124 Mr. W. B. Hardy. On the Conditions which 



would resist any movement of the particles through the fluid, because, 

 as Dorn's experiments show, electric work is done in displacing the 

 particles."^ The effect would be the same as if the viscosity of the fluid 

 was increased.! 



The stability of the system may be destroyed by altering the differ- 

 ence of potential. Free acid, added to a hydrosol in which the 

 particles are negative to pure water, will diminish the relative differ- 



Fig. 1. 

 Acetic acid toQ% 



S ^ 



Ammonia 100% 



Action of potassium sulphate in presence of varying amounts of acetic acid or 

 ammonia upon the hydrosol of gold. The abscissae represent the volume of 

 water which holds 1 gramme -molecule of the salt. The positive ordinates 

 represent the reciprocals of the volume which holds 1 gramme-molecule of the 

 acid, and the negative ordinates the reciprocals of the volume which holds 

 1 gramme-molecule of the alkali. Each division = 50,000 c.c. 



ence of potential of the water. In this case the reagent acts directly 

 on the water, and the coagulative activity of unit mass of the sub- 

 stance varies directly with its chemical activity when dissolved in 

 water. The same relation seems to hold when free alkali is added to 

 a hydrosol in which the particles are electro-positive. 



The stability of the system may also be destroyed by induction, the 

 active agents being free ions carrying a static charge. J In this case 



. ' Wied. Ann.,' vol. 10, 1880, p. 70. 



f This mode of stating the result I owe to Professor J. J. Thomson, and 

 gladly acknowledge his kindness in discussing this and kindred points with me. 

 Whetham, loc. cit. 



