of Hydrogen and some other Gases. 309 



Hence /= — ~ — IPS, 



or k — k = 



2 

 H 2 S* 



The force /is measured by a small deflexion of the balance 

 beam, which causes a vertical rotation of a small mirror 

 suspended by a bi filar system, the upper ends of the fibres 

 being attached to the lower end of the pointer and to a fixed 

 stand. This rotation of the mirror is measured as usual by 

 a scale and telescope. The above method is due to Lord 

 Kelvin. 



For a very small displacement of the suspended tube the 

 force is proportional to 8a, and consequently to the deflexion 

 of the scale $ ; hence c being a proportional constant^, 

 we get 



f=cS. 



Introducing this relation into the expression for k—k\ 

 we get 



,_ 2cB 

 K *""H*S' 

 If the intensity of the field remains constant in the range 



2c 

 of the displacement, the factor r=—^ is also constant. Let p 



denote this factor, then the above expression becomes 



k— k — pS. 



In the present experiment the measurements of the sus- 

 ceptibilities of the gases were always made relatively to 

 water or to air ; that is, for the case of air, the comparison 

 was made with distilled water, while for other gases the 

 comparison was always made with dry air. In the present 

 day the susceptibility of pure water* is accurately known, 

 its value — O720 xlO -6 was assumed in the present experi- 

 ment. The susceptibility of air was determined relatively 

 to water. 



The upper half of a glass tube separated by a glass 

 partition in the middle was filled with the gas" or liquid 

 under examination, while the air in the lower half of the 

 tube was evacuated and its lower end sealed. The tube was 

 then vertically suspended from the arm of the balance 



* P. Seve, Jour, d, Phys. (5) iii. p. 8 (1913); de Haas u. Drapier, 

 Ann. d. Phys. xlii. p. 673 (1913) ; A. Piccard, Arch, de Geneve, xxxv. 

 p. 209 (1913). 



