Force of the Elect civ Discharge through Gases. 445 



greater than that required in nitrogen or carbon monoxide 1 

 (Recent Researches, p. 152, par. 150), although the potential- 

 difference necessary to begin the discharge is much less in 

 hydrogen than in these gases. This effect, in hydrogen, may, 

 however, he due to the very rapid scattering of the ionized gas 

 produced by a discharge, from causes (1), (2) and (3) men- 

 tioned above. 



The less a gas is ionized by a discharge through it, the less 

 should its resistance be diminished for succeeding discharges, 

 and the less should the discharge in it be capable of being- 

 deflected. 



The resistance of the gas in the discharge-tube to the 

 discharge passing through it will affect its duration, since 

 the coil will continue to send the discharge until the potential- 

 difference between its terminals has fallen so much that it is 

 not able any longer to force the current oyer the opposing 

 resistance. 



The Discharge in Mercury Vapour. 



As it seemed probable that a monatomic gas would be less 

 broken up, and would consequently have its resistance less 

 changed, by the passage of a discharge through it than other 

 gases, an experiment was made to try the effect of magnetic 

 force in deflecting the discharge in mercury vapour. 



To procure a good mercury vacuum, a tube of the form 

 shown in fig. 15 (see p. 446) was employed. The vacuum-tube 

 was partly filled with clean mercury m, and connected by the 

 tube d to a mercury air-pump with the capillary tube c in a 

 vertical position ; the end of c dipped under mercury in the 

 test-tube e as shown ; c was more than 76 centims. long. 



The vacuum was made by pumping until no more air could 

 be got out, then the mercury in the tube was heated and 

 slowly distilled for about two hours, the air-pump being 

 worked at the same time. 



While the mercury was being distilled, the discharge was 

 kept going by an induction-coil and the platinum electrodes 

 a, b were raised several times to a white heat, to drive out 

 any gas occluded in them. The vacuum-tube was then sealed 

 off the air-pump at the constriction d, left for the purpose. 



After distilling the mercury left in the bulb for about three 

 hours more, so as to get rid of any traces of air left in it, the 

 bulb was then placed between the poles of the electromagnet 

 used in previous experiments, with the direction of discharge 

 horizontal and perpendicular to the magnetic force. The 

 mercury was then heated, and the effect of the magnet on the 



Phil. Mag. EL 5. Vol. 46. N T o, 282. Nov. 1898. 2 I 



