Refractivities of Air, Oxygen, Nitrogen, Argon, Sfc. 225 



9997 per cent, it behaves as a metallic conductor in possessing a 

 negative temperature coefficient. 



Similar phenomena have been observed by Arrhenius in the cases 

 of moderately dilnte solutions of hypophosphorous and phosphoric 

 acids, and explained by him by means of the ionic dissociation 

 hypothesis. It is pointed out that nitric acid of 96 — 99'97 per cent, 

 would ex hypothesi contain few, if any, free ions, and therefore the 

 theory would lead to a totally opposite conclusion. 



The results of the experiments are also discussed in relation to the 

 hydrate theory of solution, and the illustrative curves in which the 

 percentages of acid are taken as abscissae and the resistances or con- 

 ductivities in mercury units show points of discontinuity markedly 

 at percentages corresponding approximately to the composition 

 required for the hydrates HN0 3 ,2H 2 0, HN0 3 ,H 2 0, 2HN0 3 ,H 2 

 (= H 4 N 2 7 ), and less markedly for the hydrate HNO 3 ,10H 2 O. 

 Further, if the values of a X 10 4 and p X 10 6 are referred to molecular 

 proportions of water, the minima values of the former and the 

 maxima of the latter occur in the cases of 3*07, T84, 099, and 055 

 molecular proportions or very approximately HNOy,3H 2 0,HN0 3 ,2H 2 0, 

 HN0 3 ,H,0, and 2HN0 3 ,H 2 0. Further evidence is thus added by an 

 independent method to that already accumulated as to the existence 

 of definite combination of nitric acid with water. Finally, it is 

 pointed out that if a curve is plotted out in which the molecular 

 proportions of water are taken as abscissas and the values for alO 4 as 

 ordinates, there are ascending and descending branches, meeting at 

 the points corresponding to the formation of the respective hydrates ; 

 the phenomena are compared with those observed by Bakhuis- 

 Roozeboom for the solubility curves of hydrates of ferric chloride 

 and by Le Chatelier, as also by Heycock and Neville for the freezing 

 point of alloys. 



" On the Refractivities of Air, Oxygen, Nitrogen, Argon, 

 Hydrogen, and Helium." By Professor William Ramsay, 

 Ph.D., LL.D., Sc.D., F.R.S., and Morris W. Travers, 

 B.Sc. Received November 18, — Read December 9, 1897. 



In the course of a research on the nature of helium many measure- 

 ments of its refractivity referred to that of air as unity were made 

 by means of an apparatus similar to that described by Lord Rayleigh.* 

 Inasmuch as the refractivity of helium is very small it was not found 

 convenient to measure its value directly against air ; hence it was 

 compared with hydrogen, and hydrogen was compared with air. 



* 'Proceedings,' vol. 59, p. 203. 



