38 THE MECHANISM OF LIFE 



solution of similar strength of a crystalloid whose molecular 

 weight was 40. 



Not only so, but on measuring the molecular concentration, 

 the osmotic pressure, and the other constants of a colloidal 

 solution, we find values even lower than those which we should 

 expect from a consideration of its molecular weight. This is 

 probably due to the tendency of a colloid to polymerization, 

 i.e. to form groups or associations of molecules. Suppose, for 

 instance, that the molecules of a colloidal solution are aggregated 

 into groups of ten. Since each group plays the part of a 

 simple molecule, the osmotic pressure will be ten times less 

 than that corresponding to the quantity of the solute present. 

 Such a group of molecules is called by Naegeli a " micella." 



Similar phenomena of aggregation may be observed in the 

 molecules of many inorganic substances. The molecule of 

 iodine, for example, is monatomic at 1200° C, but becomes 

 diatomic at the ordinary temperature. Sulphur at 860° C. is 

 a gas with a vapour density of °/2, while at 500° C. its vapour 

 density rises to ti'ti. In both of these cases two or more 

 molecules of the element have been condensed into one as a 

 result of the fall of temperature. 



We frequently find that two successive cryoscopie observa- 

 tions on the freezing point of the same colloidal solution 

 will vary. This is due to the extreme sensitiveness of the 

 micella', which absorb or abandon their extra molecules under 

 the slightest influence. This mobility in the constitution 

 of the micellae appears to be one of the principal causes of 

 the peculiar properties of colloidal solutions. 



The phenomenon of polymerization appears to be reversible. 

 The micellae are formed under certain conditions, and are 

 disintegrated when these conditions are removed. The 

 osmotic pressure varies in the same manner, diminishing witli 

 polymerization and augmenting with the disintegration of the 

 micellae. One may easily understand what an important role 

 is played by this alternate polymerization and disintegration 

 in the phenomena of life. 



Most colloidal substances are precipitated from their solu- 

 tions by the addition of very small quantities of electrolytic 



