WEEKLY EVEXIXG MEETING, 



Friday, Januarv 25, 1918. 



Sir William Phipsox Beale, Bart., K.C. M.P., 

 Vice-President, in the Chair. 



Peofessor Johx S. Towxsexd, F.Pi.S. 



The Motions of Ions in Gases. 



Whex ions move in a gas under an electric force, and the sizes of 

 the ions are unaffected either by changes of force or of pressure, the 

 velocity of the ions in the direction of the force is proportional to 

 the ratio of the force to the pressure (X/P), provided the velocity in 

 the direction of the force is small compared with the velocity of 

 agitation of the ions. For small values of the ratio X/P the ions 

 move as if they were associated with masses which are large compared 

 with the masses of the molecules of the surrounding gas, and the 

 kinetic energy of the motion of agitation of the ions is equal to that 

 of the molecules with which they collide. 



When the pressure is reduced the mobility of the negative ions 

 increases with the force, which shows that the mass associated with 

 each ion diminishes as the force increases. 



Experiments were described from which it was shown that at a 

 certain point, depending on the ratio X/P, the electrons move freely 

 through the gas, as the ratio of the charge e to the mass m of the 

 electrified particle was found to be approximately the same as the 

 value 5 • 6 X 10^'' for electrons moving in a highly exhausted space. 



Thus in dry air at a pressure of 18 millimetres of mercury the 

 electrons move freely, unassociated with larger masses, when the 

 force is 3 or 4 volts per centimetre. A similar result is obtained at 

 higher pressures by increasing the force. When a small quantity of 

 water vapour is present in the gas a larger force is required to attain 

 the point at which the electrons move freely, which shows that the 

 electrons have a greater tendency to adhere to molecules of water 

 vapour than to molecules of oxygen or nitrogen. 



In all cases the electronic state is attained with much smaller 

 forces than those required to produce ions by collisions with molecules. 

 The latter effect, for example, could not be easily detected in air 

 at 18 millimetres pressure unless the force exceeded 30 volts per 

 centimetre. 



The experiments show that while the mass associated with the 

 electron undergoes these changes the charge remains constant. 

 Vol. XXII. (No. 112) ^ q 



