ELECTROLYTES AND THEIR ACTION 17;, 



geloster Stoffe" (1887, p. 637), he gives the evidence for the acceptance of two 

 hypotheses, which are : 



" 1. The Law of van't Hoff applies not only to the greater number of 

 substances, but to all, including those which had been considered to be exceptions 

 (electrolytes in watery solution)." 



This law of van't Hoff, which is a generalisation of Avogadro's law, has 

 already been quoted (page 148 above), but, for reference, it may be- repeated here : 



" The pressure which a gas possesses at a given temperature, when a definite 

 number of molecules are present in a definite volume, is of the same value as the 

 osmotic pressure which is exerted, . under the same conditions, by the greater 

 number of substances, when they are dissolved in any kind of liquid." 



The second hypothesis of Arrhenius states that : 



" 2. All electrolytes (in solution in water) consist partly of molecules which are 

 active (in electrolytic and chemical ^relationships), and partly of inactive molecules. 

 The latter, however, on dilution, are converted into active molecules ; so that in 

 infinitely diluted solutions only active molecules are present." 



We may now venture to call these hypotheses " laws," although objections 

 have not been wanting, as we shall see. 



Free ions, therefore, are believed to be present in all solutions of electrolytes, 

 whether a current is passing or not. Definite proof of their existence under 

 ordinary conditions is naturally desirable. Since the distinguishing property 

 of an ion is its electrical charge, the difficulty of investigating its properties 

 by methods other than electrical, which might be supposed to introduce con- 

 ditions which beg the question, is obvious. To begin with, the following reasons 

 are given by J. J. Thomson (1888, p. 294) for concluding "that the splitting 

 up of the molecules which allows the current to pass is not caused by the 

 electro-motive force, but takes place quite independently of the electric field." 

 In other words, they are already split up before the current is sent in. 



(1) The smallest electro-motive force is sufficient to start a current, so that 

 no finite electro-motive force is required to dissociate the molecules. 



(2) The experiments of Fitzgerald and Trouton (1886, p. 312) show that 

 Ohm's law is obeyed exactly, " whereas, if the electro-motive force had to break 

 up the molecules, the current would be proportional to a higher power than 

 the first of the electro-motive force." 



(3) J. J. Thomson himself was unable to detect the slightest change in the 

 osmotic pressure of a solution of an electrolyte during passage of a current 

 through it. If the number of separate systems is increased by the current, 

 the osmotic pressure must rise considerably ; in the case of dilute strong acids, 

 to nearly double. 



Let us further contrast the behaviour of an organic compound of chlorine, 

 say chloroform, CHC1 3 , with that of an inorganic chloride, CaCl.,. In the 

 first case, chlorine cannot be detected by the ordinary tests ; the molecule has 

 certain properties as a whole. In the second case, in dilute solution, the 

 behaviour to reagents is not that of an individual compound with its own 

 peculiar properties ; its reactions are simply those which are common to all 

 calcium salts, together with those which are common to all chlorides. Accord- 

 ing to the electrolytic dissociation theory, all chlorides, in dilute solution, contain 

 chlorine ions, and all calcium salts contain calcium ions. Calcium salts 'have 

 also a particular action on the muscle of the heart, and it is found that it 

 does not matter what salt is taken. Again, hydrochloric acid has no properties 

 peculiar to itself ; it tastes sour, turns litmus red, dissolves metals, inverts cane- 

 sugar, in common with all acids. It precipitates silver salts in common with 

 all chlorides. The nitrate, chloride, and bromide of copper are all blue in 

 dilute solution in water, but in alcohol, where very little dissociation is to be 

 expected, they are blue, green, and brown respectively. Ostwald (1892) has 

 shown that each ion independently contributes its share to the properties of 

 a solution, inclusive of colour, by taking photographs of the absorption spectra 

 of the permanganates of zinc, cadmium, ammonium, tin, potassium, nickel, 

 magnesium, copper, hydrogen, aluminium, sodium, barium, and cobalt in dilute 



