of Hydrogen and some other Gases. 



311 



and p Wi we have for the ratio of tlie specific susceptibilities 

 X and Xv °^ these two substances 



2k 



Xw 



or 



d Jp w 



_ d g /m ff 

 du-lm tv 



where m g and m w are the total masses of the gas and water 

 occupying the same volume of the tube respectively. The 

 same formula may also be used w r hen water is replaced by 

 air, in which case we obtain the susceptibility of a gas 

 relative to air. 



§ 3. Apparatus for Measurement. 

 (a) Magnetic Balance. 



The magnetic balance and its accessories used in measuring 

 the magnetic susceptibilities of several gases are diagram- 

 matically shown in fig. 3. 



AB is an aluminium arm of the magnetic balance, 80 cm. 

 long, C an agate knife-edge resting on a smooth plane of 

 steel, and HD an aluminium pointer, a similar counterpoise 

 being attached to the same arm, but on the opposite side of 

 the beam. DE shows the side view of the bifilar system 



