determine the Stability of Irreverdhle Hydrosoh, 



115 



Schulze's conclusions were verified by Prost,* who used sulphide 

 of cadmium, and Picton and Linder, who used the sulphide of 

 antimony.! The last-named wofkers added the important fact that 

 a small portion of the coagulating salt is decomposed, the metal being 

 entangled in the coagulum. 



The measurements which I have made with various colloidal solu- 

 tions both confirm Schulze's results, and bring out a new relation, 

 which may be stated as follows \ — 



The coagulative power of a salt is determined by the valency of one of its 

 ions. This prepotent ion is either the negative or the positive ion, according 

 to whether the colloidal particles move down or y/p the potential gradient. 

 The coagulating ion is always of the opposite electrical sign to the particle. 



The salts employed to determine this point were the sulphates of 

 aluminium, copper, magnesium, potassium, and sodium ; the chlorides 

 of copper, barium, calcium and sodium, and the nitrate of cadmium. 

 Solutions containing 1 gramme-molecule in 2000 c.c. were prepared. 



The experiments may be 'summarised as follows : — 



Silica dialysed free from Chlorides, Electro-negative. 



Concentration of Coagulating Salt 1 gramme-mol. in 120,000 c.c. 

 Temperature 16°. 

 Coagulated at once. In 10 mins. In 2 hours. In 24 hours. Still fluid. 



Al2(S04)3 CuSO^ MgSO^ K2SO4 NaCl 

 CuClg Na2S04 Control. 



Cd(N03)2 



BaCl2 



This illustrates many experiments. 



Proteid in presence of trace of Alkali, Electro-negative. 

 Temperature 16°. Coagulating Salt 1 gramme-mol. in 80,000 c.c. 

 Coagulated at once. On slightly warming. Did not coagulate. 



Al2(S04)3 MgS04 Na2S04 



Cd(N03)2 BaCl2 K2SO4 



CuSO, CaCl2 NaCl 



CuCl 



Proteid in presence of trace of Acetic^Acid, Electro-positive. 

 Coagulated instantly. No effect. 



AI2SO4 CuCl2 



CUSO4 Cd(N03)2 

 K2SO4 BaCl2 

 Na2S04 NaCl 

 MgS04 



* ' Bull, de I'Acad. Eoy. de Sci. de Belg.,' ser. 3, yoI. 14, 1887, p. 312, 

 t ' Journ. Chem. Soe.,' vol. 67, 1895, p. 63. 



