144 MICROBIOLOGY 



Production of heat. The production of heat by bacteria 

 is so slight that it does not attract attention. Fermenting 

 cultures, according to careful tests, develop a small amount 

 of heat. The increase of temperature in organic substances 

 when stored in a moist condition, such as tobacco, hay, manure, 

 etc., is partly due to the action of bacteria. 



Chemical action. The changes which substances undergo 

 that are being decomposed depend first, on the chemical na- 

 ture of the bodies involved and the conditions under which 

 they exist, and secondly, on the varieties of bacteria present. 

 Knowledge of these chemical changes is incomplete. Chemists 

 have as yet only enumerated some of the final substances 

 evolved, and described, in a few cases, the manner in which 

 they are produced. Bacteria are able to construct their body 

 substance out of various kinds of nutrient materials and also 

 to produce fermentation products, and they are able to do 

 these things either analytically or synthetically with almost 

 equal ease. This ambidextrous metabolic power exists, ac- 

 cording to Hueppe, among bacteria to an extent as yet un- 

 known in other living things. 



Hueppe has clearly set forth several groups of phenomena 

 in the building up of the bacteria bodies. " (1) Polymerization, 

 a sort of doubling up of a single compound; (2) synthesis, a 

 union of different kinds of simple compounds into one or 

 more complex substances; (3) formation of anhydride, by 

 which new substances arise from a compound through the loss 

 of water; and (4) reduction or loss of oxygen, which is 

 brought about especially by the entrance of hydrogen into 

 the molecule. The breaking down of organic bodies of com- 

 plicated molecular structure into simpler combinations take*s 

 place, on the other hand, through the loosening of the bonds 

 of polymerization, through hydration or entrance of water 

 into the molecule, and through oxidation. The chemical 

 changes are greatly influenced by the presence or absence of 

 oxygen. ' ' 



In the presence of oxygen some of the decomposition 

 products that are formed by the attack of the anaerobic bac- 



