Arrived at this unexpected turn in the road which he had hitherto pursued, Pasteur paused for an instant. Should he commit himself to the course which abruptly opened before him? His scientific instincts urged him to do so, but the prudence and reserve which show themselves to be the basis of his character, whenever he finds himself called upon to make a choice of which the necessity is not absolutely demonstrated, held him back. Was it not wiser to continue in the domain of molecular physics and chemistry? M. Biot counselled his doing so; the route had been made plain, success awaited him at each step, but an incident connected with the University triumphed over his hesitations.

He had just been nominated, at thirty-two years of age, Dean of the FacultÉ des Sciences at Lille. One of the principal industries of the DÉpartement du Nord is the fabrication of alcohol from beetroot and from corn. Pasteur resolved to devote a portion of his lectures to the study of fermentation. He felt that if he could make himself directly useful to his hearers he would thereby excite general sympathy with, and direct attention to the new FacultÉ. The young man congratulated himself on this idea, and the man of science rejoiced in it still more. He was filled by the reflections suggested to him by the strangeness of the phenomena which he had just encountered in regard to the molecular dissymmetry of the two tartaric acids, in connection with the life of a microscopic organism. He saw new light thrown upon the obscure problem of fermentation. The part so active performed by an infinitely small organism could not, he thought, be an isolated fact. Behind this phenomenon must lie some great general law.

I.

All that has lived must die, and all that is dead must be disintegrated, dissolved or gasified; the elements which are the substratum of life must enter into new cycles of life. If things were otherwise, the matter of organised beings would encumber the surface of the earth, and the law of the perpetuity of life would be compromised by the gradual exhaustion of its materials. One grand phenomenon presides over this vast work, the phenomenon of fermentation. But this is only a word, and it suggests to the mind simply the internal movements which all organised matter manifests spontaneously after death, without the intervention of the hand of man. What is, then, the cause of the processes of fermentation, of putrefaction, and of slow combustion? How is the disappearance of the dead body or of the fallen plant to be accounted for? What is the explanation of the foaming of the must in the vintage cask? of dough, which, abandoned to itself, rises and becomes sour? of milk, which curdles? of blood, which putrefies? of the heap of straw, which becomes manure? of dead leaves and plants embedded in the earth, which transform themselves into soil?

Many different attempts were made to account for this mystery before science was in a condition to approach it. In our age, and at the time when Pasteur was led to the study of the question, one theory held almost undisputed sway. It was a very ancient theory, to which Liebig, in reviving it, had given the weight of his name. 'The ferments,' said Liebig, 'are all nitrogenous substances—albumen, fibrine, caseine; or the liquids which embrace them, milk, blood, urine—in a state of alteration which they undergo in contact with the air.'

The oxygen of the air was, according to this system, the first cause of the molecular breaking up of the nitrogenous substances. The molecular motions are gradually communicated from particle to particle in the interior of the fermentable matter, which is thus resolved into new products.

These theoretic ideas regarding the part played in fermentation by the oxygen of the air were based upon experiments made in the beginning of the century by Gay-Lussac. In examining the process of Appert for the preservation of animal and vegetable substances—a process which consisted in inclosing these substances in hermetically sealed vessels and heating them afterwards to a sufficiently high temperature—Gay-Lussac had seen, for example, the must of the grape, which had been preserved without alteration during a whole year, caused to enter into a state of fermentation by the simple fact of its transference to another vessel—that is to say, by having been brought for an instant into contact with the oxygen of the air. The oxygen of the air appeared, then, to be the primum movens of fermentation.

The illustrious chemists Berzelius and Mitscherlich explained the phenomena of fermentation otherwise. They placed these phenomena in the obscure class known as phenomena of contact. The ferment, in their view, took nothing from, and added nothing to, the fermentable matter. It was an albuminoid substance, endowed with a force to which the name catalytic was given. The ferment in fact acted by its mere presence.

A very curious observation, however, had been made in France by Cagniard-Latour and in Germany by Schwann. Cagniard-Latour, however, was the first to publish this observation, which was destined to become so fruitful. One of the ferments most in use, and known as early as the leavening of dough or the turning of milk, is the deposit formed in beer barrels, which is commonly called yeast. Repeating an observation of the naturalist Leuwenhoeck, Cagniard-Latour saw this yeast, which was composed of cells, multiplying itself by budding, and he proposed to himself the question whether the fermentation of sugar was not connected with this act of cellular vegetation. But as in other fermentations the existence of an organism had not been observed even by the most careful search, the hypothesis of Cagniard-Latour of a possible relation between the organisation of the ferment and the property of being a ferment was abandoned, though not without regret by some physiologists. M. Dumas, for example, recognised that in the budding of the yeast globules there must be some clue to the phenomenon of fermentation. I, however, repeat that as nothing of the kind had been found elsewhere, and as all other fermentations presented the common character of requiring, to put them in train, organic matter in a state of decomposition, the hypothesis of Cagniard-Latour remained a simple incident, instead of having the value of a scientific principle.

Liebig, moreover, carrying general opinion along with him, contended that it is not because of its being organised that yeast is active, but because of its being in contact with air. It is the dead portion of the yeast—that which has lived and is in the course of alteration—which acts upon the sugar.

The new memoirs published on the subject agreed in rejecting the hypothesis of any influence whatever of organisation or of life in the process of fermentation. Books, memoirs, dogmatic teaching, all were favourable to the theoretic ideas of Liebig. If a few rare observers indicated the presence in certain fermentations of living organisms, this presence was, in their opinion, a purely accidental fact, which, instead of favouring the phenomenon of fermentation, was injurious to it.

From his first investigation on lactic fermentation Pasteur was led to take an entirely different view of the matter. In this fermentation he recognised the presence and the action of a living organism, which was the ferment, just as yeast was the ferment of alcoholic fermentation. The lactic ferment was formed of cells, or rather of little rods nipped at their centres, extremely small, being hardly the thousandth part of a millimeter in diameter.[8] It reproduced itself by fission—that is to say, the little rod divided itself at its middle and formed two shorter rods, which became elongated, nipped, in their turn, at their centres, each giving rise, as before, to two rods. Each of these, again, soon divided itself into two, and so on. Why had not this been observed prior to Pasteur? For the simple reason that chemists had never observed the production of lactic fermentation except in complex substances. They mixed chalk with their milk for the purpose of preserving the neutrality of the fermenting medium. They employed substances such as caseine, gluten, animal membranes, all of which, when examined by the microscope, exhibited a multitude of mineral or organic granules, with which the lactic ferment was confounded. Thus the first care of Pasteur, with the view of proving the presence of the ferment and its life, was to replace the cheesy matter and all its congeners by a soluble, nitrogenous body, which would permit of the microscopic examination of all the living cellular products.

In a memoir presented to the Academy of Sciences in 1857 Pasteur stated that there were 'cases where it is possible to recognise in lactic fermentation, as practised by chemists and manufacturers, above the deposit of chalk and the nitrogenous matter, a grey substance which forms a zone on the surface of the deposit. Its examination by the microscope hardly permits of its being distinguished from the disintegrated caseum or gluten which has served to start the fermentation. So that nothing indicates that it is a special kind of matter which had its birth during the fermentation. It is this, nevertheless, which plays the principal part.'

To isolate this substance and to prepare it in a state of purity, Pasteur boiled a little yeast with from fifteen to twenty times its weight of water. He then carefully filtered the liquid, dissolved in it about fifty grammes of sugar to the litre, and added to it some chalk. Taking then, by means of a drawn-out tube, from a good ordinary lactic fermentation a trace of the grey matter of which we have just spoken, he placed it as the seed of the ferment in the limpid saccharine solution. By the next day a lively and regular fermentation had set in, the liquid becoming turbid and the chalk disappearing, and one could distinguish a deposit which progressed continually as the chalk dissolved. This deposit was the lactic ferment.

Pasteur reproduced this experiment by substituting for the water of the yeast a clear decoction of nitrogenous plastic substances. The ferment invariably presented the same aspect and the same multiplication. These results, however, did not yet satisfy Pasteur. He desired more rigour in a subject of such theoretic importance. Might not the partisans of Liebig's theory argue, if not without subtlety yet with a semblance of justice, that the fermentation was not due to the formation and progressive growth of this feeble nitrogenous globular deposit, but rather to the nitrogenous matter dissolved during the decoction of the yeast used in the composition of the liquor? Up to a certain point it might be maintained that the dissolved matters which had been in contact with the oxygen of the air had been thrown into molecular motion, that this motion had been communicated to the fermentable matter, and that the deposit of the pretended organised ferment was but an accident—one of the physical changes or one of the precipitates so frequently observed in the modifications of albuminoid matters. In the observation of Cagniard-Latour and of Schwann as to the life of the yeast, Liebig saw nothing more. 'One cannot deny,' said he, 'the organisation of the yeast or its multiplication by budding, but these living cells are always associated with other dead cells in process of molecular alteration. It is these molecular motions which communicate themselves to the molecules of the sugar, break them up, and cause them to ferment.'

The arguments of Liebig derived great strength from the belief which was shared by all chemists that the cells of yeast perish during fermentation and form lactate of ammonia. On examining this assertion, Pasteur found that not only was there no ammonia formed during alcoholic fermentation, but that even if ammonia were added it disappeared, entering into the formation of new yeast cells. Was not this a proof of the potency of the organised ferment?

Tormented, however, by the idea that, notwithstanding all these facts, the reasonings of Liebig might still find some credit, Pasteur worked earnestly to discover new facts capable of demonstrating that Liebig's theory was absolutely false. He made two crucial experiments, the one relating to the yeast of beer, or of alcohol, and the other relating to the lactic ferment. He introduced into a pure solution of sugar a small quantity of crystallisable salt of ammonia, then some phosphates of potash and magnesia, and he sowed in this medium an imponderable quantity, if we may so express it, of fresh cells of yeast. The cells thus sown multiplied, and the sugar fermented. In other words, the phosphorus, the potassium, the magnesium of the mineral salts, united to form the substances which compose the ferment. By this experiment, so simple and yet so demonstrative, the power of the organisation of the ferment was once for all established. The contact theory of Berzelius had no longer any meaning, since it was evident that the fermentable matter here furnished to the ferment one of its essential elements, namely, carbon. Liebig's theory of communicated molecular motion, originating in a nitrogenous albuminoid substance, had no better claim, since such substances had been discarded. The whole process took place between the sugar and a ferment germ which owed its life and development to nutritive matters, the most important of which was the fermentable substance. Fermentation, in short, was simply a phenomenon of nutrition. The ferment augmented in weight, feeding upon the sugar, and its vitality was such that it contrived to build up the complex materials of its own organisation by means of sugar and purely mineral elements.

In a second experiment, Pasteur demonstrated that, notwithstanding their smallness and the possibility of confounding them with the amorphous granules of caseine and gluten, the little particles of lactic ferment were indeed alive, and that they, and they only, were the cause of lactic fermentation. He mixed with some water, sweetened with sugar, a small quantity of a salt of ammonia, some alkaline and earthy phosphates, and some pure carbonate of lime obtained by precipitation. At the end of twenty-four hours the liquid began to get turbid and to give off gas. The fermentation continued for some days. The ammonia disappeared, leaving a deposit of phosphates and calcareous salt. Some lactate of lime was formed, and at the same time one could notice the deposition of the little lactic ferment. The germs of the lactic ferment had, in this case, been derived from particles of dust adhering to the substances themselves, of which the mixtures were made, or to the vessels used, or from the surrounding air. The chapter on spontaneous generation will render this clear.

It suffices here to state that the results of this second experiment were absolutely conclusive, and that the theories of contact force or of communicated motion, which up to that time had reigned in science, were completely overthrown.

II.

The light shed by these experiments quickly extended its sphere; and Pasteur lost no time in discovering a new ferment, that of butyric acid. Having shown the absolute independence which exists between the ferment of butyric acid and the others, he found, contrary to the general belief, that the lactic ferment is incapable of giving rise to butyric acid, and that there exists a butyric fermentation having its own special ferment. This ferment consists of a species of vibrio. Little transparent cylindrical rods, rounded at their extremities, isolated or united in chains of two or three, or sometimes even more, form these vibrios. They move by gliding, the body straight, or bending and undulating. They reproduce themselves by fission, and to this mode of generation their frequent arrangement in the form of a chain is due.

Sometimes one of the little rods, with a train of others behind it, agitates itself in a lively manner as if to detach itself from the rest. Often, also, the little rod, after being broken off, holds on still to its chain by a mucous transparent thread.

These little infusoriÆ may be sown like the yeast of beer or the lactic ferment. If the medium in which they are sown is suitable for their nourishment, they will multiply to infinity; but the character most essential to be observed is, that they may be sown in a liquid which contains only ammonia and crystallisable substances, together with the fermentable substances, sugar, lactic acid, gum, &c. The butyric fermentation manifests itself as these little organisms multiply. Their weight sensibly increases, though it is always minute in comparison with the quantity of butyric acid produced; this is found to be the case in all other fermentations.

This experiment no doubt resembles those made with the alcoholic and lactic ferments. But it is distinguished from them by one circumstance eminently worthy of attention. The butyric ferment, by its motions and by its mode of generation, furnishes the irrefutable proof of its organisation and of its life. This ferment, moreover, presented to Pasteur a new and unexpected peculiarity. The vibrios live and multiply without the smallest supply of air or of free oxygen. Not only, indeed, do they live without air, but the air destroys them and arrests the fermentation which they initiate. If a current of pure carbonic acid is made to pass into the liquid where they are multiplying, their life and reproduction do not appear to be at all affected by it. If, on the contrary, instead of the current of carbonic acid we employ one of atmospheric air for only one or two hours, the vibrios fall without movement to the bottom of the vessel, and the butyric fermentation which was dependent on their existence is immediately arrested.

Pasteur designated this new class of organisms by the name of anaÉrobies; that is to say, beings which can live without air. He reserves the designation aÉrobies for all the other microscopic beings which, like the larger animals, cannot live without free oxygen. 'It matters little,' added Pasteur, 'whether the progress of science makes of this vibrio a plant or an animal; it is a living organism, endowed with motion, which is a ferment and which lives without air.'

In meditating upon these facts, and upon the general character of fermentation, Pasteur soon found himself in a position to approach more nearly to the essential nature of these mysterious phenomena. In what way do microscopic organisms provoke the phenomena of fermentation?

The organism eats, if one may say so, one part of the fermentable matter. But how does this phenomenon of nutrition differ so much from that of higher beings? In general, for a given weight of nutritive matter which the animal takes in, it assimilates a quantity of the same order. In fermentation, on the contrary, the ferment, while nourishing itself with fermentable matter, decomposes a quantity great in comparison to its own individual weight. Again, the butyric ferment lives without free oxygen. Is there not, said Pasteur, a hidden relation between the property of being a ferment and the faculty of living without free oxygen? Are not vibrios which imperatively require for their nutrition and multiplication the presence of oxygen gas those which will never have the properties of ferments?

Pasteur then contrived a series of experiments with the view of placing in parallelism these two curious physiological facts: life without air and the characteristics of ferments.

We know how wine and beer are prepared. The must of grapes and the must of beer are placed in wooden vats, or in barrels of greater or less dimensions. Whether the fermentation proceeds from germs taken from the exterior surface of the grapes, or from a small quantity of ferment sown in the must under the form of yeast, as in the fermentation of beer, the life of the ferment, its multiplication, the augmentation of its weight, are so many vital actions which to a certainty cannot borrow from the free oxygen of the external air, or from that originally dissolved in the must, an appreciable quantity of this gas. All the life of the cells of the ferment which multiplies itself indefinitely appears then to take place apart from free oxygen gas. In certain breweries in England the fermenting vats have sometimes a capacity of several thousands of hectolitres; and the fermentation liberates pure carbonic acid, a gas much heavier than atmospheric air, which rests on the surface of the liquid in the vat in a layer thick enough to protect the liquid underneath from any contact with the external air. All this liquid mass, then, is inclosed between the wooden sides of the vat and a deep layer of heavy gas which contains no trace of free oxygen. In this liquid, nevertheless, the life of the cells of the ferment and the production of all its constituents go on for several days with extraordinary activity. Here certainly we have life without air, and the ferment character expresses itself in the enormous difference between the weight of the ferment formed and collected from the vats under the name of yeast, at the end of the operation, and the weight of the sugar which has fermented, transforming itself into alcohol, carbonic acid, and various other products.

Pasteur has studied experimentally that which takes place when, without otherwise changing the conditions of these phenomena, the arrangement is so modified as to permit the introduction of the free oxygen of the atmosphere. It sufficed for this purpose to provoke a fermentation of the must of beer, or the must of grapes, upon shallow glass dishes presenting a large surface, or in a flat-bottomed wooden trough with sides a few centimeters in height, instead of in deep vats as before. In these new conditions the fermentation manifests an activity even more extraordinary than it did in the deep vats. The life of the ferment is itself singularly enhanced, but the proportion of the weight of the decomposed sugar to that of the yeast formed is absolutely different in the two cases. While, for example, in the deep vats, a kilogram of ferment sometimes decomposes seventy, eighty, one hundred, or even one hundred and fifty kilograms of sugar, in the shallow troughs one kilogram of the ferment will be found to correspond to only five or six kilograms of decomposed sugar. These proportions between the weight of the sugar which ferments and the weight of the ferment produced, constitute the measure of what one might call the ferment's character—of that character which distinguishes its mode of life from that of all other existences, great or small, in which the weight of the organising matter and the assimilated alimentary matter are about equal. In other words, the more free oxygen the yeast ferment consumes, the less is its power as a ferment. Such is the case in the shallow troughs where the extended surface is exposed to the contact of the oxygen of the air. The more, on the contrary, the life of the ferment is carried on without the presence of free oxygen, the greater is its power of decomposing and of fermenting the saccharine matter. This is the case in deep casks. The intimate co-relation then between life without air and fermentation appears complete.

The unexpected light which these facts threw upon the cause of the phenomena of fermentation made a forcible impression upon all thinking minds. 'In these infinitely small organisms,' M. Dumas said one day to M. Pasteur before the Academy of Sciences, 'you have discovered a third kingdom—the kingdom to which those organisms belong which, with all the prerogatives of animal life, do not require air for their existence, and which find the heat that is necessary for them in the chemical decompositions which they set up around them.'

The work of Pasteur, demonstrating that fermentation was always dependent on the life of a microscopic organism, continued without interruption. One of the most remarkable of his researches is that which relates to the fermentation of the tartrate of lime. The demonstration of life and of fermentation without free oxygen is in this paper carried to the utmost limits of experimental rigour and precision.

III.

But there is still another class of chemical phenomena where the life without air of microscopic organisms is fully shown. Pasteur proved that in the special fermentation which bears the name of putrefaction the primum movens of the putrefaction resides in microscopic vibrios of absolutely the same order as those which compose the butyric ferment. The fermentation of sugar, of mannite, of gums, of lactate of lime, by the butyric vibrio, so closely resembles the phenomena of putrefaction, that one might call these fermentations the putrefaction of sugar and of the other products.

If it has been thought right to call the fermentation of animal matters putrefaction, it is because at the moment of the decomposition of fibrine, of albumen, of blood, of gelatine, of the substance of the tendons, &c., the sulphur, and even the phosphorus, which enter into their composition give rise to putrid odours, due to the evil-smelling gases of sulphur and phosphorus.

The phenomena of putrefaction being then simply fermentations, differing only in regard to the chemical composition of the fermenting matters, Liebig naturally included them in his general theory of the decomposition of organic matters after death. At a period long antecedent to Pasteur's labours it had been established that there existed in putrefying matters fungi or microscopic animalculÆ, and the idea had taken shape that these creatures might have an influence in the phenomena. The proofs were wanting, but the notion of a possible relation remained. We may read in his 'Lessons on Chemistry' with what disdain Liebig mentioned these hypothetical opinions.

'Those who pretend to explain the putrefaction of animal substances by the presence of animalculÆ,' he wrote, 'reason very much like a child who would explain the rapidity of the Rhine by attributing it to the violent motions imparted to it in the direction of Bingen by the numerous wheels of the mills of Mayence. Is it possible to consider plants and animals as the causes of the destruction of other organisms when their own elements are condemned to undergo the same decompositions as the creatures which have preceded them? If the fungus is the cause of the destruction of the oak, if the microscopic animalcula is the cause of the putrefaction of the dead elephant, I would ask in my turn what is the cause which determines the putrefaction of the fungus or of the microscopic animalcula when life is withdrawn from these two organisms?'

Thirty-two years later, and after Pasteur had accumulated, during more than twenty years, proof upon proof that the theory of Liebig would not stand examination, a physician of Paris, M. Bouillaud, asked, with the insistent voice of a querulous octogenarian: 'Let M. Pasteur then tell us here, in presence of the AcadÉmie de MÉdecine, what are the ferments of the ferments.'

Before replying to this argument, which Liebig and M. Bouillaud believed to be irrefutable, Pasteur, wishing to mark all the phases of the phenomena, expounded in a short preamble the part played by atmospheric oxygen in the destruction of animal and vegetable matters after death. It is easy to understand, indeed, that fermentation and putrefaction only represent the first phase of the return to the atmosphere and to the soil of all that has lived. Fermentations and putrefactions give rise to substances which are still very complex, although they represent the products of decomposition of fermentable matters. When sugar ferments, a large proportion of it becomes gas; but alongside of the carbonic acid gas which is formed, and which is, indeed, a partial return of the sugar to the atmosphere, new substances, such as alcohol, succinic acid, glycerine, and materials of yeast, are produced. When the flesh of animals putrifies, certain products of decomposition, also very complex, are formed with the vapour of water and the other gases of putrefaction. Where, then, does nature find the agents of destruction of these secondary products?

The great fact of the destruction of animal and vegetable matters is accomplished by slow combustion, through the appropriation of atmospheric oxygen. Here, again, one must banish from science the preconceived views which assumed that the oxygen seized directly on the organic matter after death, and that this matter was consumed by purely chemical processes. It is life that presides over this work of death.

If fermentation and putrefaction are principally the work of microscopic anaÉrobies, living without free oxygen, the slow combustion is found very largely, if not exclusively, to depend upon a class of infinitely small aÉrobies. It is these last which have the property of consuming the oxygen of the air. It is these lower organisms which are the powerful agents in the return to the atmosphere of all which has lived. Mildew, mould, bacteria, which we have already noticed, monads, two thousand of which would go to make up a millimeter, all these microscopic organisms are charged with the great work of re-establishing the equilibrium of life by giving back to it all that it has formed.

To demonstrate the important part played everywhere by these microscopic organisms, Pasteur made two experiments. He first introduced into vessels air deprived of all dust. This process we shall have occasion to examine in all its details, in connection with the researches on spontaneous generation. In these vessels, containing pure air, were placed the water of yeast with sugar dissolved in it, milk, sawdust—all of which had been deprived by heat of the germs of the lower organisms. The vessels and their contents were then subjected to a temperature of twenty-five to thirty-five degrees Centigrade. In a series of parallel experiments, made under the same conditions and at the same temperature, Pasteur took no steps to prevent the germination of the little seeds of mould suspended in the air, or associated with the substances contained in the vessels, neither did he avoid other infinitely small germs of the class aÉrobies.

After some time the air of all the vessels of the two series was submitted to analysis, when, behold, a very interesting fact! In the vessels where life had been withdrawn from the organic matters—that is to say, where there were no germs—the air still contained a large proportion of oxygen. In the vessels, on the contrary, where the microscopic organisms had been allowed to develop, the oxygen was totally absent, having been replaced by carbonic acid gas. And, further, for this absorption and total consumption of the oxygen gas a few days had sufficed; while in the vessels without microscopic life there remained, after several years, a considerable quantity of oxygen in a free state, so weak is the proportion of oxygen that the organic matters consume directly and chemically when the infinitely small organisms are absent.

But can these microscopic organisms, after having decomposed or burnt up all these secondary products, be in their turn decomposed?

How, cried M. Bouillaud, repeating his question, can they be destroyed or decomposed? How can their materials, which are of the same order as those of all the living creatures of the earth, be gasified and caused to return to the atmosphere? After having been charged with the transformation of others, whose business will it be to transform them?

A ferment which has finished its work, replied Pasteur, and which for want of aliment cannot continue it, becomes in its turn an accumulation, so to speak, of dead organic matters. Such, for example, would be an accumulation of yeast exposed to the air. Leave this mass to itself in summer temperature, and you will see appear in the interior of the mass anaÉrobic vibrios and the putrefactions associated with their life when protected from contact with the air. At the same time, on the surface of the entire mass—that is to say, that which finds itself in immediate contact with the oxygen of the air—the germs of bacteria, the seeds of mould will grow, and, by fixing the oxygen, determine the slow combustions which gasify the mass. The ferments of ferments are simply ferments. As long as the aÉrobic ferments of the surface have at their disposal free oxygen, they will multiply and continue their work of destruction. The anaÉrobic vibrios perish for want of new matter to decompose, and they form, in their turn, a mass of organic matter which, by and by, becomes the prey of aÉrobies. The portion of the aÉrobies which has lived becomes the prey either of new aÉrobies of different species, or of individuals of their own species, so that from putrefaction to putrefaction, and from combustion to combustion, the organic mass with which we started finds itself reduced to an assemblage of anaÉrobic and aÉrobic germs—of those same germs which were mixed up in the original primitive organic substances.

Though a collection of germs becomes again in its turn a collection of organic matter, subject to the double action of the phenomena of putrefaction and of combustion, there need be no anxiety as to their ultimate destruction; in the final analysis they represent life under its eternal form, for life is the germ, and the germ is life.

Thus in the destruction of that which has lived, all reduces itself to the simultaneous action of these three great natural phenomena—fermentation, putrefaction, and slow combustion. A living organism dies—animal, or plant, or the remains of one or the other. It is exposed to the contact of the air. To the life which has quitted it succeeds life under other forms. In the superficial parts, which the air can reach, the germs of the infinitely small aÉrobies hatch and multiply themselves. The carbon, the hydrogen, and the nitrogen of the organic matters are transformed by the oxygen of the air, and under the influence of the life of these aÉrobies, into carbonic acid, vapour of water, and ammonia gas. As long as organic matter and air are present, these combustions will continue. While these superficial combustions are going on, fermentation and putrefaction are doing their work in the interior of the mass by the developed germs of the anaÉrobies, which not only do not require oxygen for their life, but which oxygen actually kills. Little by little, at length, by this work of fermentation and slow combustion, the phenomenon is accomplished. Whether in the free atmosphere, or under the earth, which is always more or less impregnated with air, all animal and vegetable matters end by disappearing. To arrest these phenomena an extremely low temperature is required. It is thus that in the ice of the Polar regions antediluvian elephants have been found perfectly intact. The microscopic organisms could not live in so cold a temperature. These facts still further strengthen all the new ideas as to the important part performed by these infinitely small organisms, which are, in fact, the masters of the world. If we could suppress their work, which is always going on, the surface of the globe, encumbered with organic matters, would soon become uninhabitable.