106 Messrs. V. H. Veley and J. J. Manley. 



SECTION II. The Refractive Indices of Nitric Acid.. 

 Introductory. 



The earliest observation on record upon any one sample of nitric acid 

 appears to be that of Baden-Powell,* namely, /V 8 ' 8 1-4026, but the 

 percentage concentration is not given. Twenty-seven years later- 

 van der Willigenf published values of //, for various spectrum lines 

 from A to H determined at the same temperature from which the 

 absolute index (Cauchy's method) is deduced from the formula 



n = 1 -385967 + 0-527631 A---0-18099(10 (1 ) A -4. 



The values of ^ for the same line at different temperatures are also 

 given, from which A/A/ A;! = O'000424/l is deduced. 



Owing to experimental difficulties, especially in the measurement of 

 /* 4 , only one particular sample was fully examined by v. d. Willigen, 

 namely, of concentration 58*89 per cent. 



His work will, however, be alluded to frequently in the sequel for 

 the following reasons : (i) his instrument was by the same maker and 

 of the same construction ; (ii) his methods present certain points of 

 similarity ; and (iii) his more detailed observations on mixtures of 

 sulphuric acid and water showed that Biot's and Arago's formula 



( 1 00 - p) (p? - 1 )/(l +p (/*'-' - 1 )/d' = 1 00 (p"* - 1 )/d" 



was inadmissible in such a case, not of admixture, but of presumed 

 chemical combination, but that variations of //. for the same liquid at 

 different temperatures can be expressed by a differential : 



Gladstone, I in the course of his prolonged investigations on Kefrac- 

 tion Equivalents M(/x -l)d, has from time to time given values for 

 different spectrum lines ; these results in the particular cases of R a , R D , 

 and R, for samples of percentage values varying from 6 to 98 -7 per 

 cent, have been collated together by the above-mentioned observer 

 with Hibbert, from which the average rate of change per difference 

 of percentage, namely, R 2 - Ki/pi -p->, is calculated. Ten samples 

 from two specimens of percentage values varying from 6 to 98 - 7 per 

 cent, have been examined, and the experimental results illustrated by 

 diagrams of curves. 



The general conclusion arrived at is " that the refraction equivalent 



* ' The Undulatory Theory,' p. 112 (London, 1841). 



t ' Archives Musee Teyler' (1), 79 (Haarlem, 1868), and (2), 238 (1869). 

 I ' Roy. Soc. Proc.,' vol. 16, p. 439 ; ' Phil. Trans.,' 1870 ; Journ. Chem. Soc.,' 

 (Trans.), 1891, p. 589. 



'Journ. Chem. Soc.' (Trans.), 1895, p. 831. 



