CHARACTERS OF ORGANISMS. 31 



with hydrogen and nitrogen as well as oxygen in various 

 proportions. The constancy of carbon as an ingredient of 

 organic bodies is so great that what formerly was called 

 organic chemistry is now often called the chemistry of the 

 carbon compounds. 



These complex associations of many atoms of carbon with 

 many atoms of other elements, are readily dissociated when ex- 

 posed to the air under even slightly disturbing influences. When 

 heated to a certain degree they burn, i. e., unite rapidly with the 

 oxygen of the air ; and in the presence of minute organisms they 

 putrefy. Thus instability is a general feature commonly met 

 with in most substances of organic origin. 



Chemical instability reaches the highest pitch in tissues which 

 are actually alive and engaged in vital processes. So long as any 

 texture lives it must constantly undergo certain chemical changes, 

 one of which is regarded as a kind of decomposition, tending to 

 produce disintegration, and the other, a reintegration by means 

 of new chemical associations with fresh materials. A tissue may 

 then be said to deserve the term living, only as long as it under- 

 goes these antagonistic chemical changes. The tendency to de- 

 structive oxidation or disintegration is intimately connected with 

 the functional activity of the living texture and increases with 

 this activity. The reintegration or constructive process requires 

 the presence of suitable materials with which the texture may 

 combine, in order to make up for the loss. Thus living tissues 

 are ever on the point of destruction, which can only be warded 

 off by the timely reconstruction of their chemical ingredients by 

 suitable fresh materials. This reconstruction by means of fresh 

 matter from without is called assimilation, and forms the most, if 

 not the only, satisfactory criterion by which adequately to dis- 

 tinguish living beings from inorganic matters. 



The object of assimilation is to supply suitable fresh materials 

 to the various textures for the chemical processes required for 

 their function when living. This will be found to form a great 

 part of physiological study. Further, the energy manifested in 

 the living activity of the textures depends upon the various oxi- 

 dizing processes, and the exact laws which govern these combus- 



