DECOMPOSITION OF ORGANIC COMPOUNDS. 253 



requires six atoms of oxygen, when it is converted into carbon 

 dioxide and water: 



C 2 H 6 + 60 = 2CO 2 + 3H 2 0. 



But alcohol may also undergo slow oxidation, in which case 

 oxygen first removes hydrogen, with which it combines to form 

 water, whilst at the same time a compound known as acetic 

 aldehyde, C 2 H 4 0, is formed: 



C 2 H 6 + O = C 2 H 4 O + H 2 0. 



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 rrydrogen, 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 mole- 

 cules 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 20 and 40, but they may take place at lower or higher 



