454 MR. GEORGP] AV. AVALKER ON TEIE DEPENDENCE OF THE 
The following table gives a compaiisoii of the results with those of former 
observers* :— 
Refractive Index for Na Light at 7G centims. Pressure and 0° C. 
Obseiver. 
Air. 
Hydrogen. 
Carbon dioxide. 
Ammonia. 
Sul 2 )hur dioxide. 
Present .... 
1-0002928 
+ 3 
1-0001407 
+ 15 
1-0004510 
+ 5 
1-0003793 
+ 5 
1-0006758 ' 
-f 5 
Mascart .... 
1-0002927 
1-000139 
1 - 000454 
1-000379 
1-0007038 
Lorenz .... 
— 
1-000139 
— 
1-000373 
KETTEI.ER . . . 
— 
1-000143 
1-000449 
_ 
1-000686 
Dulong .... 
1-000294 
1-000138 
1-000449 
1 - 000385 
1-000685 
Tlie followdng table gives a comparison of Mascart’s temperature coefficients with 
those obtained in this paper:— 
Air. 
Hydrogen. 
Car]:)on 
dioxide. 
Ammonia. 
Sulpliur 
dioxide. 
Coefficient of exjjansion . . 
-00367 
-00366 
-00371 
-00390 
Mascart, refractive index 
coefficient. 
-00382 
-00378 
-00406 
_ 
-00460 
Present . 
-00360 
-00350 
-00380 
-00390 
-00416 
±3 
i 3 
+ 3 
± 3 
±2 
1 he values of the temperature coefficient of refractive index obtained are, in every 
case, less than those olitained l)y Mascart. It is somewhat futile to attempt to 
exj)lain the diflerence ; Init perhaps the following points are worthy of attention. In 
my apparatus the tubes were aliout 1 metre long and the two rubber washers 
together about f inillim. thick, while Mascart used tubes about 25 centims. loim 
Jiiiu his riil)]) 0 r washers were probably 1 inillim. thick each. He does not nieiitioii 
the thickness, but Lorenz, who appears to have used an almost identical apparatus, 
used washers 1-^ unllims. thick each. The somewhat irregular behaviour of I'uliber 
under varying conditions of temperature and pressure may have produced errors in 
Mascart s observations, from whicli I consider that mine are entirely free. 
I have already referred to the apparent escape of impuiities from the walls of the 
apjiaratus. Mascart makes no reference to this point, arid gives no indication of 
iiow he tested the constancy of composition of tlie gas during the experiments. It is 
true he analysed the gas chemically after the experiments, but this is hardly accurate 
enough for the point in view. 
* A very useful table of the results of different observers is given b}" Bruiil, ‘ Zeitschrift fiir 
I’hysikalischc Cheniie,’ vol. 7 , 1891. 
