64 ADVANCED PHYSIOLOGY 



Further than this, we should note that a substance which 

 has undergone one fermentative process and been broken up 

 chemically into two or more different substances, may be still 

 further subjected to changes by other ferments. As an illus- 

 tration: starch changes into sugar; sugar may be broken by 

 fermentation into alcohol and carbon dioxid; alcohol by 

 fermentation will yield acetic acid (the acid of vinegar) and 

 water; acetic acid by fermentation is resolved into carbon 

 dioxid and water. 



It is noticeable that the change is always from what is com- 

 plex to things which are simpler; fermentation never produces 

 the opposite result, i. e., is never constructive. In other 

 words, to use a chemical term, ferments are catalytic agents. 



These two examples are illustrations of different types 

 of fermentation, one produced by the growth of living, micro- 

 scopic plants; the other by non-living enzymes. 



While these fermenting agents are unlike in many respects, 

 their activities agree in the following points, which are, there- 

 fore, characteristic of fermentations. 1. They are progres- 

 sive chemical changes which take place in organic bodies. By 

 "progressive" is meant that when once begun they continue 

 until the fermenting body is used up or until something stops 

 the action. 2. They never take place spontaneously, but 

 are brought about by the addition to the fermentable body 

 of substances called ferments. 3. They are stopped by the 

 influence of heat, and by the presence of some chemicals. 

 4. The substance which starts fermentation does not appear 

 to be used up in the process. 



When a substance ferments, its nature is totally changed; 

 alcohol and carbon dioxid are quite different from sugar, 

 and sugar is unlike starch. Fermentation always forms new 

 substances, and these new substances are sometimes good and 

 sometimes bad in their effects. Fermentations may produce 

 poisons from perfectly harmless substances. When starch 

 is turned into sugar, the product is a useful food, but when 

 sugar is turned into alcohol, the product is dangerous and 



