EVOLUTION OF COLLOIDS 129 



the multi-molecule, becomes experimentally 

 almost impossible. It is easy to discover 

 by analysis whether a mono-molecule contains 

 three or four atoms of a given element, but 

 the query as to whether a given multi-molecule 

 contains sixty or sixty-one molecules of a 

 given substance is obviously more difficult 

 of solution. The same difficulty is found 

 in the region of the mono-molecule, such as 

 those dealt with in the last chapter. When 

 there are over twenty atoms of hydrogen 

 in the molecule of a fatty acid, the chemist 

 is often unable to determine from the results 

 of an analysis, whether the figure assigned 

 in the formula should be, say, twenty or 

 twenty-one. Finally, it is to be remembered 

 that even in inorganic chemistry, compounds 

 are to be found in a similarly constituted 

 series with varying numbers of atoms, and 

 accordingly the occurrence of a series of 

 molecular combinations between two saturated 

 molecules with varying relative numbers of 

 molecules is no good evidence against the 

 chemical nature of molecular combination. 

 All molecular combinations do not lead to 

 the formation of colloids : indeed, molecular 

 combination in the case of the greater half 

 of the inorganic substances known as crystal- 

 loids (as distinguished from colloids) forms 



