THE CHEMISTBY OF BACTEKIA. 113 



serve as the sole food for the herbivorous animal. The 

 carnivorous animal derives its food from the latter, and 

 man, as an omnivorous being, is consequently, directly or 

 indirectly, dependant upon the vegetable kingdom for food. 

 The nitrogen which as ammonia, nitrites and nitrates was 

 present in the soil, and the carbon, which as carbonic acid 

 was present in the air, have been built up into the complex 

 protoplasm of the animal cell. On the death of the plant 

 or animal, these elements would reniain indefinitely in this 

 combination were it not for the action of micro-organisms. 

 These induce the decay or putrefaction, which results in the 

 complete cleavage of these complex chemical substances, 

 so that the elements are transformed into the simple com- 

 pounds in which they originally existed, in the soil and air. 



The carbonic acid, which is thus liberated in the fer- 

 mentation or putrefaction of dead matter, is utilizable by a 

 growing plant. As long as the carbon was present in the 

 cellulose, starch, fat or protein molecule it wJLs as useless to 

 new plant life as that contained in the limestone deposits 

 of the earth. The same is equally true of the nitrogen 

 stored up in the protein molecule. It is only when brought 

 back to the inorganic form that it can serve as plant food. 

 The microscopic workers, therefore, in their r61e of scaven- 

 gers prevent the accumulation of dead animal and plant 

 matter, and, at the same time, maintain a cycle of life in 

 which they themselves form an important link, and thus 

 justify the axiom "Death is Life." 



The bacteria which produce putrefaction have been de- 

 signated as saprogenic. These same organisms, however, 

 when grown on media containing sugar or other carbohy- 

 drates may give rise to fermentations, so that in the latter 

 case they would also be designated as zymogenic. It is 

 evident, therefore, that putrefaction is not a specific change 

 due to a single organism, or even to a special group of 

 organisms. It is induced by any one of a' large number of 

 bacteria, whenever these are grown in solutions of protein 



