220 RELATION OF PHYSICAL PROPERTIES OF AQUEOUS SOLUTIONS 



test the applicability to sufficiently dilute solutions, of such an 



expression, viz., 



P = P w + k (I a) n + Ian, (1) 



where P is the numerical value of any property (density, &c.), 

 P w that of the same property of water under the same physical 

 conditions, n the molecular concentration of the solution, i. e., 

 the number of gramme-equivalents of the dissolved substance 

 per unit volume of the solution, a the ionization-coefficient cm 

 and (1 a)n consequently the numbers of dissociated and undis- 

 sociated gramme-equivalents per unit of volume respectively, 

 and k and I constants, which may be spoken of as ionization- 

 constants, which will vary with the solvent, the substance 

 dissolved, the property to which they apply, the temperature, 

 and the pressure, but not with the concentration of the solution. 

 The formula can obviously apply only to properties for 

 which P has a finite value. Thus it is inapplicable to electrical 

 resistance, for which P^ would have a practically infinite value. 



SIMPLE SOLUTIONS. 



In order to test the applicability of the above expression I 

 have determined the ionization-constants for the density, thermal 

 expansion, viscosity, surface-tension, and refractive index of 

 solutions of Sodium and Potassium Chlorides, by the aid of 

 observations made by Bender *, Bruckner f, and Rother J. 1 

 selected these observations as a first instalment, not because of 

 their precision (for in one or two cases more exact observations 

 are available), but because these observers, in all cases but one, 

 determined the values of the above properties for mixtures of 

 solutions as well as for simple solutions. I selected the above 

 chlorides partly because I thought it well to begin with salts of 

 simple molecular structure, but largely also because, for the pur- 

 pose of calculating the conductivity of mixtures of them (as 

 described in my paper on this subject ), I had already obtained 

 interpolation formulae and curves which, judged by the results 



*Wied. Ann. vol. xxii. (1884) p, 184, and vol. xxxix. (1890) p. 89. 



t Ibid. vol. xlii. (1891) p. 293. t Ibid. vol. xxi. (1884) p. 576. 



Trans. N. S. Inst. Sci. ix. (1896) p. 101 ; and Phil. Mag. [5] xli. (1896) p. 276. 



