PHYSIOLOGICAL 361 



that a ferment or enzyme secreted by the yeast — made by it in its 

 one-three-thousandth of an inch laboratory — can bring about 

 fermentation without there being any living yeast cells in the 

 substance that is fermented — that is, the extract of the yeast will 

 do the same work as the living yeast. 



Beyond this, however, investigators are pressing, and bringing us 

 back again to Pasteur; for they have proof that the living yeast- 

 plants work far better than the non-living yeast juice. The fermenta- 

 tion induced by the living organism is more rapid and also more 

 regular — that is to say, less liable to be disturbed by extraneous 

 influences. Another line of inquiry concerns the tracking back of 

 the ferment (zymase) to the probable parent substance (certain 

 definite granules) formed in the living matter of the yeast cell. It is 

 indeed usual in ferment-making cells that the ferment should first 

 appear in a masked or inactive form called zymogen. But we do not 

 yet know how the ferment achieves what it does. 



From the general natural history point of view the salient fact is 

 that the yeast-plant feeds on the sugar. It is on the strength of the 

 sugar that it multiplies prodigiously by budding, quickly leavening 

 the whole lump. But it cannot utilize the sugar as a source of energy 

 by oxidation or combustion as we, for instance, do; it utilises it in 

 a wasteful way by fermentation, which leaves about 95 per cent, in 

 the form of alcohol. The carbon dioxide liberated in the fermentative 

 change makes the bread "light", and it accumulates in large quanti- 

 ties in the process of brewing. As a heavy gas it sinks in air, 

 and we used to read in old books that it would put out a candle 

 low'ered into the emptied vat, and so make it dangerous or even fatal 

 to a workman descending into it. 



Of great practical importance is the modern precision which has 

 made it possible to distinguish many different kinds of yeast-plant. 

 An expert like Mr. Chapman separates out not merely species, but 

 genera, which differ from one another in shape and size, in their 

 mode of multiplying, and in the way they work. The Irewer's and 

 distiller's yeast, that has. been cultivated for so long, is probably 

 one species {Saccharomyces cerevisecB), but of this there are many 

 races and varieties, differing in the rapidity with which they bring 

 about fermentation and in the flavour of their products. It was a 

 great step when Hansen, in 1879, showed how it was practicable to 

 isolate a single yeast cell and thus start a pure culture. It must be 

 kept in mind that there are many "wild" yeasts, such as those that 

 wait in the vineyard soil till the grapes are ripe, and it is very plain 

 that in industrial processes the microscope must guard the door 

 against intruders which are unsuitable for the particular end in 

 view. Science is always eliminating chance. 



Ferments. — Man's knowledge of ferments is very modern, but 

 his knowledge of fermenting is very ancient. Vines were cultivated 



