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.
