438 Mr. A. J. Dempster on the Ionization 



curve of the 7 rajs is obtained directly. Again, if the 

 divergence between the value here obtained for fi ('66 cm. -1 ) 

 and that found by Rutherford and by Richardson (*50 cm. -1 ) 

 is a genuine one, this is further evidence that the hard 

 ft rays are mainly due, not to the hardest 7 rays reflected at 

 43' from rock-salt, but to those reflected at 1° 0'. 



There are two possible disturbing factors to be reckoned 

 with, primary ft rays producing secondary or scattered ft rays, 

 and secondary (scattered) 7 rays from the radiator. The 

 former, however, must be almost negligible, even for no 

 extra screen A7, since the primary radiation has already 

 passed through the platinum and glass in which the radium 

 is contained. In fact, the curves themselves show that the 

 primary ft rays have been already almost totally absorbed. 

 The effect of the secondary 7 rays, on the other hand, is by no 

 means negligible, though that it is small compared with that 

 of the ft rays is shown by the closeness with which the curves 

 of fig. 3 approach to zero. We may, in fact, safely assume 

 that the values found for /j, really represent the falling-off of 

 the secondary ft radiation. 



I wish to thank Sir Ernest Rutherford for suggesting this 

 research and for the kind interest which he took in it while 

 work was in progress. 



LII. 2 he Ionization and Dissociation of Hydrogen Molecides. 

 and the Formation of H z . By A. J. Dempster, late 1851 

 Exhibition Scholar of the University of Toronto *. 



BY the analysis of positive rays, J. J. Thomson has shown 

 that in a discharge-tube containing hydrogen there are 

 present charged atoms, charged molecules, and sometimes 

 a constituent with a mass three times that of the atom of 

 hydrogen. The pressure used was about *003 mm. of mer- 

 cury, and consequently the potential necessary was of the 

 order of 20,000 volts. In the following experiments a 

 different method of getting the positive rays was used.. 

 Electrons from a Wehnelt cathode C are accelerated in the 

 field CA. They ionize the gas, and the positive particles 

 produced are given a velocity which carries them past the 

 edge of C (2 mm. wide) and through the narrow tube T. 

 These positive particles are then deflected by magnetic and 

 electric fields, and fall on a screen which has a parabolic 



* Communicated by Prof. R. A. Millikan. 



