THE PROPERTIES OF COLLOIDS H7 



ferric hydrate, K 2 S0 4 , with a divalent S0 4 ", is 400 times ... M 



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 



MgSO 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 S0 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 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 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. 



The factors involved in the formation of adsorption or absorption coi 

 binations are therefore : 



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



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



with a surface of 22 sq. cm., if reduced to a fine powder consisting o 



of -00000025 cm. in diameter, will have a surface of 20,000,( 



i.e. nearly half an acre. At the whole of this surface adsorpti 



take place, involving the concentration of dissolved ele 



gases. 



(2) Chemical nature of particle. 



(3) Electric charge on the surface. The sign of this may be det 



