LIFE AND ENERGY 33 



there is another phenomenon which takes part in the maintaining 

 of the particles in suspension, and is also of importance in other 

 ways. If we place a colloidal solution between electrodes, 

 connected to a battery so that there is a fairly high difference 

 of potential between them, we shall find, in nearly all cases, that 

 the colloidal particles are carried either to the positive or to 

 the negative pole, and deposited there. In the former case, 

 they must have a negative charge ; in the latter, a positive one 

 (E., p. 177). 



We may ask, what is the effect of this charge on the surface 

 tension ? Remembering that charges of the same sign repel each 

 other, we may look upon the surface of each particle as made 

 up of areas charged with the same sign ; the parts of the surface 

 mutually repel one another, so that the surface tends to increase 

 its area. This is in opposition to the direction of the ordinary 

 surface tension, due to internal pressure, and the result is a 

 favourable one on the state of suspension of the colloid. The 

 mutual repulsion of the particles themselves also plays a part 

 in keeping them from aggregation and deposition. 



How is the presence of this electrical charge on the surface of 

 substances In contact with water to be accounted for? There is no 

 doubt that, in the majority of cases, it is due to electrolytic dissocia- 

 tion of the material at the surface of the particle itself. This takes 

 place in two somewhat different ways, according to the dimensions 

 of the molecule of the chemical compound concerned, giving rise, 

 on the one hand, to what have been called "'electrolytic colloids" 

 or, on the other hand, to " electrolytically dissociated colloids" As an 

 instance of the former, in which the particles consist of a large 

 number of small molecules aggregated together, let us take silicic 

 acid in the colloidal state. This substance is usually regarded as 

 being insoluble in water, but it is not absolutely so, as indeed no 

 substance is. When in solution, silicic acid, like all other acids, 

 dissociates into hydrogen-ions, which are freely soluble, and anions 

 of silicon oxide, which are practically insoluble. Consider now the 

 state of affairs at the surface of a particle of silicic acid in water. 

 The molecules of the surface layer are electrolytically dissociated. 

 The hydrogen- ions pass into the water, leaving behind on the 

 surface the insoluble silicic anions. These latter possess negative 

 charges, so that the particle, as a whole, will have a negative charge 

 consisting of the sum of the charges of the anions on its surface. 

 The particle becomes a kind of large composite ion, and may be 

 called a "colloidal ion" ; but it must be remembered that such ions 

 vary greatly in the number of molecules they contain, so that the 

 charge is not composed of a definite number of electrons, like that 

 of the true ion is. Similar considerations apply to particles of 



