132 THE ROYAL SOCIETY OF CANADA 



substituting these values in the relation given by the equation (3) the 

 mobilities for both positive and negative ions were deduced for the 

 corresponding pressures. These mobilities are all collected in Table 

 II and they are plotted in Fig. 3 against the pressures as abscissae. 



Mobilities calculated according to the inverse pressure law on the 

 basis of the mobilities for positive and negative ions at atmospheric 

 pressure being respectively 1 • 34 and 1 • 89 cm. a second per volt a cm. 

 are also given in Table II. The dotted curves represent the calcu- 

 lated mobilities and the smooth curves the mobilities determined in the 

 present investigation. As both the table and the figure show, the 

 mobilities did not decrease as the pressure rose so rapidly as was 

 demanded by the inverse pressure law. Moreover, it will also be 

 seen from the table and the diagram that the mobility of the positive 

 ion approached that of the negative ion as the higher pressures were 

 reached, the ratio of the mobility of the negative ion to the positive 

 ion dropping from 1-43 at 66-86 atmospheres to 1-31 at 181-5 at- 

 mospheres. The departure from the inverse pressure law, however, 

 was not very great. 



It will be recalled that Greinacher^, in his experiments on the 

 ionisation of paraffin oil and of petrol ether by alpha rays, found that 

 the mobilities of the positive and negative ions produced in these 

 liquids were practically identical. In this connection it is interesting 

 to see that our results indicate that very probably the same equality 

 would apply to the mobilities of positive and negative ions in liquid 

 air. Measurements on the mobilities of ions in air at pressures still 

 higher than those used in this investigation would be required, however, 

 to show whether this surmise were correct or not. 



In closing we desire to express our appreciation of the services 

 of Mr. P. Blackman, who assisted us in taking many of the readings 

 in this investigation. 



The Physical Laboratory, 



University of Toronto, 

 May 1st, 1915. 



'Greinacher. Phys. Zeit., 10 Jahr., No. 25, p. 986. 



