48 HOLISM AND EVOLUTION chap. 



involving two types of chemical combination and structure. 

 The one type, which prevails among the salts, acids, and 

 bases of inorganic chemistry, is a much looser, less rigid 

 combination or union than the other, which prevails among 

 the carbon compounds of organic chemistry. Thus common 

 salt, which is a combination of sodium and chlorine, is now 

 understood to be a more or less loose aggregation of free 

 positive sodium atoms or ions held in equilibrium by an 

 equal number of free negative chlorine atoms or ions; the 

 equilibrium being fairly stable, without any actual union 

 of the atoms such as was assumed by ortliodox chemists. 

 In organic compounds, however, the linkage of the constit- 

 uent atoms is real, and the compound is not a system of free 

 ions in equilibrium, but a real combination or fixed structure 

 of the atoms concerned. Organic compounds thus display an 

 advance in respect of chemical structure in substances. 

 While in inorganic salts and similar substances the looser 

 arrangement of the atoms or ions approximates to the 

 type of " physical'^ combination, in organic substances, 

 on the other hand, the chemical union is more thorough 

 and intense, and leads to a closer structural character, 

 linked together by common electrons. In this connection 

 it is important to remember that organic compounds are 

 the mechanisms of life: we may therefore say that as we 

 approach life we witness a more intense element of structure 

 in chemical compounds. 



Life may have arisen in — at least it now uses as its 

 mechanisms — chemical substances of a subtler structure than 

 that which characterises inorganic compounds. 



In connection with the explanation of the structure of the 

 atom given above the question arises whether the structure 

 of the atom is really as above indicated, or whether we have 

 merely to do with a hypothesis to explain the known facts. 

 The question is important, because it raises one instance of 

 the general method of scientific explanation. Science deals 

 with sensible phenomena and tries to co-ordinate them in 

 accordance with known physical laws, and in doing so has 

 often to interpret the sensible phenomena in a particular way 



