314 Sir J. Conroy. Refractive Indices and Densities of 



refractive indices was found to be 4 in the sixth place, and for the- 

 densities 2 in the fifth place. 



The sensitiveness to change of temperature (i.e., the decrease in the 

 value of the index for an increase of 1) as calculated from the values 

 given in the table is nearly the same for both the normal and semi- 

 normal solutions, the mean value being 0*000095. The value for 

 water as calculated from the values for the indices at 15 and 20% 

 given by eight different observers, being 0*000080.* 



The solutions not being accurately normal or semi-normal, the values 

 for the refractive indices and densities of normal and semi-normal 

 solutions at 18* were calculated from the observed values given in 

 Table I, on the assumption that the differences between the indices and 

 densities of the solutions and those of water are, over this small range, 

 proportional to the weights of salt present in the unit volumes. The 

 values so obtained are given in Table II, those for the potassium and 

 sodium chlorides being the mean values for solutions A and B. 



Table II. 



Table II shows that both the densities and the refractive indices as a 

 rule increase with the molecular weight, but that in the latter case 

 there is one remarkable exception. The refractive index of potassium 

 chloride is slightly lower than that of sodium chloride. 



This fact has already been noticed by Benderf and Borgesius.J: 

 Bender's observations were made by the hollow prism method, with 

 solutions containing from 0*5 to 4'5 gram-molecules in the litre, and at 

 temperatures between 16 and 21 ; he states (p. 92) that the solutions 

 of potassium and sodium chloride had nearly identical refractive 

 indices, those of the latter being slightly the greater, especially with 

 the more concentrated solutions. 



Borgesius's observations were made by an interference method, and 



* Conf. Dufet, ' Eecueil de Donuees Numeriques,' p. 87. 

 f ' Wied. Ann.,' vol. 39, p. 88. 

 J ' Wied. Ann.,' vol. 54, p. 221. 



