[ 435 
XI. 0?i the Dependence of the Refnictive Index of Gases on Temperature. 
By George W. Walker, M.A., A.E.C.Sc., Fellow of Trinity College, Cambridge. 
Communicated by Professor J. J. Thomson, F.R.S. 
Received February 26,—Read March 26, 1903. 
The importance of this question was first imjrresseci on me in the course of some 
theoretical investigations on refraction in gases and the closely related property of 
electric susceptibility. 
A comparison of the actual temperature effect on a pi’operty of a body, with a 
theoretical formula professing to explain the property, is a very severe test, and one 
which has proved fatal to many theories. 
According to Gladstone and Dale’s law, of which most theories of refraction are 
particular cases, the refractive power of a gas is proportional to its density ; or, as a 
formula, 
g — 1 = Kp, 
where g is the refractive index, 
p is the density, 
and K a constant depending on the gas, but independent of temperature. If, then, 
the gas closely obeys Boyle’s and Charles’ laws, we must have 
{n — 1) ( 1 ~h _ {po — 1) (1 d~ ^ tp) 
P Po 
where p) is the pressure, t is the temperature, and a the coefficient of expansion of the 
gas at constant pressure. 
If the pressure is kept constant, we must have 
g — la 
1 —h at 
( 1 ). 
Several observers liave attempted to test this point, 
Mascart,^ Lorenz,! Beno1t,| Von Lange§ made observations on the refractive 
* ‘ Aimales de I’Ecole Normale Superieure,’ Series 2, vol. 6, 1877, p. 9. 
t Wiedemann, ‘ Annalen der Physik,’ vol. 11, 1880. 
J ‘ Travaux et Memoires du Bureau International des Poids et Mesures,’ vol. 6 1888. 
§ PoGGENDORFF, ‘ Annalen der Physik,’ vol. 153, p. 488. 
VOL. CCI.-A 341. 3 K 2 
14.7.03 
