114 Mr. W. B. Hardy. On the Conditions vjJiich 



acids, or alkalis. One may, therefore, approach the point by the addi- 

 tion of, say, acid or alkali, and use a salt to measure the stability of 

 the system, as in the experiment described later. In such experiments, 

 however, the colloid particles are immersed in a complicated system of 

 three components, the conditions of equilibrium of which cannot be 

 arrived at from existing data. The conditions could be simplified by 

 using, say, KHO or H2SO4 to approach the iso-electric point, and 

 K2SO4 as a measure of the change of stability. A series of determi- 

 nations with different systems of this kind may afford the requisite 

 measurements. 



A direct and conclusive proof that stability does decrease as the 

 iso-electric point is approached was however obtained in two ways. The 

 iso-electric point can be approached in the case of the hydrosol of proteid 

 by the withdrawal of either the free acid or the free alkali, as the case 

 may be. As it is neared, the proteid particles increase in size, so that 

 instead of scattering blue light, they scatter white light ; thus the sur- 

 face of contact of fluid and solid gradually diminishes as the point is 

 neared. The second experiment, though not a quantitative one, is very 

 convincing. A hydrosol of gum mastic dialysed as pure as possible is 

 not destroyed by mechanical agitation even when long continued. If, 

 however, a salt is added in an amount so small that it just fails to 

 coagulate the hydrosol, the latter is rendered so unstable that it is 

 destroyed by shaking. 



Experiments were made to determine whether the particles actually 

 carry a charge. An electric field which was practically uniform was 

 made by using flat electrodes of the same size, which were placed 

 parallel to one another at the ends of a straight tube. The particles 

 were found to move in all parts of the field ; they therefore carry a 

 definite charge which, according to Quincke's theory of the movement 

 of particles in an electric field, would be a surface charge, each particle 

 being surrounded by a double layer of electricity. 



Action of Salts. — The power possessed by salts of destroying colloidal 

 solutions was noticed by Graham. The subject was, however, first 

 accurately investigated by H. Schulze."^ He showed that the power 

 which various salts possess of precipitating a hydrosol of sulphide of 

 arsenic is related to the valency of the metal, while the valency of the 

 acid has little influence. The increase in the precipitating or coagu- 

 lating power produced by increase in valency is very great. If 

 coagulative power be defined as the inverse of the concentration in 

 gramme-molecules per litre necessary to convert a given hydrosol into a 

 hydrogel, then from Schulze's measurements the coagulative power of 

 metals of different valency is : — 



R' : E" : E'" = 1 : 30 : 1650. 



* ' Journ. f. prakt. Chemie,' vol. 25, 1882, p. 431. 



