EXAMPLES OF EEEMENTATION 201 



are known as malt. When malt is placed in water the work of con- 

 version takes place rapidly, and a sugary solution, called wort, is the 

 result. In this case the gain to the plant by the secretion of the 

 ferment is that a digestible substance (sugar) is placed at its disposal 

 in place of an indigestible substance, viz. starch. 



Acetic Acid Fermentation. Our third illustration is introduced 

 because in it and in similar cases we see fermentations that apparently 

 take place without the secretion of ferments. When beer or wine is 

 left exposed to the atmosphere, after a few days a thin skin or pellicle 

 is found on the surface. If the liquid be examined later, it will be 

 found that the alcohol has been changed into acetic acid, the conversion 

 being brought about by bacteria which thrive in the beer or wine, and 

 effect the change. The skin is made up of millions of bacteria, which 

 correspond to the yeast and barley in the two fermentations that we 

 have just considered. But whereas in those cases definite ferments 

 are secreted, in this case there is no indication of such bodies, and 

 the conclusion that must be arrived at, is, that the change which we 

 call fermentation is effected by the organism as a whole, and not 

 through the agency of a ferment secreted by it. At the same time, it 

 is not beyond the bounds of possibility that the ferment is there, 

 but that modern methods of dealing with ferments have not succeeded 

 in establishing its existence. 



§4. DEFINITION OF FERMENT AND OF FERMENTATION— 

 PROPERTIES OF FERMENT. 



We are now in a position to define what is meant by these two terms. 

 Fermentation is the term applied to the decomposition of substances 

 into simpler forms through the agency of living organisms. If there 

 is a special secretion for this purpose, the secretion is called a ferment. 



It must be added, however, that this definition is not universally 

 accepted. It is claimed, for instance, that some ferments can not only 

 decompose complicated substances, but can also build them up from 

 simpler materials. Even if this were proved in the case of a few 

 ferments, it would not seem advisable to extend the conception of 

 fermentation to this length, for the logical outcome of it would be that 

 M-e should have to dignify by this name any process of any kind 

 performed through the agency of a living organism. Such an extension 

 of the meaning of the term would serve no useful purpose, and would 

 result in some confusion, for it would make the wall of partition 

 between fermentation and metabolism extremely thin. 



