150TANICAL JilOLUGV. 417 



uecessary Jbr the niitrilion of the yeast piotophisiu. In piiie sugar tlie 

 yeast starves, isext, Seliwaini found that known protoi)lasmic i)oisons, 

 by killing the yeast-cells, would i)rohibit 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 whicli the yeast secretes. It is true that pure cane- 

 sugar can not be fermented, and that yeast effects the inversion of this, 

 as it is called, into glucose and hevulose. It does this by a ferment 

 which can be extracted from it, and Avhich is often present in plants, 

 lint you can extract nothing from yeast whicli will do its pecular work 

 ai)art from itself. Helmholtz made tlie crucial experiment of suspend- 

 ing 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 '• ferments" or zymases we have 

 to do with a chemical — i. c, a purely physical process ; wliile 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 fa(!t whicli had some time been known, that the production of alcohol 

 from sugar is a result of which yeast has not the monopoly. If ripen- 

 ing fruits — such as plums — are kept in an atnios{)here 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 witJi oxygen, 

 it feeds ou the sugar of a fermentable tiuid without producing alcohol. 

 But under the ordinary circumstance of fermentation, its access to 

 oxj'gen 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 aujount 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 of grape- 

 sugar. If the protoplasm of the yeast could, with the aid of atmos- 

 pheric 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 developme}it. 



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, qnit«i independently of 

 whether the sugar is used njt or not. In other cases of iermentation 

 the same inhibiting effect of the products of fermentation is met with. 

 H. Mis. 224 27 



