Dissociation into Ions, and its Consequences. 25 



kinetic energy of the dissolved substance, K(K + C1), can, 

 according to the osmotic-pressure theory, be calculated. With 

 a monatomic gas no intramolecular work is possible, and 

 therefore the pressure caused by that gas is a measure of its 

 total kinetic energy (cf. Ostwald, loc. cit. p. 76) ; the pressure 

 of each ion is in this case, ex. hyp., the same as that which an 

 ordinary molecule produces, and the pressure-producing energy 

 of a gram-molecular proportion of such a gas is 34,008,000,000 

 ergs, or 819 cal., so that 



K(K+Cl)=2x819eal., 



and P(KC1) =P(K + CI) -3646 cal.-K(KCl); 



thus the potential energy of the ions must be considerably 

 greater than that of the solid molecule. 



The same substance might have been taken as an instance 

 of a gas dissolving with evolution of heat and a solid with 

 absorption of heat ; and in such a case the potential energy 

 of the substance in solution must be intermediate between 

 that of the gaseous and solid molecules. It is certainly 

 difficult to imagine that this substance can consist of uncom- 

 bined atoms. 



But a still greater difficulty arises in some cases : a solution 

 of calcium nitrate, for instance, absorbs heat on dilution, and, 

 as the only change produced by diluting an already dilute 

 solution is, according to the theory, to dissociate into ions 

 some of the molecules still remaining intact, and as these mole- 

 cules are present in the uncombined and gaseous condition, it 

 follows that the dissociation of gaseous calcium nitrate mole- 

 cules must absorb heat ; therefore the gaseous salt on beino- 

 dissolved in excess of water must absorb heat, the liquid salt 

 would absorb more than the gas (by an amount equivalent to 

 the heat of vaporization of this salt) , and still more would be 

 absorbed by the solid ; yet direct experiment shows that this 

 last, instead of giving a large absorption, actually evolves 4000 

 cal. when it is dissolved. 



The conception that salts &c. in solution are entirely de- 

 composed into ions has been regarded as a development of the 

 theory of Clausius that a few free ions exist at any given moment 

 in a mass of solution. But these two views appear to me to 

 be radically different. Clausius's conception (Phil. Mag. 1858, 

 vol. xv. p. 100) was of a two-fold nature. ( 1) That the molecules 

 in any fluid, being at different temperatures at different times, 

 owing to the impacts to which they have been subjected, some 

 of them may occasionally be so hot as to be above their dis- 

 sociation temperature, and that some temporarily free atoms 

 would therefore be present. (2) That two similar molecules 



