Mobility of the Negative Ion. 457 



with the strength of the field, and when the opposite force 

 due to the positive ions begins to prevail, the pressure is 

 reversed and assumes a negative value. In the case when 

 the positive ions are moving toward the grating the wind- 

 pressure produced by them remains constant, but since the 

 opposite force due to the negative ions gradually diminishes, 

 the resultant pressure measured by the gauge increases with 

 the electric force. At atmospheric pressure the negative 

 direction of the electric wind can be produced only with 



large electric forces exceeding 3000 , at 50 mm. 



pressure a force of 80 is required for the same effect. 



It is of interest to note that with small electric forces 

 applied within the plates the wind-pressure increases with 

 diminution of pressure of the gas down to a certain limit. 

 This is due to the fact that the iouization-current, which is 

 in this case far from saturation at atmospheric pressure, 

 gradually approaches it when the pressure is reduced. 



It would not be out of place to cousider these results in 

 connexion with the observations made by Joly * on the 

 motion of radium in an electric field. Almost all the peculiar 

 properties of radium described in that paper are easily 

 explained, in the light of these results, by the electric wind 

 produced by radium. 



On the Nature of the Negative Carrier. 



In the discussion of the results given above the existence 

 was assumed of a transition stage between a negative ion 

 and an electron, at which the average mass associated with 

 the ion gradually diminishes with increasing electric forces. 

 Whilst these experiments were in progress a new paper on 

 the mobility of the negative ion appeared f, in which a 

 radically different point of view as to the nature of the ion 

 is put forward. According to E. M. Wellisch there are two 

 distinctly different kinds of negative carriers in a gas : 

 normal ions and free electrons, the proportion of the latter 

 increasing with diminution of pressure ; the conception of 

 an intermediate stage between an ion and an electron is 

 erroneous and is due merely to the attempt at averaging the 

 different properties of these two kinds of ions. 



* Joly, Phil. Mag. vii. p. 303 (1904}. 



t E. M. Wellisch, Arner. Jour, of Science, xxxix. p. 583 (1915). 



Phil. Mag. S. 6. Vol. 32. No. 191. Nov. 1916. 2 I 



