34 INTRODUCTION TO GENERAL PHYSIOLOGY 



basic substances, such as those of aluminium hydroxide. In these 

 cases the soluble ions which go away into the water are OH ions, 

 so that the particle is left with a positive charge. 



Turning now to those substances which are present in solution 

 in single molecules, but are colloidal on account of one of the ions 

 being of large dimensions and insoluble, it is clear that these ions 

 will have charges of a definite number of electrons, according to 

 their valency. Otherwise, their behaviour is the same as that of 

 the previous kind. To distinguish them, however, they may be 

 called " electrolytically dissociated colloids" They are met with, 

 especially amongst complex organic electrolytes ; many of the 

 aniline dyes and the proteins are examples. The behaviour of the 

 latter is of special interest, and will be described in the next 

 chapter. 



There is one point about the electrical state of such systems as 

 those just referred to that must not be left unmentioned. When 

 the diffusible ion goes into solution in the water surrounding the 

 particles it is endowed with kinetic energy, of course, as all the 

 other molecules of the liquid. In virtue of this, it naturally tends 

 to wander away into the solution. But this is prevented by the 

 powerful electrical attraction exerted by the oppositely charged 

 solid particle. The soluble ion can only go so far as the balance- 

 between its kinetic energy and the electrostatic attraction permits 

 it. A number of them form, thus, a sheath or layer at a very short 

 distance away from the particle. Such an arrangement is known 

 as the " Helmholtz double layer" and we shall have occasion to 

 return to it again later. 



In certain cases where the dispersed phase shows on investiga- 

 tion that it has an electrical charge, it is not an easy matter to 

 explain it by electrolytic dissociation, although this may ultimately 

 turn out to be the case. Droplets of paraffin oil in water are 

 negatively charged. It has been suggested that this charge has 

 an origin similar to that of frictional electricity. 



There is, again, a further cause of an electric charge on inert 

 particles in solutions of electrolytes. If particles of carbon are 

 suspended in water, surface tension is present at their contact 

 surfaces with the liquid. By the deposition of ions on this surface, 

 adsorption, in fact, the surface energy can be lowered. This may 

 be either in the mechanical way, or by imparting an electric charge. 

 It is a matter of experiment that if acid is present in the liquid 

 phase, hydrogen ions are deposited on the surface, giving it a 

 positive charge. If alkali is present, the surface becomes negative 

 by deposition of OH ions. There must be some reason why the 

 H and OH ions are deposited in preference to these of opposite 

 charge which are always present. It may be that the greater 



