46 HOLISM AND EVOLUTION chap. 



throughout another in very minute particles which are yet 

 larger than molecules. Originally substances were divided 

 into colloids and non-colloids ; but more recently it has been 

 shown that non-colloids (like mineral salts) can under cer- 

 tain conditions be reduced to the colloid state. And now this 

 division has been abandoned, and the colloid state is recog- 

 nised as a fourth form of material aggregation applying to 

 substances generally under certain conditions. Much of the 

 earth and the air exists in the colloid state; but the colloid 

 state is specially important because it seems to be distinctive 

 of all life-forms — the protoplasm of all organic cells being 

 organised in the colloid state. The protoplasm of the cells 

 contains solid substances in most minute form dispersed 

 throughout its jelly-like fluid, and this colloid state seems to 

 link the inorganic with the organic elements in the cell. 



Owing to their minute size, particles in the colloid state 

 expose the maximum surface area in comparison with their 

 mass; and the colloid state in consequence brings into action 

 the play of surface energies more than any other phase of 

 matter. In all forms of aggregation the surface molecules 

 of matter are specially orientated; the active sides of the 

 molecules being turned inwards, and the outer surface thus 

 consisting of the weak ends of these molecules. This 

 orientation affects the surface tensions, chemical behaviour 

 and energies of the surface molecules; and as colloids expose 

 a maximum of such surfaces they show properties which are 

 of a distinctive character. Thus at these surfaces loose 

 unstable combinations with other special substances are 

 easily formed, and colloids appear in consequence to have a 

 peculiar and almost mysterious selective action for other sub- 

 stances. The phenomenon is called " adsorption "; the se- 

 lected substances being adsorbed at the colloid surfaces. Col- 

 loids are thus used in many industrial processes to separate 

 other substances from each other, to remove impurities, and 

 in other ways to act as a selective separator of mixed sub- 

 stances. They also act as catalysts; that is to say, at their 

 surfaces chemical actions take place and combinations are 

 effected which otherwise would not be brought about. The 



