determine the Stahility of Irreversible Hydrosols. 



117 



salt on the hydrosol is as a rule small compared with the effect of the 

 metal ion. Thus the stability of a hydrosol of electro-positive proteid 

 is increased by free acid, yet the acid salts find their proper place in 

 the scale of valency. Again, ferric hydrate is coagulated by nitric acid 

 when the concentration reaches 1 gramme-mol. in 2500 c.c. ; yet the 

 cadmium salt of this acid is not much more potent than the " neutral " 

 salts MgS04, BaClg. 



Temperature 16°. Concentration necessary to Coagulate 



Ferric Hydrate. 



Salt. 



K2SO4 1 gramme-mol. in 4,000,000 c.c. 



MgS04 „ „ 4,000,000 „ 



BaCla „ „ 10,000 „ 



NaCl „ „ 30,000 „ 



Cd(N03)2 ... „ „ 50,000 „ 



The extraordinary rise in coagulative power with an increase in 

 valency, which was observed by Schulze, Prost, and Picton and 

 Linder, holds in all cases. In order to measure it for ferric hydrate, 

 I used Schulze's method, in which a drop of the hydrosol is allowed 

 to fall into a large volume of the solution of the salt. A number of 

 experiments were made until the concentration of the salt was found 

 which just sufficed to coagulate the drop. In the case of gold and 

 mastic the process was reversed, the salt solution being added drop 

 by drop to a measured quantity of the hydrosol. I append the 

 results : — 



Gum Mastic, Neutral. Temperature 40°. 



BaCl2 1 gramme-mol. in 86,000 c.c. 



NaCl „ „ 8,000 „ 



MgS04 „ „ 68,000 „ 



K2SO4 „ „ 8,000 „ 



Gold, very faintly acid. Temperature 16°. 



NaCl 1 gramme-mol. in 72,000 c.c. 



BaCl2 „ „ 500,000 „ 



' K2SO4 „ „ 75,000 „ 



The figures for ferric hydrate have already been given. It has been 

 pointed out that if specific molecular coagulative power be defined as 

 the inverse of the volume occupied by one gramme-molecule of a sub- 

 stance when it just suffices to bring about coagulation, then this value 

 (K) varies with the valency of the active ion approximately according 

 to the square and cube : — 



K' : R" : E'" = K : : K^. 



