COLLOIDS AND ADSORPTION 27 



though others, gas in liquid (foams), liquid in gas (fogs), 

 solid in gas (smokes) and solid in solid are equally import- 

 ant in other fields of work. The particles which constitute 

 the disperse phase may be collections of large numbers 

 of atoms or molecules, as is the case with gold sols and 

 cadmium sulphide sols, or they may consist of single very 

 large molecules or of a comparatively few molecules, 

 which is usually the case with the proteins and poly- 

 saccharides (of molecular w^eight of the order of 15,000 

 to 100,000 or higher) w^hose molecules' are so large that 

 they fall within the colloidal range of sizes . If the particles 

 of the disperse phase are much larger than 200 m/x they 

 tend to settle out comparatively rapidly under the 

 influence of gravity, whilst if they are much smaller than 

 10 m/x they cease to behave as colloids and show the 

 properties of crystalloid solutions. 



In a stable colloid system the minute particles are 

 prevented from cohering and coagulating by two pro- 

 cesses. In the first place they are constantly bombarded 

 by the molecules of the continuous phase which keeps 

 them in the ceaseless zig-zag motion loiown as Brownian 

 movement, and secondly the particles, as a rule, carry 

 an electric charge which may be positive or negative 

 according to the system. Since all the particles in any 

 one system carry a like charge they tend to repel one 

 another, and so remain uniformly distributed throughout 

 the continuous phase. The charge carried by the particles 

 may be due to the adsorption of ions from the solution, 

 or it may be due to ionisation of the particles themselves 

 or to a combination of both factors. 



Colloids can be divided into two large classes, the 

 lyophobic (solvent -hating) sols and the lyophiUc (solvent - 

 loving) sols. The first group contains those systems in 

 which the disperse phase has little if any attraction for 

 the continuous phase. To this group belong most 

 inorganic sols, like gold and the sulphides, and also 

 emulsions of oil in water. They show no tendency 



