22 



THE ROYAL SOCIETY OF CANADA 



(No-1) P 



NtP-l = 



where 



_ (l+at) 76 



No = refractive index at 0°C and 76 cms. 

 a = 0-00365 

 The refractive indices of the 1% hydrogen-air and 1% helium-air 

 mixtures at 0° and 76 cms. were found by using the ordinary additive 

 law for the refractive index of a mixture, the validity of which has 

 been established for similar gas mixtures by several observers.^ 



The factors Na and Nx in the formula (Na — Nx)L = f A were thus 

 obtained; / was therefore calculated for various temperatures and 

 pressures at which readings were to be made. 



As (Na — Nx) is proportional to the percentage of hydrogen or 

 of helium in the mixture, such percentages were plotted directly 

 against the values obtained for/ as shown in Fig. 3. 



The following refractive indices were employed in the various 

 calculations : 



Refractive index at 

 0° & 760 mm. for Y =5461x 10-^ cm. Observer Reference 



Air 



Helium 



Hydrogen 



1-0002936. 



1-000034525 



1-000139 



Cuthbertson 



W. Burton 



Cuthbertson 



Proc. Roy, Soc. A. 

 83-151-1909. 

 Proc. Roy. Soc. A. 

 80-411-1908. 

 Proc. Roy. Soc. A. 

 80-390-1908. 

 Proc. Roy. Soc. A. 

 81-444-1908. 



Arrangement of Apparatus 



The arrangement of apparatus is shown diagrammatically in 

 Fig. 1. It consisted essentially of the interferometer, spectroscope 

 to observe the bands and the necessary connecting tubes. 



The Jamin interferometer — made by Adam Hilger Ltd., London 

 — consisted of the usual two parallel glass blocks of equal thickness, 

 silvered at the back and mounted on a heavy metal base. The two 

 25 cm. glass tubes, fitted with plane parallel ends, supported in rests 

 between the mirrors were each joined by side tubes to the respective 

 inlet and outlet taps, drying tubes and manometer, as shown in the 

 figure. 



A parallel beam of light from an Ediswan Pointolite lamp after 

 reflection from the mirrors and passage through the tubes fell on the 



^ Ramsay and Travers. 

 Jones and Partington. 



Proc. Roy. Soc. 62-225, 1897. 

 Phil. Mag. 28-29-1915. 



