determine the Stability of Irreversible Hydrosols. 



113 



electrode — the acceleration due to the electric field being about 5 mn:. 

 an hour. The suspended particles of ferric hydrate show, therefore, an 

 exceedingly slight movement in a direction opposite to that which they 

 manifest when in colloidal solution. In the latter condition they are 

 markedly electro-positive ; in the former they are exceedingly faintly 

 electro-negative. An exceedingly faint electro -negative character is 

 also conferred upon the ferric hydrate when the hydrosol is coagulated 

 by ammonia, 1 gramme-molecule of the latter being present in 

 100,000 c.c. 



If a fresh gel of silica is broken up in distilled water and carefully 

 washed to free it from still uncoagulated silica, and from impurities, it 

 is completely iso-electric with the water. It becomes markedly electro- 

 negative, however, on the addition of the minutest trace of free 

 alkali. 



Gum mastic precipitated from a dilute hydrosol by adding barium 

 chloride until the concentration is 1 gramme-molecule in 600,000 cm. is 

 found to be iso-electric with the fluid. It is markedly electro-negative 

 when in colloidal- solution. 



Picton and Linder have shown that the particles in these hydrosols 

 gradually grow in size as the coagulation or precipitation point 

 is neared."^ It might, therefore, be urged that, as the movement 

 of the particles in the electric field is, on Quincke's theory of electric 

 endosmose, due to surface action, the fact that they do not move when 

 in simple suspension as opposed to colloidal solution may be due to 

 the diminution of the impelling force acting on a given volume.! This 

 is, however, negatived by the character of the experiments. The 

 addition of a minute amount of free alkali to a mass of particles of 

 coagulated silica which have settled to form a " mud " cannot alter 

 the size of these relatively very large masses to any appreciable 

 extent. And since in the case of ferric hydrate and proteid, the sign 

 of the charge which the particles carry in the electric field is different 

 on each side of the actual point of precipitation, that point must of 

 necessity be an iso-electric point. 



If the stability of the hydrosol is dependent upon a difference in 

 electrical potential between the solid particles and the fluid, then one 

 would expect that for, at any rate a short distance from the iso-electric 

 point, the stability would vary simultaneously with the variation in 

 the difference of potential. The experimental investigation of this 

 question is beset by many difficulties. At present I know of no way 

 of approaching the iso-electric point other than by the addition of salts, 



* ' Joum. of Chem. Soc.,' vol. 61, 1892, p. 148. 



t As a matter of fact, Lamb finds that the velocity of a partic'.e is independent 

 of its size or shape, provided that its dimensions are large compared with the 

 slip, so perhaps tlie objection scarcely needs discussion. Lamb, ' Brit. Assoc. 

 Eeport,' 1887, p. 502. 



