[MCLENNAN-EL WORTHY] JAMIN INTERFEROMETER 23 



slit of a Hilger glass prism spectroscope and the bands were observed 

 in the field of the micrometer eyepiece. 



The telescope of the spectroscope was set in such a position that 

 1;he moveable cross hair in the micrometer eyepiece when set at 500 

 on the scale, was focussed on the green mercury line, A = 5461X10~'^ 

 cm., which served as the standard line. 



Procedure in Taking Readings 



For measurements, both interferometer tubes were initially 

 filled at atmospheric pressure and temperature with dry air. The 

 spectroscope was then adjusted so that the centre of a band coincided 

 with the cross hair, set at 500. 



The gas "mixture to be examined, was then passed through the 

 gas tube of the interferometer until the air was completely swept out. 

 When this was accomplished the bands remained stationary. Usually 

 the gas flow was continued for three to five minutes. The inlet taps 

 of both gas and air tubes were then closed, and the outlet taps left 

 open so that atmospheric pressure was established in both tubes. 

 The movement of the bands, the centre of which coincided with the 

 standard line when both tubes contained air, was then measured as 

 well as the distance between the centres of two bands adjacent to the 

 zero position. The shift was calculated as a percentage of the dis- 

 tance between the two bands. 



From the appropriate curve, shown in Fig. 3, the percentage of 

 gas was obtained. 



Calibration 



The theoretical method of calibration employed has already been 

 discussed under "Method Adopted." As a check on this method a 

 further one, proposed by J. D. Edwards^ was employed. 



After dry air had been passed through both tubes and atmos- 

 pheric pressure established, the gas tube was closed and the pressure 

 in the tubes lowered by successive amounts by letting mercury slowly 

 run out of the bulb in Fig. 2, through the screw slip. The shift of 

 bands for each pressure change was measured. The refractive index 

 of the air at each pressure was calculated and also the composition 

 of the helium-air or hydrogen-air mixture that would have a similar 

 refractive index at the initial atmospheric pressure and temperature. 

 The percentage of helium or hydrogen thus found was plotted against 

 the shift of bands. The curves so plotted agreed with those obtained 

 by the first method. 



1 J. D. Edwards. Jour. Amer. Chem. Soc, 2382-1917. 



