1 70 PRINCIPLES OF GENERAL PHYSIOLOGY 



Avogadro's law, they are said to be " dissociated." In the same way we 

 may speak of the molecules in our solutions with anomalous osmotic pressures 

 as being "dissociated." 



But what sort of dissociation are we to suppose that such a salt as potassium 

 chloride undergoes in water? It is plain that hydrolysis into hydrochloric acid 

 and potassium hydroxide is not to be thought of ; these cannot exist together 

 in solution. Moreover, such a hypothesis would not explain the phenomena in 

 the case of acids or bases, which behave in the same way as salts. Again, it 

 cannot be potassium and chlorine in their ordinary state, since potassium is 

 immediately converted by water into its hydroxide, and chlorine would be easily 

 detected if present. Looking at the lists of substances in the two classes 

 referred to, we are at once struck by the fact that those substances which 

 give an abnormally high osmotic pressure and are, as we have seen, " dissociated " 

 in solution in water, are all good conductors of electricity, whereas the " normal " 

 ones are non-conductors. Further, it is found that if a substance which gives an 

 anomalous osmotic pressure in water and is a good conductor is dissolved in 

 a solvent in which it no longer conducts electricity, as, for example, hydrochloric 

 acid in benzene, its osmotic pressure is normal, or, at any rate, not greater 

 than normal. 



Suppose that we take a solution of hydrochloric acid in water and pass an 

 electric current through it, we find free chlorine is separated at the anode, 

 where the current enters, and free hydrogen at the cathode, where the current 

 leaves the solution. One of the constituents of the solute, which is called an 

 " electrolyte " when it conducts electricity, " wanders " in one direction, the other 

 in the other direction. Faraday was the first to use the name "electrolyte," 

 and he showed that this is actually the way in which the electric current is 

 carried through the solution of a substance which is capable of conducting it. Each 

 constituent of the electrolyte carries a definite quantity of electricity ; in our 

 case, the hydrogen carries positive electricity from the anode to the cathode, and 

 the chlorine carries negative electricity from the cathode to the anode. The 

 name used by Faraday (1834, pp. 78 and 79, and 1839, I. pp. 197, 198) for these 

 electrically charged atoms, or molecules, was " ions " (iwv, participle of tipi, 

 "going"), those carrying a positive charge, which they give up at the cathode, 

 being "cations" (Kara down), and those with a negative charge "anions" 

 (ai'a = up), in accordance with the direction in which they move in relation to 

 the current, regarded as of positive electricity. The electrodes are " anode " and 

 "cathode" (68ds = way). A portrait of Faraday will be found in Fig. 51. 

 In order to conduct a current, then, the solute must be decomposed into positively 

 and negatively charged parts, that is, " electrolytically dissociated." 



We may note here that, according to Nernst (1911, p. 356), the word " ionisation," 

 sometimes used, is better reserved for the case of the gas ions, which are produced by X-rays, 

 ultra-violet light, etc., and consist of a number of molecules of the gas grouped around a 

 single electron. 



It is obvious from the facts of electrolytic conduction that hydrogen and 

 chlorine are capable of existence in forms which have quite different properties 

 from those which they possess in their ordinary familiar forms. While they are 

 engaged in carrying electric charges through the solution in which a current passes, 

 they cannot be recognised as hydrogen and chlorine. 



The actual amount of electricity carried by a univalent ion is, as was shown by 

 Faraday's work, a definite quantity, and is now known by the name suggested by 

 Johnstone Stoney as an " electron." A bivalent ion carries two electrons and so 

 on. Helmholtz (1881) put forward the view that electricity itself has an atomistic 

 structure, so that, in a certain sense, we may look upon positive and negative 

 electrons as two new univalent elements. Thus a positive electron may be said 

 to replace Cl in HC1, forming hydrogen ion instead of hydrogen chloride (Nernst, 

 1911, p. 395). If this be so, it is not surprising that hydrogen ion is completely 

 different from hydrogen itself, since it is a new chemical compound, and the 

 essence of chemical combination consists in the manifestation of properties unlike 

 those of the constituents. 



