Thus Lavoisier thought he had demonstrated that the carbonic acid and the alcohol which are produced by the process of fermentation, are equal in weight to the sugar which disappears; but the application of the more refined methods of modern chemistry to the investigation of the products of fermentation by Pasteur, in 1860, proved that this is not exactly true, and that there is a deficit of from 5 to 7 per cent.
of the sugar which is not covered by the alcohol and carbonic acid evolved. The greater part of this deficit is accounted for by the discovery of two substances, glycerine and succinic acid, of the existence of which Lavoisier was unaware, in the fermented liquid.
But about 1-1/2 per cent. still remains to be made good. According to Pasteur, it has been appropriated by the yeast, but the fact that such appropriation takes place cannot be said to be actually proved.
However this may be, there can be no doubt that the const.i.tuent elements of fully 98 per cent. of the sugar which has vanished during fermentation have simply undergone rearrangement; like the soldiers of a brigade, who at the word of command divide themselves into the independent regiments to which they belong. The brigade is sugar, the regiments are carbonic acid, succinic acid, alcohol, and glycerine.
From the time of Fabroni, onwards, it has been admitted that the agent by which this surprising rearrangement of the particles of the sugar is effected is the yeast. But the first thoroughly conclusive evidence of the necessity of yeast for the fermentation of sugar was furnished by Appert, whose method of preserving perishable articles of food excited so much attention in France at the beginning of this century.
Gay-Lussac, in his "Memoire sur la Fermentation,"[1] alludes to Appert"s method of preserving beer-wort unfermented for an indefinite time, by simply boiling the wort and closing the vessel in which the boiling fluid is contained, in such a way as thoroughly to exclude air; and he shows that, if a little yeast be introduced into such wort, after it has cooled, the wort at once begins to ferment, even though every precaution be taken to exclude air. And this statement has since received full confirmation from Pasteur.
[Footnote 1: "Annales de Chimie," 1810.]
On the other hand, Schwann, Schroeder and Dusch, and Pasteur, have amply proved that air may be allowed to have free access to beer-wort, without exciting fermentation, if only efficient precautions are taken to prevent the entry of particles of yeast along with the air.
Thus, the truth that the fermentation of a simple solution of sugar in water depends upon the presence of yeast, rests upon an una.s.sailable foundation; and the inquiry into the exact nature of the substance which possesses such a wonderful chemical influence becomes profoundly interesting.
The first step towards the solution of this problem was made two centuries ago by the patient and painstaking Dutch naturalist, Leeuwenhoek, who in the year 1680 wrote thus:--
"Saep.i.s.simo examinavi fermentum cerevisiae, semperque hoc ex globulis per materiam pellucidam fluitantibus, quam cerevisiam esse censui, constare observavi: vidi etiam evidentissime, unumquemque hujus fermenti globulum denuo ex s.e.x distinctis globullis constare, accurate eidem quant.i.tate et formae, cui globulis sanguinis nostri, respondentibus.
"Verum talis mini de horum origine et formatione conceptus formabam; globulis nempe ex quibus farina Tritici, Hordei, Avenae, f.a.gotritici, se constat aquae calore dissolvi et aquae commisceri; hac, vero aqua, quam cerevisiam vocare licet, refrigescente, multos ex minimis particulis in cerevisia coadunari, et hoc pacto efficere particulam sive globulum, quae s.e.xta pars est globuli faecis, et iterum s.e.x ex hisce globulis conjungi."[1]
[Footnote 1: Leeuwenhoek, "Arcana Naturae Detecta." Ed. Nov., 1721.]
Thus Leeuwenhoek discovered that yeast consists of globules floating in a fluid; but he thought that they were merely the starchy particles of the grain from which the wort was made, re-arranged. He discovered the fact that yeast had a definite structure, but not the meaning of the fact. A century and a half elapsed, and the investigation of yeast was recommenced almost simultaneously by Cagniard de la Tour in France, and by Schwann and Kutzing in Germany. The French observer was the first to publish his results; and the subject received at his hands and at those of his colleague, the botanist Turpin, full and satisfactory investigation.
The main conclusions at which they arrived are these. The globular, or oval, corpuscles which float so thickly in the yeast as to make it muddy, though the largest are not more than one two-thousandth of an inch in diameter, and the smallest may measure less than one seven-thousandth of an inch, are living organisms. They multiply with great rapidity, by giving off minute buds, which soon attain the size of their parent, and then either become detached or remain united, forming the compound globules of which Leeuwenhoek speaks, though the constancy of their arrangement in sixes existed only in the worthy Dutchman"s imagination.
It was very soon made out that these yeast organisms, to which Turpin gave the name of _Torula cerevisiae_, were more nearly allied to the lower Fungi than to anything else. Indeed Turpin, and subsequently Berkeley and Hoffmann, believed that they had traced the development of the _Torula_ into the well-known and very common mould--the _Penicillium glauc.u.m_. Other observers have not succeeded in verifying these statements; and my own observations lead me to believe, that while the connection between _Torula_ and the moulds is a very close one, it is of a different nature from that which has been supposed. I have never been able to trace the development of _Torula_ into a true mould; but it is quite easy to prove that species of true mould, such as _Penicillium_, when sown in an appropriate nidus, such as a solution of tartrate of ammonia and yeast-ash, in water, with or without sugar, give rise to _Torulae_, similar in all respects to _T.
cerevisiae_, except that they are, on the average, smaller. Moreover, Bail has observed the development of a _Torula_ larger than _T.
cerevisiae_, from a _Mucor_, a mould allied to _Penicillium_.
It follows, therefore, that the _Torulae_, or organisms of yeast, are veritable plants; and conclusive experiments have proved that the power which causes the rearrangement of the molecules of the sugar is intimately connected with the life and growth of the plant. In fact, whatever arrests the vital activity of the plant also prevents it from exciting fermentation.
Such being the facts with regard to the nature of yeast, and the changes which it effects in sugar, how are they to be accounted for?
Before modern chemistry had come into existence, Stahl, stumbling, with the stride of genius, upon the conception which lies at the bottom of all modern views of the process, put forward the notion that the ferment, being in a state of internal motion, communicated that motion to the sugar, and thus caused its resolution into new substances. And Lavoisier, as we have seen, adopts substantially the same view, (But Fabroni, full of the then novel conception of acids and bases and double decompositions, propounded the hypothesis that sugar is an oxide with two bases, and the ferment a carbonate with two bases; that the carbon of the ferment unites with the oxygen of the sugar, and gives rise to carbonic acid; while the sugar, uniting with the nitrogen of the ferment, produces a new substance a.n.a.logous to opium. This is decomposed by distillation, and gives rise to alcohol.) Next, in 1803, Thenard propounded a hypothesis which partakes somewhat of the nature of both Stahl"s and Fabroni"s views. "I do not believe with Lavoisier," he says, "that all the carbonic acid formed proceeds from the sugar. How, in that case, could we conceive the action of the ferment on it? I think that the first portions of the acid are due to a combination of the carbon of the ferment with the oxygen of the sugar, and that it is by carrying off a portion of oxygen from the last that the ferment causes the fermentation to commence--the equilibrium between the principles of the sugar being disturbed, they combine afresh to form carbonic acid and alcohol."
The three views here before us may be familiarly exemplified by supposing the sugar to be a card-house. According to Stahl, the ferment is somebody who knocks the table, and shakes the card-house down; according to Fabroni, the ferment takes out some cards, but puts others in their places; according to Thenard, the ferment simply takes a card out of the bottom story, the result of which is that all the others fall.
As chemistry advanced, facts came to light which put a new face upon Stahl"s hypothesis, and gave it a safer foundation than it previously possessed. The general nature of these phenomena may be thus stated:--A body, A, without giving to, or taking from, another body, B, any material particles, causes B to decompose into other substances, C, D, E, the sum of the weights of which is equal to the weight of B, which decomposes.
Thus, bitter almonds contain two substances, amygdalin and synaptase, which can be extracted, in a separate state, from the bitter almonds.
The amygdalin thus obtained, if dissolved in water, undergoes no change; but if a little synaptase be added to the solution, the amygdalin splits up into bitter almond oil, prussic acid, and a kind of sugar.
A short time after Cagniard de la Tour discovered the yeast plant, Liebig, struck with the similarity between this and other such processes and the fermentation of sugar, put forward the hypothesis that yeast contains a substance which acts upon sugar, as synaptase acts upon amygdalin. And as the synaptase is certainly neither organized nor alive, but a mere chemical substance, Liebig treated Cagniard de la Tour"s discovery with no small contempt, and, from that time to the present, has steadily repudiated the notion that the decomposition of the sugar is, in any sense, the result of the vital activity of the _Torula_. But, though the notion that the _Torula_ is a creature which eats sugar and excretes carbonic acid and alcohol, which is not unjustly ridiculed in the most surprising paper that ever made its appearance in a grave scientific journal[1], may be untenable, the fact that the _Torulae_ are alive, and that yeast does not excite fermentation unless it contains living _Torulae_, stands fast. Moreover, of late years, the essential partic.i.p.ation of living organisms in fermentation other than the alcoholic, has been clearly made out by Pasteur and other chemists.
[Footnote 1: "Das entrathselte Geheimniss der geistigen Gahrung (Vorlaufige briefliche Mittheilung)" is the t.i.tle of an anonymous contribution, to Wohler and Liebig"s "Annalen der Pharmacie" for 1839, in which a somewhat Rabelaisian imaginary description of the organization of the "yeast animals" and of the manner in which their functions are performed, is given with a circ.u.mstantiality worthy of the author of Gulliver"s Travels. As a specimen of the writer"s humour, his account of what happens when fermentation comes to an end may suffice. "Sobald namlich die Thiere keinen Zucker mehr vorfinden, so fressen sie sich gegenseitig selbst auf, was durch eine eigene Manipulation geschicht; alles wird verdaut bis auf die Eier, welche unverandert durch den Darmka.n.a.l hineingehen; man hat zuletzt wieder gahrungsfahige Hefe, namlich den Saamen der Thiere, der ubrig bleibt."]
However, it may be asked, is there any necessary opposition between the so-called "vital" and the strictly physico-chemical views of fermentation? It is quite possible that the living _Torula_ may excite fermentation in sugar, because it constantly produces, as an essential part of its vital manifestations, some substance which acts upon the sugar, just as the synaptase acts upon the amygdalin. Or it may be, that, without the formation of any such special substance, the physical condition of the living tissue of the yeast plant is sufficient to effect that small disturbance of the equilibrium of the particles of the sugar, which Lavoisier thought sufficient to effect its decomposition.
Platinum in a very fine state of division--known as platinum black, or _noir de platine_--has the very singular property of causing alcohol to change into acetic acid with great rapidity. The vinegar plant, which is closely allied to the yeast plant, has a similar effect upon dilute alcohol, causing it to absorb the oxygen of the air, and become converted into vinegar; and Liebig"s eminent opponent, Pasteur, who has done so much for the theory and the practice of vinegar-making, himself suggests that in this case--
"La cause du phenomene physique qui accompagne la vie de la plante reside dans un etat physique propre, a.n.a.logue a celui du noir de platine. Mais il est essentiel de remarquer que cet etat physique de la plante est etroitement lie avec la vie de cette plante."[1]
[Footnote 1: "Etudes sur les Mycodermes," Comptes-Rendus, liv., 1862.]
Now, if the vinegar plant gives rise to the oxidation of alcohol, on account of its merely physical const.i.tution, it is at any rate possible that the physical const.i.tution of the yeast plant may exert a decomposing influence on sugar.
But, without presuming to discuss a question which leads us into the very arcana of chemistry, the present state of speculation upon the _modus operandi_ of the yeast plant in producing fermentation is represented, on the one hand, by the Stahlian doctrine, supported by Liebig, according to which the atoms of the sugar are shaken into new combinations, either directly by the _Torulae_, or indirectly, by some substance formed by them; and, on the other hand, by the Thenardian doctrine, supported by Pasteur, according to which the yeast plant a.s.similates part of the sugar, and, in so doing, disturbs the rest, and determines its resolution into the products of fermentation.
Perhaps the two views are not so much opposed as they seem at first sight to be.
But the interest which attaches to the influence of the yeast plants upon the medium in which they live and grow does not arise solely from its bearing upon the theory of fermentation. So long ago as 1838, Turpin compared the _Torulae_ to the ultimate elements of the tissues of animals and plants--"Les organes elementaires de leurs tissus, comparables aux pet.i.ts vegetaux des levures ordinaires, sont aussi les decompositeurs des substances qui les environnent."
Almost at the same time, and, probably, equally guided by his study of yeast, Schwann was engaged in those remarkable investigations into the form and development of the ultimate structural elements of the tissues of animals, which led him to recognize their fundamental ident.i.ty with the ultimate structural elements of vegetable organisms.
The yeast plant is a mere sac, or "cell," containing a semi-fluid matter, and Schwann"s microscopic a.n.a.lysis resolved all living organisms, in the long run, into an aggregation of such sacs or cells, variously modified; and tended to show, that all, whatever their ultimate complication, begin their existence in the condition of such simple cells.
In his famous "Mikroskopische Untersuchungen," Schwann speaks of _Torula_ as a "cell;" and, in a remarkable note to the pa.s.sage in which he refers to the yeast plant, Schwann says:--
"I have been unable to avoid mentioning fermentation, because it is the most fully and exactly known operation of cells, and represents, in the simplest fashion, the process which is repeated by every cell of the living body."
In other words, Schwann conceives that every cell of the living body exerts an influence on the matter which surrounds and permeates it, a.n.a.logous to that which a _Torula_ exerts on the saccharine solution by which it is bathed. A wonderfully suggestive thought, opening up views of the nature of the chemical processes of the living body, which have hardly yet received all the development of which they are capable.
Kant defined the special peculiarity of the living body to be that the parts exist for the sake of the whole and the whole for the sake of the parts. But when Turpin and Schwann resolved the living body into an aggregation of quasi-independent cells, each, like a _Torula_, leading its own life and having its own laws of growth and development, the aggregation being dominated and kept working towards a definite end only by a certain harmony among these units, or by the superaddition of a controlling apparatus, such as a nervous system, this conception ceased to be tenable. The cell lives for its own sake, as well as for the sake of the whole organism; and the cells, which float in the blood, live at its expense, and profoundly modify it, are almost as much independent organisms as the _Torulae_ which float in beer-wort.
Schwann burdened his enunciation of the "cell theory" with two false suppositions; the one, that the structures he called "nucleus" and "cell-wall" are essential to a cell; the other, that cells are usually formed independently of other cells; but, in 1839, it was a vast and clear gain to arrive at the conception, that the vital functions of all the higher animals and plants are the resultant of the forces inherent in the innumerable minute cells of which they are composed, and that each of them is, itself, an equivalent of one of the lowest and simplest of independent living beings--the _Torula._
From purely morphological investigations, Turpin and Schwann, as we have seen, arrived at the notion of the fundamental unity of structure of living beings. And, before long, the researches of chemists gradually led up to the conception of the fundamental unity of their composition.
So far back as 1803, Thenard pointed out, in most distinct terms, the important fact that yeast contains a nitrogenous "animal" substance; and that such a substance is contained in all ferments. Before him, Fabroni and Fourcroy speak of the "vegeto-animal" matter of yeast.
In 1844 Mulder endeavoured to demonstrate that a peculiar substance, which he called "protein," was essentially characteristic of living matter. In 1846, Payen writes:--
"Enfin, une loi sans exception me semble apparaitre dans les faits nombreux que j"ai observes et conduire a envisager sous un nouveau jour la vie vegetale; si je ne m"abuse, tout ce que dans les tissus vegetaux la vue directe ou amplifiee nous permet de discerner sous la forme de cellules et de vaisseaux, ne represente autre chose que les enveloppes protectrices, les reservoirs et les conduits, a l"aide desquels les corps animes qui les secretent et les faconnent, se logent, puisent et charriant leurs aliments, deposent et isolent les matieres excretees."
And again:--
"A fin de completer aujourd"hui l"enonce du fait general, je rappellerai que les corps, doue des fonctions accomplies dans les tissus des plantes, sont formes des elements qui const.i.tuent, en proportion peu variable, les organismes animaux; qu"ainsi l"on est conduit a reconnaitre une immense unite de composition elementaire dans tous les corps vivants de la nature."[1]
[Footnote 1: "Mem. sur les Developpements des Vegetaux," &c.--"Mem.
Presentees." ix. 1846.]
In the year (1846) in which these remarkable pa.s.sages were published, the eminent German botanist, Von Mohl, invented the word "protoplasm,"
as a name for one portion of those nitrogenous contents of the cells of living plants, the close chemical resemblance of which to the essential const.i.tuents of living animals is so strongly indicated by Payen. And through the twenty-five years that have pa.s.sed, since the matter of life was first called protoplasm, a host of investigators, among whom Cohn, Max Schulze, and Kuhne must be named as leaders, have acc.u.mulated evidence, morphological, physiological, and chemical, in favour of that "immense unite de composition elementaire dans tous les corps vivants de la nature," into which Payen had, so early, a clear insight.
As far back as 1850, Cohn wrote, apparently without any knowledge of what Payen had said before him:--
"The protoplasm of the botanist, and the contractile substance and sarcode of the zoologist, must be, if not identical, yet in a high degree a.n.a.logous substances. Hence, from this point of view, the difference between animals and plants consists in this; that, in the latter, the contractile substance, as a primordial utricle, is enclosed within an inert cellulose membrane, which permits it only to exhibit an internal motion, expressed by the phenomena of rotation and circulation, while, in the former, it is not so enclosed. The protoplasm in the form of the primordial utricle is, as it were, the animal element in the plant, but which is imprisoned, and only becomes free in the animal; _or_, to strip off the metaphor which obscures simple thought, the energy of organic vitality which is manifested in movement is especially exhibited by a nitrogenous contractile substance, which in plants is limited and fettered by an inert membrane, in animals not so."[1]
[Footnote 1: Cohn, "Ueber Protococcus pluvialis," in the "Nova Acta"