THE PROPERTIES OF COLLOIDS 163 



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 x/ , 

 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/ 7 , is 400 times as effective as 

 BaCl 2 . 



AMOUNT OF SALT NECESSARY TO PRECIPITATE COLLOIDAL 

 SOLUTIONS 



To coagulate Fe. 2 



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

 MgS0 4 4,000,000 

 BaCl 2 10,000 

 NaCl 30,000 



To coagulate Gold 

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

 NaCl 72,000 

 K 2 SO 4 75,000 



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 the 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 combinations, 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 absorption compounds. Since, how- 

 ever, 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 aggregates of 

 molecules which distinguish the colloidal state form a system with 

 a considerable inertia, so that we have a tendency to the establish- 

 ment 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 21 C., but does not melt until 

 warmed to 29-6 C. Solutions of agar in water set at about 35 C., but 

 do not melt under 90 C. A gel of gelatin takes twenty-four hours 

 after setting to attain a constant melting-point. 



