1908.] 



Electrolytes and Colloids. 



43 



The development of such a potential difference between colloid particles 

 and fluid has been accounted for in two ways. The first, which may be 

 described as a purely physical hypothesis, ascribes it to differences in the 

 speed of the ions of electrolytes present. The colloid particles at any moment 

 contain within themselves an excess of the most penetrating and rapidly 

 moving ions present, and they therefore have the charge of that ion. In 

 presence of acid they will have the charge of the hydrogen ion, in the 

 presence of alkali that of the hydroxyl ion. This hypothesis was advanced 

 by one of us to explain the properties of certain proteins of the globulin 

 class when in solution.* It was also advanced independently by Perrin to> 

 explain the electrical properties of colloidal solutions in general.f 



The second hypothesis is frankly chemical in nature, and, as applied to 

 proteins, it may be put as follows : — The protein molecule contains H and 

 OH groups. Proteins, therefore, as a class are, like their chemical allies the 

 amino-acids, amphoteric electrolytes. They react with acids and alkalis to 

 form salts, but the reactions are not precise, an indefinite number of salts of 

 the form (B)„BHA being formed where the value of n is determined by con- 

 ditions of temperature, and concentration, and of inertia due to electrification 

 of internal surfaces within the solution. 



The salt so formed is ionised by the water. Positive or negative ions, as 

 the case may be, leave the protein face to enter the water face, and form an 

 electric layer there, while the protein face is left charged respectively 

 negative or positive.* On this view, in the particular case under consideration, 

 the decrease and final disappearance of the potential difference which occurs 

 when the concentration of acid rises above a certain value would be due to a 

 suppression of the feeble ionisation by the excess of acid. 



The first view seems to be incompatible with certain experimental facts — 

 such, for instance, as the fact that salts such as LiCl or LiBr, the velocities 

 of whose ions are in the ratio of about 1 to 2, do not confer any change on 

 proteins, nor, as Perrin noticed, do they produce any contact difference of 

 potential between a water and a solid wetted by it. It also ignores the purely 

 chemical nature of the conditions which govern the formation of colloidal 

 solutions of metals.§ 



* Hardy, ' Journ. Physiol.,' vol. 29, p. xxvii, 1903. 



t Perrin, ' Journ. de Chim. Physique,' vol. 2, p. 601, 1904 ; vol. 3, p. 50, 1905. 



% Hardy, ' Journ. Physiol.,' vol. 33, p. 251, 1905 ; ' Roy. Soc. Proc.,' vol. 79, p. 413, 1907. 



§ Burton, 'PhiL Mag.,' vol. 11, p. 425, 1906. 



