Sept. 13, 1 888 J 



NATURE 



479 



already remarked, have very little to do with one another ; and 

 it would save confusion and emphasize the fact if we ceased to 

 speak of ferments but used some of the alternative names which 

 have teen proposed for them, such as zymases or enzymes. 



The classical case of fermentation, which is the root of our 

 whole knowledge of the subject, is that of the conversion of 

 sugar into alcohol. Its discovery has everywhere accompanied 

 the first stages of civilization in the human race. Its details are 

 now taught in our text-books ; and I should hardly hope to be 

 excused for referring to it in any detail if it were not nec e ssary 

 for my purpose to draw your attention more particularly to one 

 or two points connected with it. 



Let us trace what happens in a fermenting liquid. It becomes 

 turbid, it froths and effervesces, the temperature sensibly in- 

 creases ; this is the first stage. After this it begins to clear, the 

 turbidity subsides as a sediment ; the sugar which the fluid at first 

 contained has in great part disappeared, and a new ingredient, 

 alcohol, is found in its place. 



It is just fifty years ago that the great Dutch biologist Schwann 

 made a series of investigations which incontrovertibly demon- 

 strated that both fermentation and putrefaction were due to the 

 presence of minute organisms which live and propagate at the 

 expense of the liquids in which they produce as a result these 

 extraordinary changes. The labours of Pasteur have confirmed 

 Schwann's results, and — what could not have been foreseen — 

 have extended the possibilities of this field of investigation to 

 those disturbances in the vital phenomena of living organisms 

 themselves which we include under the name of "disease," and 

 which, no one will dispute, are matters of the deepest concern 

 to every one of us. 



Now, at first sight, the conversion of starch into sugar by 

 means of diastase seems strikingly analogous to the conversion 

 of sugar into alcohol. It is for this reason that the phenomena 

 have been so long associated But it is easy to show that they 

 are strikingly different. Diastase is a chemical substance of 

 obscure composition it is true, but inert and destitute of any vital 

 properties, nor is it affected by the changes it induces. Yeast, 

 on the other hand, which is the active agent in alcoholic 

 fermentation, is a definite organism ; it enormously increases 

 during the process, and it appears to me impossible to resist the 

 conclusion that fermentation is a necessary concomitant of the 

 peculiar conditions of its life. Let me give you a few facts 

 which go to prove this. In the first place, you cannot ferment 

 a perfectly pure solution of sugar. The fermentable fluid must 

 contain saline and nitrogenous matters necessary for the nutri- 

 tion of the yeast protoplasm. In pure sugar the yeast starves. 

 Next, Schwann found that known protoplasmic poisons by 

 killing the yeast-cells, would prohibit fermentation, lie found 

 the same result to hold good of putrefaction, and this is the 

 basis of the w hole theory of antiseptics. Nor can the action of 

 yeast be attributed to any ferment which the yeast secretes. It 

 is true that pure cane-sugar cannot be fermented, and that yeast 

 effects the inversion of this, as it is called, into glucose and 

 laevulose. It does this by a ferment which can be extracted from 

 it, and which is often present in plants. But you can extract 

 nothing from yeast which will do its peculiar work apart from 

 itself. Ilelmholtz made the crucial experiment of suspending a 

 bladder full of boiled grape-juice in a vat of fermenting must ; 

 it underwent no change ; and even a film of blotting-paper has 

 been found a sufficient obstacle to its action. We are driven, 

 then, necessarily to the conclusion that in the action of "fer- 

 ments" or zymases we have to do with a chemical — i.e. a purely 

 physical process ; while in the case of yeast we encounter a purely 

 physiological one. 



How, then, is this action to be explained ?, Pasteur has laid 

 stress on a fact which had some time been known, that the pro- 

 duction of alcohol from sugar is a result of which yeast has not 

 the monopoly. If ripening fruits, such as plums, are kept in an 

 atmosphere free from oxygen, Berard found that they, too, 

 exhibit this remarkable transformation ; their sugar is converted 

 appreciably into alcohol. On the other hand, Pasteur has shown 

 that, if yeast is abundantly supplied with oxygen, it feeds on the 

 sugar of a fermentable fluid without producing alcohol. But, 

 under the ordinary circumstances of fermentation, its access to 

 oxygen is practically cut off; the yeast, then, is in exactly the 

 same predicament as the fruit in Berard's experiment. Sugar is 

 broken up into carbon dioxide and alcohol in an amount far in 

 excess of the needs of mere nutrition. In this dissociation it 

 can be shown that an amount of energy is set free in the form of 

 heat equal to about one-tenth of what would be produced by the 



total combustion of an equivalent amount of grape-sugar. If 

 the protoplasm of the yeast could, with the aid of atmospheric 

 oxygen, completely decompose a unit of grape-sugar, it would 

 get ten times as much energy in the shape of heat as it could 

 get by breaking it up into alcohol and carbon dioxide. It 

 follows, then, that to do the same amount of growth in either 

 case, it must break up ten times as much sugar without a supply of 

 oxygen as with it. And this throws light on what has always 

 been one of the most remarkable facts about fermentation— the 

 enormous amount of change which the yeast manages to effect in 

 proportion to its own development. 



There are still two points about yeast which deserve attention 

 before we dismiss it. When a fermenting liquid comes to contain 

 about 14 per cent, of alcohol, the activity of the yeast ceases, 

 quite independently of whether the sugar is used up or not. In 

 other cases of fermentation the same inhibiting effect of the 

 products of fermentation is met with. Thus, lactic fermentation 

 soon come-; to an end unless calcium carbonate or some similar 

 substance be added, which removes the lactic acid from the 

 solution as fast as it is formed. 



The other point is that in all fermentations, besides what may- 

 be termed the primary products of the process, other bodies are 

 produced. In the case of alcoholic fermentation the primary 

 bodies are alcohol and carbon dioxide ; the secondary, succinic 

 acid and glycerine. Delpino has suggested that these last 

 are residual products derived from that portion of the fer- 

 mentable matter which is directly applied to the nutrition of the 

 protoplasm. 



Yeast, itself the organism which effects the remarkable 

 changes on which I have dwelt, is somewhat of a problem. It 

 is clear that it i-; a fungus, the germs of which must be ubiqui- 

 tous in the atmosphere. It is difficult to believe that the simple 

 facts, which are all we know about it, constitute its entire 

 life-history. It is probably a transitory stage of some more 

 complicated organism. 



1 can only briefly refer to putrefaction. This is a far more 

 complex process than that wdiich I have traced in the case of 

 alcoholic fermentation. In that, nitrogen is absent, while it is an 

 essential ingredient in albuminoids, which are the substances 

 which undergo putrefactive changes. But the general principles 

 are the same. Here, too, we owe to Schwann the demonstration 

 of the fact that the effective agents in the process are living 

 organisms. If we put into a flask a putrescible liquid such as 

 broth, boil it for some time, and during the process of boiling 

 plug the mouth with some cotton-wool, we know that the broth 

 will remain long unchanged, while if we remove the wool 

 putrescence soon begins. Tyndall has shown that, if we 

 conduct the experiment on one of the high glaciers of the Alps, 

 the cotton-wool may be di-pensed with. We may infer, then, 

 that the germs of the organisms which produce putrefaction are 

 abundant in the lower levels of the atmosphere and are absent 

 from the higher. They are wafted about by currents of air ; 

 but they are not imponderable, and in still air they gradually 

 subside. Dr. Lodge has shown that air is rapidly cleared of 

 suspended dust by an electric discharge, and this, no doubt, 

 affords a simple explanation of the popular belief that thunderous 

 weather is favourable to putrefactive changes. 



Cohn believes that putrefaction is due to due to an organism 

 called Bacterium termo, which plays in it the same part that 

 yeast does in fermentation. This is probably too simple a 

 statement ; but the general phenomena are nevertheless similar. 

 There is the same breaking down of complex into simpler 

 molecules ; the same evolution of gas, especially carbon dioxide ; 

 the same rise of temperature. The more or less stable products 

 of the process are infinitely more varied, and it is difficult, if not 

 impossible, to say, in the present state of our knowledge, 

 whether in most cases they are the direct outcome of the putre- 

 factive process, or residual products of the protoplasmic activity 

 of the organisms which induce it. Perhaps, on the analogy of 

 the higher plants, in which some of them also occur, we may 

 attribute to the latter category certain bodies closely resembling 

 vegetable alkaloids ; these are called ptomaines, and are extremely 

 poisonous. Besides such bodies, Bacteria undoubtedly generate 

 true ferments and peculiar colouring-matters. But there are in 

 most cases of putrefaction a profusion of other substances, 

 which represent the various stages of the breaking up of the 

 complex proteid molecule, and are often themselves the outcome 

 of subsidiary fermentations. 



These results are of great interest from a scientific point of 

 view. But their importance at the present moment in the study 



