138 ORIGIN AND NATURE OF LIFE 



An example of interest to the biological 

 chemist, is the molecular union of one molecule 

 each of common salt and grape sugar, which 

 separates in crystalline form from a common 

 solution of these two substances, and this is 

 but one instance amongst many occurring 

 in the body, too specialized to give in detail 

 in a general volume. 



With this outline sketch we may leave the 

 question of molecular unions amongst crystal- 

 loids, and pass on to the remaining properties 

 of colloids. 



All the known properties of colloids can be 

 traced to feeble molecular affinities between 

 the molecules themselves, causing them to 

 unite into multi-molecules or " solution aggre- 

 gates," and to a balance between such affini- 

 ties and similar feeble affinities for crystalloids 

 in common solution with them, and for the 

 molecules of the solvent. The whole essence 

 of the colloidal condition is that of a balance 

 of play of energies in the most delicate 

 equilibrium. Quantitative differences exist, 

 giving classes of colloids which differ from one 

 another in stability. Some are upset with 

 the greatest ease by either adding a slight ex- 

 cess of crystalloid to the solution, or by 

 heating the solution a few degrees in tempera- 

 ture, others more delicate still even undergo 



