698 EEPORX— 1888. 



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 iucontrovertibly demonstrated that hoth 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 residts, and — what 

 could not have been foreseen — have extended the possibilities of this field of investi- 

 gation 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 subtance 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 piu-e solution of sugar. The fermentible fluid must contain 

 saline and nitrogenous matters necessary for the nutrition 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. He found the same 

 result to hold good of putrefaction, and this is the basis of the whole 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 lajvulose. 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. Helmholtz made the crucial experiment of suspending a bladder full of 

 boiled grape-juice in a vat of fermenting must; it imderwent 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 ' ferments' 

 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 P Pasteur has laid stress on a fact 

 which had some time been known, that the production of alcohol fi'om 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, B6rard found that the}', 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 fermentible fluid without producing alcohol. But, under 

 the ordinary circumstances of fermentation, its access to oxygen is practically cut 

 off"; the yeast, then, is iu exactly the same predicament as the fruit in Berard's ex- 

 periment. 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 fer- 

 mentation — the enormous amount of change which the yeast manages to eflect in 

 proportion to its own development. 



There are still two points about yeast which deserve attention before we dismiss 



