N(JTES ON TIIR 11KFIIA(;T[ VIPIES OK 'l'IIK rXACTIVE GA.SES. 357 



ma-iionieter aiul ;i iiioval)le réservoir witli capillary tubiiig. So tliat on 

 raisiug or lowering tlie réservoir the ])ressure of tlie gases can be so adjusted 

 that the iiiterference-baiuls formée! in the lower of the field cnn be accurately 

 brought iuto liiie with the stationary bands in the upper half. Readings 

 of pressnre are taken on both raauometers at pressures not differing 

 greatly from that of the atmosphère; then, on lowering the réservoirs, 

 readings on botli manometers are taken at lower pressures, the bands 

 being again madc to coincide in position with the upper fiducial bands, 

 The ratio of the refractivities is inversely as the ditferences of pressure 

 in the two gases. The influence of température does not appear, for 

 the tubes of the manometer lie side by side, and may be regarded as 

 equally affected by variations of température. 



The accuracy of the method varies with the value of the refractivity 

 of the gas. For, if the gas has a low refractivity, then a great différence 

 of ])ressure produces the ])assage of fewer bands across the field than if 

 it has a liigh one; and, as the accuracy of reading may safely be taken 

 as the twenty-tifth of a band, and as between thirty and forty bands 

 pnssed the field with snch gases as oxygen, nitrogen, and argon, the 

 error may be taken in such cases as from 1 in 750 to 1 in 1000. 



The tubes containing the gases to be examined \vere connected with 

 a Tôpler's pump; and before admission of gas each tube was pumped 

 empty, so that in an attached Pr-iic'KER''s tube there was brilliant ])hos- 

 phorescence. The tubes were then washed ont with the gases to be 

 admitted, the a])paratus again evacuated, aiul the final ([uantity of gas 

 allowed to enter by a contrivance a description of wliich is to be found 

 in the „Trans. Cheni. Soc", vol. 67, p. 086. 



In determining the refractivity of such a gas as hélium or néon, 

 hydrogen was used as a standard of comparison ; in order to ensure the 

 purity of the hydrogen it was made by warining a tube containing pal- 

 ladium-hydrogen which had been prepared by admitting hydrogen made 

 from pure zinc and sulphuric acid into contact with s})ongy palladium. 

 The tube was ])umped empty in the cold and then gently warmed; it 

 was again allowed to cdol and again pumped empty. The hydrogen was 

 then collected, passing slowly through a tube, filled with phosphoric 

 anhydride, into the expérimental tube. For the refractivities of the 

 heavier gases the comparison was made with air. 



Determined in this way, the refractivity of the inactive gases com- 

 pared with air was; 



