THE CAUSES AND MECHANISM OF COMBUSTION 543 



An optimal oxygen-pressure exists for every organism, but the course of this 

 curve is not necessarily coincident with that for growth. As the result of a change 

 of oxygen-pressure certain after effects may ensue such as have already been 

 mentioned as following the removal of free oxygen. Thus Johannsen found 

 that after remaining for a time under a high partial-pressure of oxygen, respiration 

 was temporarily more than normally active when the plant was returned to ordinary 

 air. The action of sudden changes has not yet been determined, and hence it 

 is uncertain whether an anaerobe is injured more rapidly by a sudden supply of 

 oxygen than by one which gradually increases to the same level. 



SECTION 101. The Causes and Mechanism of Physiological 



Combustion (continued}. 



A clear comprehension of the phenomena of physiological combustion 

 has not yet been obtained, and the genetic relationship between aerobic and 

 anaerobic respiration simply shows that the oxidation is a metabolic process 

 regulated by the vital activity of the protoplast. Neutral oxygen is drawn 

 into metabolism by means of the affinities which the latter creates, and 

 without being oxonized, for no such oxidizing effects are produced upon 

 pigments or chromogens present within normal living protoplasts, as are 

 induced by simple treatment with a weak oxidizing agent such as peroxide 

 of hydrogen. Protoplasm does not act therefore as a general oxidizing 

 agent, although almost any substances which enter into metabolism may 

 undergo oxidation when necessary. Thus certain nitro-bacteria oxidize 

 ammonia into nitrites, while others which are unable to oxidize ammonia 

 complete the process of oxidation into nitrates. Similarly in the process of 

 respiration, organic acids and other substances which are capable of still 

 further oxidation may be produced in spite of the presence of an abundance 

 of oxygen (Sects. 85 and 86). 



Those organisms in which energy is obtained partially or entirely 

 by the oxidation of nitrites, sulphur, &c., are of the utmost importance 

 in the study of the general characteristics of physiological combustion 

 (Sect. 96). for in them the means by which oxidation is induced may 

 be revealed. It is, however, possible that the process of oxidation is 

 different in different organisms and according to the substance oxidized, 

 for facultative anaerobes afford examples to show that energy need 

 not always be obtained in precisely the same manner. It is, moreover, 

 uncertain whether oxidizable substances are produced by metabolism and 

 that these combine with oxygen as rapidly as they are formed, or whether 

 the protoplasm itself induces the oxidation of compounds upon which 

 oxygen exercises no direct effect. In the latter case the oxidizing action 



