THE CHEMISTRY AND PHYSICS OF THE CELL 47 



heterogeneous media when in contact. The nature of the charge 

 depends upon both the suspended substance and the fluid ; e. g., 

 sulphur or graphite particles suspended in water assume a nega- 

 tive charge and move toward the anode, but when suspended 

 in oil of turpentine they become positively charged and move 

 toward the cathode. Water has such a high dielectric constant 

 that most substances in water become negatively charged as com- 

 pared with the water, and move toward the positive pole or 

 anode. 



Hardy has observed that colloidal solutions of coagulated 

 proteids move toward the anode when in alkaline solution, and 

 toward the cathode when in acid solution. This peculiar prop- 

 erty of proteids suggests that perhaps simple surface phenom- 

 ena do not suffice to account for the electrification of all colloid 

 particles. Knowing the peculiar amphoteric character of pro- 

 teids, which is probably due to the presence of both NH 2 and 

 COOH groups in the molecule, we can readily believe that in 

 an acid solution the NH 2 radicles are combined with the acid, 

 leaving the COOH radicles free. The molecule would then have 

 acid properties, and could dissociate into an acid H ion and a 

 basic or electrically positive colloid ion. The colloid ion would 

 then go toward the negative pole slowly, because of its great 

 size. Were this true, however, we might expect the colloidal 

 solutions to show more conductivity than they do, but possibly 

 on account of the very large size of the proteid molecule too 

 few H ions are liberated to produce much effect, and also, ioni- 

 zation may be much less in a neutral solution than in an acid or 

 alkaline one. Electrification of suspensions of platinum, gold, 

 or powdered glass could hardly be explained on this basis, unless 

 we assume that the water or other solvent united with the par- 

 ticles becomes ionized. Quite possibly we have both ionization 

 and cataphoresis occurring, the former in the case of some com- 

 pounds, the last in the case of elements or perfectly insoluble 

 substances held in suspension. 



Surface tension, which may be described as the force with 

 which a fluid is striving to reduce its free surface to a minimum, is 

 highly exhibited by colloids as compared with crystalloids. The 

 phenomenon of cataphoresis depends upon the existence of a high 

 surface tension, and it is this same property that explains the 

 ability of colloidal particles to stay suspended in a fluid which 

 has a much lower specific gravity than the solid. The forma- 

 tion of emulsions and the spreading out of oil upon the surface 

 of water also depend upon surface tension. Ameboid move- 

 ment may be attributed to changes in surface tension, as also 



