THE PROPERTIES OF COLLOIDS 145 



electro-positive ferric hydrate the acid ion of the salt is the determining 

 factor, the coagulative power increasing rapidly with the valency of the acid.. 

 On the other hand, in the precipitation of a gold sol the electro-positive ion 

 is the effective agent, and here again the coagulative effect is enormously 

 increased by a rise in valency. This is shown in the following Tables, 

 where it will be seen that in the coagulation of gold, barium chloride with 

 the divalent Ba", is seven times as powerful as K 2 S0 4 containing the 

 univalent K'. On the other hand, in the precipitation of the electro-positive 

 ferric hydrate, K 2 S0 4 with a divalent SO/', is 400 times as effective as 

 BaCL. 





AMOUNT OF SALT NECESSARY TO PRECIPITATE COLLOIDAL SOLUTIONS 

 To coagulate Gold To coagulate Fe 2 3 





K 2 SO 4 1 g. mol. in 4,000,000 c.c. 

 MgS0 4 4,000,000 

 BaCl 2 10,000 



BaCl 2 1 g. mol. in 500,000 c.c. 

 NaCl 72,000 

 K 2 S0 4 75,000 



NaCl 30,000 



The presence of a charge is not however a necessary condition for the 

 stability of a colloidal solution. Thus the proteins of serum, globulin in a 

 weak saline solution, or gelatin, present no drift when exposed to a strong 

 electric field. In such cases one must assume the stability of the solution 

 to be determined by the absence of any surface tension between the two 

 phases in the solution, or between the particles of solute and solvent. 

 Thus no force is present tending to cause aggregation of the particles. 



The charged condition of a colloidal particle makes it behave in an 

 electric field in much the same way as a charged ion of an electrolyte, and 

 this similarity extends also to its chemical behaviour, so that we have a 

 class of compounds formed resembling in many respects chemical com- 

 binations, but differing from these in the absence of definite quantitative 

 relations between the reacting substances. This class of continuously 

 varying chemical compounds has been designated by Van Bemmelen absorp- 

 tion compounds. Since however the interaction must take place at the 

 surface layer bounding the charged particles, it will be perhaps better, as 

 Bayliss has done, to use the term adsorption. The huge molecules or aggre- 

 gates of molecules which distinguish the colloidal state form a system with 

 a considerable inertia, so that we have a tendency to the establishment 

 of conditions of false equilibrium. Once a configuration is established, it 

 is necessary, in consequence of the inertia, to overstep widely the conditions 

 of its formation in order to destroy it. Thus a 10 per cent, gelatin solution 

 sets at 21C., but does not melt until warmed to 29-6C. Solutions of agar 

 in water set at about 35C., but do not melt under 90C. A gel of 

 gelatin takes twenty-four hours after setting to attain a constant melting- 

 point. 



The factors involved in the formation of adsorption or absorption com- 

 binations are therefore : 



(1) Extent of surface. In a colloidal solution this must be enormous 

 in proportion to the mass of substance in solution. Thus a 10 c.c. sphere 



10 



