The Refractive Indices of the Elements. 283 



that may be attributed to the destruction by the soda of the ice- 

 molecules present in the water. In the formula 



pt = 



which represents the influence of temperature on the density of water 

 and aqueous solutions of soda, the coefficient of fi vanishes when a 

 concentration of 12 per cent. NaOH is reached, whilst the coefficient of 

 t 2 vanishes at 42 per cent. NaOH ; at the latter concentration there is 

 a simple linear relationship between density and temperature. 



(7) The molecular volume of sodium hydroxide in dilute aqueous 

 solution has a large negative value, a litre of water dissolving 140 

 grammes of sodium hydroxide at 0, 100 grammes at 18, or 60 grammes 

 at 50, without increasing in volume. It is noteworthy that the 

 molecular volume does not increase continuously as the temperature 

 rises, but reaches a maximum value at about 70 C. In a 50-per-cent. 

 solution, however, the temperature has little effect on the molecular 

 volume, the extreme variation being only about 10 per cent. 



" The Eefractive Indices of the Elements." By OLIVE CUTHBERTSON. 

 Communicated by Professor F. T. TROUTON, F.E.S. Eeceived 

 October 18, Eead November 24, 1904. 



(Abstract.) 



In a letter addressed to 'Nature,' in October, 1902, attention was 

 drawn to the fact that the refractivities of the five inert gases of the 

 atmosphere, He, Ne, A, Kr, and X, as determined by Eamsay and 

 Travers, were, within narrow limits of accuracy, in the proportion of 

 1, 2, 8, 12 and 20; or, more simply, of J, J, 2, 3, and 5. 



In a second letter it was shown that the refractivities of the halogens, 

 Cl, Br, and I, stand also in the relation of 2, 3, and 5 to the same 

 degree of accuracy; but it was pointed out that the figures for P,. 

 As, and S, as measured by M. Le Eoux in 1861, did not show any 

 similar relation ; and it was observed that a redetermination of them 

 would be interesting. 



With a Jamin's refractometer, adapted for use with high tem- 

 peratures, results have now been obtained for Hg, P, and S, which 

 differ widely from those of M. Le Eoux. The index of mercury,, 

 calculated for a molecule containing two atoms, is placed at 1 '001 857, 

 a number which agrees closely with the value given by the refractive 

 equivalent of Gladstone. The index of P 2 is found to be 1-001197 and 

 that of S 2 is 1-001101. 



In all three cases it is estimated that the margin of error does not 



VOL. LXXIV. z 



