450 



Mr. S. Ratner on the 



some other impurities in the air, and this supposition is 

 supported by the fact that, as shown below, even imperceptible 

 traces of chloroform vapour change completely the aspect 

 of the mobility curves. At lower pressures, when the 

 critical force Xj is small, another difficulty arises, as the 

 ionization-current and the corresponding wind-pressure de- 

 crease with the potential difference between the plates and 

 become, in this case, too small to be measured with accuracy. 



"V" 



In the Table III. the values of Xi and — - at different 



P . 

 pressures are given, X x being the mean approximate value 



from a great number of experiments. 

 Table III. 



p> 



X v 





750 



1800 



24 



600 



1000 



17 



500 



700 



1-4 



400 



450 



11 



300 



250 



•8 



200 



100 



"5 



150 



60 



•4 



100 



30 



•3 



75 



20 



•26 



It is seen from the Table that — - decreases rapidly with 



diminution of pressure, so that at high pressures the negative 



ion may attain comparatively large velocities before it begins 



to assume an electronic state, and that is the reason why 



the abnormal mobility could not be observed at high pressures 



by other methods. It is, however, worth noticing that even 



at atmospheric pressure the largest velocity attained by the 



cm 

 normal negative ion, viz. 3200 — - , is small compared with 



sec. 



its velocity of thermal agitation. 



2. Interesting mobility curves are obtained with larger 



values of — . It has been already noticed from the curves 



p J 



(fig. 4) that at large electric forces the mobility tends to 

 remain constant. When the force is still further increased 



