to be functions of purely chemical reagents.

A. Fermentations proved to be purely chemical reactions.

These are conveniently arranged according to the respective catalytic agents.

1. Acids.—Many organic substances, when boiled with water and a small quantity of sulphuric, muriatic, or other strono, mineral acid, undergo hydration and decomposition, or other chemical trans-mutation, the acid remaining ultimately in its original condition. Thus, under the circumstances named,—

(1.) Cane sugar is "inverted," i.e., converted into dextrose and levulose, thus:

C12H2.2On + H20 = C6H1206 + C6H1208.

(2.) The same equation applies to the case of milk sugar, one of the products C6H120,. in this case being a peculiar substance called "galactose."

(3.) Starch passes into dextrine (a kind of gum) and dextrose, thus:
C24H40O20 + 3H20 = C6H10O5 + 3C6H120„
Starch. Water. Dextrine. Dextrose.

(4.) Salicine (a bitter principle in willow bark) breaks up into glucose and " saligenine," thus :
C13H1807 + H20 = C6H1206 + C7H80„
Salicine. Glucose. Saligenine.

Many other "glucosides" (native substances containing potential glucose) behave in a similar manner. The exact mechanism of these reactions is scarcely understood ; pending exact investigations, they may be explained, according to Lyon Playfair, by a tendency of the acid to combine with the elements of one of the products, which tendency, although sufficient to sever these elements from the rest, is defeated ultimately by the stability of the compound formed by their union with one another.

2. Diastase is a peculiar substance which is formed in the germination of grain (in malting), and which has the power of convert-ing many times its weight of starch into dextrine and dextrose when made to act on it in the presence of water at about 66° C. Diastase has not yet been isolated in the pure state. In the process referred to it is changed ; but it is not known into what.

3. Emulsine is a constituent of almonds (both of bitter and sweet), which is known chiefly for its power of decomposing amygdaline (a crystalline substance contained in bitter almonds, and extractable therefrom by alcohol), with formation of bitter-almond oil and glucose, thus :
O20H27rTOn + 2H20 = 2CeHI0Of) + C7H60. NCH

Amygdaline. Glucose. Volatile oil.

The oil, as the formula shows, is a (loose) compound of prussio acid, NCH, and benzaldehyde, C7H60. The common idea that bitter almonds contain prussic acid is erroneous ; that acid, like the benzaldehyde, is present only potentially, viz., as amygdaline, which, when the almond meal is treated with water, undergoes the above ferm antative change. Many other glucosides are decomposed by emulsine, as they are by dilute acids. It may be said, in pass-ing, that the acrid volatile oil contained in table-mustard is not found ready formed in the mustard seed, but is produced from a constituent of the seed by a fermentative action closely analogous to that we have just been explaining.

4. Soluble Yeast Ferment.—It has already been stated that an aqueous extract of yeast, though devoid of the power of inducing vinous fermentation, converts cane sugar into dextrose and levulose. The ferment, as Berthelot showed, can be precipitated from the liquid, in an impure state, by addition of alcohol.

5. Pepsine.—Stomach digestion (in man and animals nearly re-lated to man) consists mainly in this that the gastric juice dis-solves the albuminoids of the food, as hydrochlorates of peptones, the only form, it seems, in which they can be assimilated by the system. The juice owes this power to the presence in it of small percentages of two things, namely, of free hydrochloric acid and of " pepsine," both of which are continuously produced by the mucous membrane. Real pepsine has never been seen ; but an impure substance, possessing the specific properties of the ferment, can be extracted from the mucous membrane of the stomach by a .laborious process which we have no space to describe. Highly dilute hydrochloric acid alone dissolves certain albuminoids, but it does not convert them into peptones ; it acquires this property by the addition to it of a small quantity of the preparation named. It is as well to state in passing, that the so-called pepsine of the pharmaceutist is only a very poor apology for even the pepsine of the physiological chemist.
6. Pancreatine.—What pepsine is to the gastric juice pancreatine is to the secretion of the pancreas gland, whose function it is to digest the starchy and fatty portions of the food. This "pancrea-tine" seems to include three ferments, namely, a kind of diastase (see above), a ferment similar in its functions to pepsine, and a fer-ment which has the power of converting fats into fatty acids and glycerine. If one of these has been isolated.

7. Erythrozyme.—This is a peculiar ferment which Edward Schunck, in 1854, extracted from madder-root, and which was found by him to possess the power of inducing vinous fermentation in solutions of sugar,—a most important discovery, which ought to be further investigated.

Nate.—All these ferments, the acids of course excepted, lose theif efficacy at temperatures near 100° C. in presence of water. In. the dry state they may survive boiling heat.

B. Fermentations which are known only as Physiological Processes.

L Vinous Fermentation.—This case having already been considered, we confine ourselves here to a few additions and qualifications. Vinous fermentation, as we see it going on in the brewers' vats and in the wine-producers' casks, is a function of Sacclm-romyces, a genus of fungi, consisting of minute cells, which sometimes are isolated from one another, sometimes grouped to-gether in a variety of forms, but never united into an organized tissue. There is a variety of species, of which S. cerevisim (the main constituent of ordinary yeast, as produced in the high fermen-tation of beer) is the most important. It consists of cells of about xii millimetre diameter. According to Pasteur, saccharomyces thrives best when immersed in grape juice or wort, or similar liquids. It multiplies only by budding, never by sporification. In pure sugar-water it lives, so to say, at its own expense, and gradually becomes exhausted ; but on addition of phosphates (yeast-ash works best) and ammonia salt to the sugar, the plant thrives as well almost as in native sugar juices. When saccharomyces is not fully immersed in the liquor, and otherwise constrained to live under abnormal conditions, it passes into " aerobiotic" forms which are similar to mucors and mucedos (mould plants), and which, like these, live on atmospheric oxygen. But these abnormal forms, when re-immersed in wort, &c, always relapse into the non-aerobiotic form of saccharomyces. Real mucedos, &c., for instance Myeoderma vini and eerevisioe, which by nature are aerobiotic, when immersed in wort or grape juice, and thus placed in what to them is an abnormal condition, assume non-aerobiotic forms, and produce vinous fermenta-tion, but (contrary to what was formerly assumed by Pasteur himself) they are never converted into saccharomyces, and their fermentative power soon comes to an end, unless they are occasionally revived by re-exposure to the atmosphere.

The power of inducing vinous fermentation, however, is by no means confined to microscopic organisms. It has long been known, from the experiments of Dobereiner and others, that sweet fruit, when kept within an inert atmosphere devoid of free oxygen, evolves carbonic acid with formation of alcohol, and it has been proved by Pasteur that this fermentation, which may extend to a jonsiderable portion of the sugar present, is not accompanied by the development of any microscopic species. Closely related to this fact is the well-established experience that large quantities of sugar may be made to ferment by means of yeast without the latter multiplying to any noteworthy extent. On the other hand, large growths of yeast may be obtained (and as a matter of fact are obtained every day by the makers of German barm) without pro-ducing much alcohol. Oskar Brefeld, by means of a peculiar arti-fice, succeeded in growing saccharomyces in brewers' wort, without producing a trace of alcohol. From these experiences we must conclude that vinous fermentation, far from being the characteristic life-function of healthy saccharomyces, is dependent on a certain pathologic condition of "non-photobiotic" plant-cells (i.e., cells which habitually live in darkness) generally, which is brought about by immersing them in sacchariferous fluids and shutting them out from the oxygen-gas which they need for their healthy development. Hence, even in the ordinary cases of fermentation, the normal life of the yeast-plant on the one hand, and the dis-sociation of the sugar on the other, are not only not necessarily re-lated, but, in the individual cell, positively exclude each other. In any given mass of yeast, healthy cells and diseased cells are in general mixed up together, and thus, in practice, the two pheno-mena come to be accidental concomitants. But this brings us back almost precisely to the later views of Liebig, as set forth in his last memoir on the subject.

In regard to the genesis of the yeast plant little is known. According to Pasteur's experiments and observations the yeast which forms spontaneously in grape juice is derived chiefly from certain germs which abound about harvest time on the grapes, and still more on the grape-stalks. These germs are largely diffused also through the atmosphere of breweries, wine cellars, and labora-tories where fermentation experiments are carried on, but they are not by any means widely diffused through the atmosphere generally.

2. Lactic Fermentation.—Milk when left to itself in warm weather, as everybody knows, soon turns sour, the main feature in the chemical process being the transmutation of the milk sugar into lactic acid, as expressed by the equation

C]2H22On.H20 = 4C3He03

Hydrated Milk sugar. Lactic acid.

The milk sugar, before assuming the form of lactic acid, probably passes through the condition of glucose. At any rate, ordinary-glucose, when dissolved in milk, ferments into lactic acid along with the milk sugar originally present. But in this case, if the total percentage of sugar goes beyond a certain limit, the reaction comes to a stop as soon as the acidity of the liquid has attained a certain limit-value. Addition of chalk or carbonate of soda, i. e., conversion of the lactic acid into a neutral lactate, then revives the process. A solution of "invert-sugar" (as produced by boiling cane sugar water with a little vitriol), when mixed with excess of chalk and some putrid cheese, and kept at 30°-35° C., soon ferments, with formation of large quantities of lactate of lime (Bensch). Lactic fermentation, according to Pasteur, is caused by the development in the mass of a microscopic fungus, consisting of cylindrical cells which are far smaller than those of saccharomyces. We are not aware that this "lactic ferment" has ever been seen in ordinary sour milk; in Bensch's process it is produced largely as a greyish deposit on the chalk, from which pure growths of the fungus may be obtained by Pasteur's method (see above). The lactic ferment, to the annoyance of brewers, frequently occurs in ordinary yeast as an impurity.

There is no doubt that that fungus which Pasteur calls the lactic ferment is capable of inducing lactic fermentation ; but it does not by any means follow that it is the thing which actually causes the souring of milk under ordinary circumstances. On the con-trary, from a remarkable set of experiments made by Lister in 1873, this appears not to be the case. According to him, milk can be completely purged of germs by exposing it (within a germ-less flask) to the temperature of boiling water for some hours, and, when protected against atmospheric germs by a slightly carbolized stopper of cotton wool, keeps sweet for an indefinite time. Speci-mens of such germless milk, when exposed to the atmosphere of his study, were found by Lister to undergo a variety of fermentative changes, accompanied sometimes by the development of an acid reaction, but none of them set into sour milk. A specimen of ordinary unboiled dairy milk when kept in the same room did get sour as usual, and, when examined under the miscroscope, was found to contain, not Pasteur's fungus, but a kind of motionless bacterium which Lister calls B. lactis, because the introduction of it (or rather of a trace of the sour milk containing it) into the germless milk determined normal lactic fermentation, the Bacterium lactis multiplying at the same time. The same bacterium, when made to pass successively through germless urine and other germless organic liquids, underwent a series of metamorphoses, but, when ultimately put back into milk, caused normal lactic fermentation. The germs of this bacterium must be assumed to abound in the atmosphere of cows' stables and dairies, although they do not seem to be abundantly diffused through the atmosphere generally.

3. Viscous Fermentation is a peculiar change which has long been known to occasionally accompany vinous fermentation, and which manifests itself in this that the wine becomes thick and viscous, so that, wdien poured from one vessel into another, it draws into long threads. This property is caused by the presence of a kind of gum (of the composition C12H20O10) which is invariably accompanied by mannite, a sweet crystalline substance of the composition C6H1406 (i. e. containing the elements of glucose, C6H1206 + those of hydrogen, H2). The exact nature of the reaction is not established; in fact, we do not know whether it is one reaction or a set of reactions going on simultaneously. According to the usually adopted equation, 100 parts of cane sugar should yield 61 of mannite, 45'5 of gum, and 6 of carbonic acid. According to Peligot (supported by Pasteur) the "viscous ferment" is a fungus consisting of very minute spherical cells (of O'OOl to 0'0014 millimetre diameter).
4. Butyric Fermentation.—In the lactic fermentation of glucose, as induced by milk or cheese in the presence of chalk, the lactate of lime is no sooner formed than it undergoes itself a further change, which, chemically, is represented approximately by the equation

2C3H603 = C4H802 + 2C02 + 2H2 Lactic acid. Butyric acid. Carbonic acid. Hydrogen gas

The temperature most favourable to the change lies near 40° C. A number of similar changes (of other organic acids than lactic) are known, but they are passed over here, being of a more purely scientific interest. According to Pasteur, butyric fermentation is caused by the development in the mass of a special kind of vibrio, a worm-shaped animalcule, consisting of a number of longitudinal cells, each absnt 0'002 millimetre thick, and from 0'002 to 0'02 mm. long. Butyric fermentation, strictly speaking, is only one of a large genus of changes customarily summed up under the generic name of putrefaction.

5. Putrefaction.—The scientific meaning of this term coincides pretty much with its popular acceptation, except that it must be understood to be exclusive of all cases of oxidation. In olden times it was assumed that organized matter (the tissues of plants and animals, blood corpuscles, &c.) could hold together even chemically only as long as supported by the vital force. But this is a long exploded notion. In absolute absence of water, or at very low temperatures, dead organized matter remains chemically (and even structurally) unchanged. In support of this assertion we need only refer to that well known case of the mammoth of the Siberian cave, which was found sweet and fresh thousands of years after the extinction of life. And since the time of Appert (who discovered the now so extensively used process of preserving meats in sealed-up tins) we know that prolonged exposure to boiling heat and subsequent absolute exclusion of air prevent putrefaction, even in presence of liquid water and at the ordinary temperature, as long as the air remains excluded. Chemically speaking, ordinary putrefaction is a most complex phenomenon, alwavs involving the simultaneous on-going of a multiplicity of chemical reactions. A comparatively simple case is the putrefaction of urine, which substantially consists in this that the urea, by assimilating water, passes into carbonate of ammonia, just as it does when heated by itself with pure water to high temperatures. In the case of animal tissues, which, broadly speaking, may be said to consist of fats and albuminoids, the latter always give way first. Their decomposition is a most complex set of successive reactions,—leucine, tyrosine, fatty acids, and many other things appearing as primary products, ammonia, compound ammonias, sulphuretted hydrogen, hydrogen, and nitrogen as second-ary ones. Less rapidly, but none the less constantly, the fats are changed, being decomposed, in the first instance, into fatty acids and glycerine, which latter undergoes further transmutations, while the former survive for a considerable time. The "adipocere" which is so well known as one of the constant ultimate products of the decay of dead bodies that have been kept from the air, and which consists of palmitate and stearate of lime (Hoppe-Seyler), well illustrates the high stability of fatty acids. Regarding the causes of putrefaction, we can scarcely do more than refer to what we have quoted from the researches of Schwann and his followers. From these researches two things are clear, namely, (1) that putrefaction is not pos-sible under conditions precluding the development of life, or, in other words, that there is no putrefaction where there is not at least potential life ; and (2) that in 999 out of a thousand cases this potential life assumes the actual form of bacteria and vibriones.

Putrefactions going on in presence of air are always accompanied by processes of oxidation, the effects of which are difficult to differentiate absolutely from those of putrefaction pure and simple.

C. Cases of Oxidation.

1. Acetous Fermentation is the best known of these. Every-body knows that weak fermented liquors, when exposed to the air, soon turn sour and ultimately become vinegar'. The chemical re-action involved consists of two stages. In the first the alcohol in the liquor absorbs oxygen from the air, with formation of alde-hyde and water, according to the equation

C2H60 + O = C2H40 J- H.,0.

Alcohol. Aldehyde. Water.

This aldehyde, however, in ordinary acetous fermentation never actually appears, being at once oxidized by the direct action of the air into acetic acid, thus :

C2H40 + O = C2H402

Aldehyde. Acetic acid.

These two reactions can be realized in all their chemical simpli-city, not, it is true, by oxygen-gas pure and simple, but easily by oxygen as condensed on platinum-black ; and as the deoxi-dized platinum-black readily reabsorbs oxygen from the air, a small quantity of this reagent suffices to oxidize large quantities of alcohol into acetic acid by means of atmospheric oxygen. Upon this observation of Dobereiner's Schiitzenbach based a rapid and practical method of vinegar-making, which consists in this, that the dilute alcohol is made to trickle through a tower of beech-wood shavings, packed into a tall barrel, constructed so as to draw in an ascending current of air. When a temperature of 20° C. is main-tained in the room, and the spirit introduced having a temperature of 26°, the temperature within the barrel is found to rise to 38°-40°, the heat being produced by the rapid oxidation of the alcohol into acetic acid. What, in the old method of vinegar-making, required weeks or months is thus accomplished in a day or even less. It is difficult to avoid the conclusion that in Schützenbach's process the wood-shavings, besides serving to spread the alcohol over an immense surface, act exactly as the platinum-black does in Dober-einer's experiment, condensing oxygen in their pores in order to hand it over to the alcohol. And, supposing this theory to be correct, the old process, which consists in exposing the wine to the air in half-filled tubs or casks, would appear to rest on the same principle,—the wood of the cask acting as the shavings do in Schützenbach's process, only far more slowly. But then it is an old experience of vinegar-makers that the old process at least always involves the formation of two organized products, namely, that of a kind of mould which appears on the surface as a membrane, and goes by the name of "flowers of vinegar," and that of a mucilaginous mass within the- liquid, called "mother of vinegar;" and it has always been admitted that the presence of these substances ma-terially accelerates the process of oxidation. This, however, is no contradiction to the theory ; it would only prove that the mould-membrane and the mucilaginous mass are more effective carriers of oxygen than the wood of the tub. Besides, in Schiitzenbach's pro-cess, the shavings, according to experience, work the better the freer they are from organized deposits. Hence, one might say, with Liebig, that the efficacy of these substances is a function only of their physical and chemical condition, the presence of life in the mould plant being purely accidental and immaterial to the process. According to Pasteur this view is a mistake. With him it is the membranous mould on the surface of the fermenting liquid which hands the oxygen to the alcohol, and it does so only when it consists of living specimens of a certain species of mould-plant which he calls Mycoderma aceti. Other moulds or dead Mycoderma aceti do not work. Mother of vinegar, according to Pasteur, is the "non-aero-biotic ' ' form of the mycoderma. Only the aerobiotic form acts. To keep it alive we must take care that the liquid contains the phos-phates and the albuminoids or ammonia, which it needs as food. But, if it is to produce vinegar, it must not be fed too liberally, because, when in a vigorous state of health, it oxidizes the alcohol into carbonic acid and water. If the oxidation is to stop at acetic acid, the mycoderma must be in a peculiar abnormal condition,which may be ensured by the presence in the liquid of a certain limit percentage of alcohol. In the case of Schutzenbach's process, Pasteur maintains (in spite of apparently contrary experience) that it is the very same Mycoderma aceti wdiich enables the wood shavings to act. One of his arguments is the acknowledged fact that Schutzen-bach's towers require to be started with ordinary vinegar,although they can be worked with distilled spirits. A more powerful argu-ment he derives from the following experiment. A very slow current of dilute alcohol was caused to trickle down a long string suspended in a room kept at the most favourable temperature to acetous fermentation. The alcohol failed to assume an acid reaction. But the slightest coating of Mycoderma aceti attached to the string caused it to act exactly as the shavings do in the Schiitzenbach casks. After all, however, it is a little difficult to believe that the many pounds of alcohol which, in the course of a day, pass through a Schiitzenbach tower and come out below as acetic acid, have all been under the direct influence of the few grains of Mycoderma aceti which a microscopist might hunt up amongst the wood shavings. It appears far more rational to assume that the mycoderma acts only indirectly, perhaps by converting a small portion of the alcohol into aldehyde, which diffuses itself through the whole mass of the alcohol, and, through its inherent attraction for oxygen atoms, which is assisted by a similar tendency in the porous wood, reduces the stability in a far larger number of oxygen molecules than it needs itself, to such an extent that these so to say half-liberated oxygen atoms become available for the oxidation of alcohol, the more readily as the reaction itself involves a considerable liberation of energy.

2. Ercmacansis.—Animal and vegetable matters, besides being liable to putrefy, are known, on exposure to air and water, to undergo a slow process of oxidation which ultimately converts them into products of simpler atomic constitution. Speaking quite generally, the process, in the case of wood and vegetable tissues generally, consists in this, that the hydrogen is eliminated more and more completely—most of it in the form of water, a small portion in the form of marsh gas (CH4), the rest of the carbon passing gradually into substances more and more closely allied to charcoal. In the case of albuminoids eremacausis is always preceded and accompanied by putrefaction,—the most general effect of the former being that the ammonia produced by the latter is turned into nitrogen and water, or, in the presence of basic substances, such as lime, carbonate of lime, carbonates of alkalies, &c., is oxidized into nitrites and nitrates successively. This process of "nitrification" is going on in all porous soils and waters contaminated with nitrogenous organic matter, and, under favourable circumstances, progresses at a great rate. When sewage is filtered through a bed of porous soil, the drainage waters are generally found almost free from ammonia, the whole of the nitrogen having been oxidized into nitrites and nitrates. According to recent researches by Schlôsing and Miintz (which were confirmed by Warrington) nitrification, like acetous fermentation, is determined by the presence in the soil of organized ferments, the chief arguments being that a purely fnorganic soil does not act, and secondly, that nitrification in a soil is stopped, or at least very effectually checked, by antiseptic vapours such as those of bisulphide of carbon, chloroform, and carbolic acid. (W. D.)


According to Schutzenberger, who, however, does not quote his authority, arsenious acid does not impede vinous fermentation.

Bacteria are microscopic organisms, composed of two elongated cells united end to end, possessed of varied powers of motion, and almost without exception '' rather in themselves reproductive organs " (Lister) than beings possessed of powers of reproduction. The vibriones are seemingly nothing more than polymerized bacteria with intensified powers of locomotion. With regard to their position in the world of life present evidence leaves it uncertain whether they are plants or animals.

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