THE COLLOIDAL STATE 89 



An instructive model to illustrate the phenomena of resonance has been 

 designed by Burch (1913, p. 490). 



THE ELECTRICAL CHARGE 



The fact that contact surfaces between phases are usually the seat of differences 

 of electrical potential has been referred to in the previous chapter. It is not 

 surprising, therefore, to find that such charges play a large part in the properties 

 of the colloidal state. 



The origin of these charges is clearly, in many cases, electrolytic dissociation. 

 Imagine a particle of silicic acid in water. This particle consists of a very great 

 number of molecules. Silicic acid must be supposed to be not wholly insoluble 

 in water. The outer layer of molecules will, therefore, be dissociated. H- ions 

 will travel off, in accordance with their great mobility, while the silicate anions, 

 probably on account of their relative insolubility, remain as a layer on the outer 

 surface of the particle. This particle will then have the negative charges 

 corresponding to a large number of dissociated molecules and will behave as 

 a multivalent anion. Similar considerations will apply to all acidic substances 

 in the colloidal state. If basic, such as aluminium hydroxide, OH' ions will 

 be given off, leaving a multivalent cation. Substances of the kind here described 

 are called by Hardy (1910) "electrolytic colloids" and the huge aggregate, 

 partially dissociated, a " pseudo-ion " or, preferably, a " colloidal ion." 



When such a colloidal solution is examined by the ultra-microscope, the particles are found 

 to be of various sizes, but, if exposed to the field between oppositely charged electrodes, they 

 all move at the same rate. The differences of potential between them and the external water 

 phase must therefore be the same for all. It follows that the charge must be directly pro- 

 portional to their size. While a true ion, of the same chemical composition, always carries 

 the same charge, these colloidal ions carry variable charges, although the chemical nature is 

 unaltered. If the charge be due to surface dissociation, as described, it is natural that more 

 ions should be produced on a large surface than on a smaller one. 



An interesting class of colloids is that of certain salts, which do not pass 

 through parchment paper, but yet, according to measurements of electrical 

 conductivity, are electrolytically dissociated in solution, except in very con- 

 centrated ones, to very nearly the same degree as salts like sodium chloride are. 

 Dyes with a large molecular weight, such as Congo-red, belong to this class. 

 The precise nature of their solutions is not yet clear, since the osmotic pressure 

 is less than would be expected from their conductivity (see my work on . Congo- 

 red, etc., Bayliss, 1911). Salts of proteins with a strong acid or base, such as 

 sodium caseinogenate, or globulin hydrochloride, belong to this class. 



Now, Congo-red is a sodium salt and presumably, on dissociation, Na - ions will 

 be formed. These ions can readily pass through parchment paper, as shown 

 by the diffusion through it of sodium chloride. But, in the presence of the 

 colloidal anion, they are held back. How 1 ? The answer is, by electrostatic 

 attraction. An ion cannot leave the immediate neighbourhood of an oppositely 

 charged ion, unless much work is done in overcoming the attraction. For this 

 reason, the H- ions in the case of silicic acid are held in close proximity to the 

 oppositely charged particle, forming, in fact, one component of a Helmholtz 

 double layer. Certain important phenomena due to colloidal salts bounded by 

 membranes are due to the same fact, as will be seen in the following chapter. 



Substances like Congo-red and salts of caseinogen may be called " electrolytically 

 dissociated " colloids, to distinguish them from the electrolytic colloids of Hardy. 

 At the same time it may turn out that the two are essentially the same, since 

 the large colloidal ion may really consist of aggregates of ions in both cases, 

 although in the former, these aggregates, if present, are too small to be resolved 

 by the ultra-microscope ; the utmost that can be seen is a faint haze. 



There are certain facts, however, which cannot be neglected,- not readily to 

 be explained on the basis of electrolytic dissociation. Quincke (1898, p. 217) 

 noticed that a great variety of inert substances, paper, charcoal and so on, have 

 a negative charge in water. The similar charge on drops of petroleum (Lewis) 

 and of aniline (Ridsdale Ellis) has already been mentioned (page 53 above), and 

 the difficulty of explanation on a purely chemical basis was pointed out. On the 



