140 ORIGIN AND NATURE OF LIFE 



called a reversible colloid ; while in the 

 other two cases the hydrogel (or coagulum, 

 as it is also called), is permanent, and a 

 hydrosol is not again formed as the tempera- 

 ture falls. 



In the condition of the hydrosol, the 

 multi-molecules, or solution aggregates, are 

 each free and microscopically invisible, and 

 so far as the properties of solution are con- 

 cerned, each behaves as would a single unit, 

 or molecule in solution. When gel formation 

 occurs, the multi-molecules aggregate together 

 and form a system made up of two distinct 

 kinds. It is called a diphasic system, and is 

 no longer a true solution. It is in this way 

 that there arise foam structures, and the 

 protoplasmic networks and structures seen 

 in living cells, and also as artifacts, or products 

 of death and fixation by chemicals, in dead 

 cells. When the colloidal solution, as a result 

 of changes in environment, forms two phases, 

 it does so by separating into two systems, in 

 one of which there is still a true solution, 

 but a comparatively dilute one, of the smaller 

 multi-molecules, and, in the other, relatively 

 huge molecular complexes have separated 

 with comparatively little of the solvent. 

 In the limit, the fluid part may be pure 

 solvent and the particulate part pure colloidal 





