594 Prof. J. S. Townsend and Mr. V. A. Bailey on 



investigations with argon, which are of particular interest, as 

 they show that the molecules o£ this gas are very different 

 from those of the other gases. 



Thus when an electron moving with a velocity of the 

 order of 10 8 centimetres per second collides with a molecule 

 of argon it loses about one ten- thousandth part of its energy, 

 but when it collides with molecules of the other gases it 

 loses more than one per cent, of its energy. Also the mean 

 free path of an electron moving with these velocities in 

 argon is about ten times as long as its mean free path in 

 the other gases at the same pressure. 



2. The experiments with argon were made with the 

 apparatus described in the previous paper. Some prelimi- 

 nary investigations were made with impure argon which 

 had been dried for several days in a vessel containing 

 phosphorus pentoxide. The gas was supplied in a cylinder 

 by the British Oxygen Company, an'd was said to contain 

 88 per cent, of argon, 10*5 per cent, of nitrogen, and 1*5 

 per cent, of oxygen. It was found that the lateral diver- 

 gence of a stream of electrons in the impure argon was 

 remarkably large, and for a given electric force X and gas 

 pressure p the velocity in the direction of the electric 

 force and the velocity of agitation of the electrons were 

 greater than in nitrogen or hydrogen. The gas was then 

 purified by Rayleigh's method. It was admitted to a 

 pressure of about 50 centimetres into a flask containing 

 a solution of caustic potash, and oxygen was admitted in 

 excess of the amount required to combine with the nitrogen. 

 Two platinum electrodes were sealed in the flask and a 

 discharge was passed through the gas for several hours. 

 An approximate estimate of the rate of combination of the 

 oxygen and nitrogen was made by observing the change of 

 pressure of the gas, and when the proportion of nitrogen 

 was reduced to about one per cent, of the argon the gas was 

 passed over hot copper in long quartz tubes to remove 

 all the oxygen, and into a vessel containing phosphorus 

 pentoxide, where it was dried for several days before 

 measurements were made of its electrical properties. 



As it was important that no air should leak into the gas 

 all the apparatus, including the quartz tubes, had been 

 exhausted by a mercury pump down to one-hundredth of a 

 millimetre, and tested for leaks by a McLeod gauge. Also 

 before using the copper it was heated for several hours in 

 order to expel the occluded gases. This first purification did 

 not have an appreciable effect on the velocity in the direction 

 of the electric force, but the divergence of the stream of 

 electrons became wider than in the more impure argon. 



