The modern development of the science of electrons does not belong to the subject of 

 this book; those interested may consult the short work by Ramsay (1912) on "Elements 

 and Electrons." It is well, however, to refer to one point. The existence of two kinds of 

 electrons, positive and negative, has been assumed above. The question is not yet definitely 

 decided as to whether there is only one kind, the negative, and whether an apparent positive 

 charge is really only the absence of a negative one. This does not affect the argument, and 

 there is evidence in favour of the existence of positive electricity. 



Since a hydrogen atom is a very different thing from a hydrogen ion it is not 

 permissible to use the same symbol for both. It is generally agreed to use a dot 

 for a single positive charge and a dash for a single negative one, repeating them 

 as many times as the valency of the ion requires. Thus, H', Ca", NH 4 ' are 

 positive ions, and Cl', SO 4 ", 

 PO 4 '" are negative ions. 

 The signs + and - , used at 

 one time, are no doubt 

 more expressive, but cause 

 difficulties to the printer, 

 when added to the top of a 

 symbol and, in other posi- 

 tions, would be liable to 

 cause confusion. 



Although it is most conveni- 

 ent to speak of the possession 

 of positive and negative charges, 

 it should be remembered that it 

 is possible that the apparent pre- 

 sence of a positive charge may 

 mean simply the absence of a 

 negative one, so that, for ex- 

 ample, H' means that the hydro- 

 gen ion has one less negative 

 electron than an "uncharged" 

 atom and two less than OH'. 



So far we have spoken 

 only of the ions present in a 

 solution through which an 

 electrical current is actually 

 passing. Now Clausius 

 (1857) pointed out that, in 

 order to explain the pheno- 

 mena of electrolysis, a part 

 of the molecules of the 

 electrolyte must be assumed 

 to be already dissociated 

 into ions, which possess 

 movements independent of 

 one another. In the case of 



the solutions with anomalous osmotic pressures we notice, in the table given on 

 page 169 above, that the "isotonic coefficient," or van't Hoff's factor i, that is the 

 ratio between the actual osmotic pressure of a solution of an electrolyte, and that 

 which it would have if it contained only non-dissociated molecules, is not a whole 

 number^ although in dilute solutions of strong acids and bases it is very near being 

 so. In dilute hydrochloric acid it is practically 2, but in. sodium chloride of O'l 

 per cent, it is only 1 -9. Measurements of electrical conductivity show the same 

 ratio between the part of the solute that carries the current, i.e., the ions, and 

 the non-dissociated fraction which takes no part in the process, as the table 

 referred to shows. 



Arrhenius (1887), on considering these various facts, was led to see that the 

 anomalous osmotic pressures of solutions of electrolytes could be very simply 

 explained by the assumption that the dissociation into ions is not merely the state 

 during the passage of a current, but is the normal condition of the solution of an 



FIG. 51. PORTRAIT OF MICHAEL FARADAY. 



