DECOMPOSITION OF ORGANIC COMPOUNDS. 455 



C 2 H 6 -f O C 2 H 4 -f H a O. 



This aldehyde, when further acted upon by oxygen, takes up an 

 atom of this element, thereby forming acetic acid : 



C 2 H 4 + O = : C 2 H 4 2 



The three instances given above illustrate the action of oxygen 

 upon organic substances, which action may consist in a mere removal 

 of hydrogen, in a replacement of hydrogen by oxygen, or in an 

 oxidation of both the carbon and hydrogen, and also of sulphur and 

 phosphorus, if they be present. 



An organic substance, when perfectly dry and exposed to dry air 

 only, may not suffer decay for a long time (not even for centuries), 

 but in the presence of moisture and air this oxidizing action takes 

 place almost invariably. 



Besides the slow oxidation or decay which all dead organic matter 

 undergoes in the presence of moisture, there is another kind of slow 

 oxidation, called respiration, which takes place in the living animal ; 

 this process will be more fully considered in the physiological part of 

 this book. 



Fermentation and putrefaction. These terms are applied to 

 peculiar kinds of decomposition, by which the molecules of certain 

 organic substances are split up into two or more molecules of a less 

 complicated composition. These decompositions take place when 

 three factors are simultaneously acting upon the organic substance. 

 These factors are : presence of moisture, favorable temperature, and 

 presence of a substance generally termed ferment. 



The most favorable temperature for these decompositions lies 

 between 25 and 40 C. (77 and 104 F.), but they may take place 

 at lower or higher temperatures. No substance, however, will either 

 ferment or putrefy at or below the freezing-point, or at or above the 

 boiling-point of water. 



The nature of the various ferments differs widely, and their true 

 action cannot, in many cases, be explained; what we do know is, 

 that the presence of comparatively small (often minute) quantities of 

 one substance (the ferment) is sufficient to cause the decomposition of 

 large quantities of certain organic substances, the ferment itself suf- 

 fering often no apparent change during this decomposition. 



Ferments have been divided into two classes : 1. Organized fer- 

 ments (sometimes called true ferments), being unicellular living micro- 

 organisms chiefly of vegetable origin. 2. Soluble ferments, unorganized 



