90 PRINCIPLES OF GENERAL PHYSIOLOGY 



other hand, as Hardy has shown (1912, p. 632), a mere trace of a chemically- 

 active substance, present as impurity, such as an ester or olcic acid, is sufficient 

 to cause the spreading on water of a heavy hydrocarbon oil which, when pure, 

 does not do so. This being so, a chemical explanation of all the above cases must 

 not be too hastily set aside. 



But also, it must not be forgotten that electrical charges can be conferred 

 by other means than electrolytic dissociation in the usual sense. It will be 

 sufficient to refer to the phenomena of frictional electricity. The separation of 

 positive and negative electricity here, and the source of the electrical energy 

 resulting, must be looked for in the mechanical work of tearing apart the 

 constituents of the double layer, although the way the double layer itself is 

 produced is not quite clear. 



Rudge (1914) finds that dust, blown up so as to make a cloud, becomes highly charged, and 

 that the sign of the charge depends on the chemical nature of the particles. "Acidic" 

 substances, such as sand or molybdic acid, become negative, " basic " substances, such as coal, 

 Hour, red lead or alkaloids, become positive. The facts show that the charges of frictional 

 electricity may, after all, be due to electrolytic dissociation. 



The various phenomena connected with the electrification of gases and the 

 action of ultra violet light are also to be remembered. It is possibly such facts 

 that caused Lewis to suggest an " electronic " origin for the charge in certain 

 cases. It appears to be the point of view taken by Perrin (1904 and 1905) in 

 his work on electrification at the surface of contact between solids and liquids 

 This observer found that no electrical charges are present except in ionising 

 liquids, such as water, alcohol, etc. None was found in ether, chloroform, 

 turpentine, etc. But, as Hardy points out (1910, p. 193), the absence of migra- 

 tion in a non-conductor does not necessarily prove the absence of potential 

 difference between the phases. 



Although many of the cases described by Perrin can be explained on the basis of 

 electrolytic colloids, as stated above, it must be admitted that in such cases as charcoal, 

 carborundum, cellulose, etc., the hypothesis of ionisation seems rather forced. It does not assist 

 matters greatly to point to the existence of graphitic acid in the case of charcoal, while the 

 sign of the charge on aniline is opposite to that which one would expect from electrolytic 

 dissociation. The existence of any charge on petroleum drops is, moreover, a difficulty. How 

 far the presence of impurities may account for some of these facts, as in Hardy's experiments 

 on surface tension (1912, p. 632), is at present uncertain. It would he interesting to know 

 whether Hardy's pure hydrocarbon oil, which does not spread on water, has any charge on its 

 surface of contact with water. 



An experiment of Gee and Harrison (1910, p. 46) is interesting in this connection. 

 Alizarin (one part in 10,000) forms a colloidal solution in 2'5 per cent, alcohol, and a 

 true solution in 50 per cent, alcohol. When a current is passed through this latter solution, 

 no migration of the dye occurs, so that it is not ionised, nor has it any charge at all. 

 In the colloidal solution, along with the formation of a contact surface, the particles have 

 a charge and move in the electric field. Apparently, then, this charge cannot be due to 

 electrolytic dissociation, since the molecules in true solution are not so dissociated. In 

 strengths of alcohol intermediate between the above, the rates of migration of the particles 

 showed all intermediate stages. The interpretation of this experiment, as it seems to nu . 

 is not quite simple. Owing to the lower dielectric constant of alcohol, a less charge would 

 be expected, and, moreover, I have found that the temperature coefficient of conduct i.vity 

 of a suspension of well-washed alizarin in water amounts to 3'29, a value greater than 

 that which would be given by a trace of foreign electrolyte, in fact 28 per cent, more 

 than that of potassium chloride, and indicating some slight true solubility and; electrolytic 

 dissociation of alizarin itself (see page 77 above). If this is so, the electric charge may 

 well be due to surface ionisation, with production of colloidal negative ions, similar to 

 those of silicic acid. We know, indeed, that alizarin does behave as a weak acid. 



On the whole, the question of the origin of the charge in certain cases requires 

 further investigation, although it seems that Perrin's view of contact electrification 

 has considerable justification. In the majority of cases, there is no doubt that 

 electrolytic dissociation is the cause of the charge. 



One possibility should be referred to, although the experiments of Elissafov, 

 to be described in the next section, do not support it. If, at the contact of 

 an " insoluble substance " with water, there is surface tension of the ordinary 

 mechanical kind and a trace of an electrolyte be added to the water, it is 

 conceivable that one of the ions into which the electrolyte dissociates may produce 

 a greater diminution of surface energy than the other one. This ion would then 



