430 



Professor J. J. Thomson 



[April 30, 



the rays outside tlie tube is independent of the density and chemical 

 composition of the gas outside the tube, though it varies very much 

 with the pressure of the gas inside the tube. The cathode rays could 

 be started by an electric impulse which would depend entirely on 

 what was going on inside the tube ; since the impulse is the same 

 the momeutam acquired by the particles outside would be the same ; 

 and as the curvature of the path only depends on the momentum, tbe 

 path of these particles outside the tube would only depend on the 

 state of aifairs inside the tube. 



The investigation by Lenard on the absorption of these rays shows 

 that there is more in his experiment than is covered by this considera- 

 tion. Lenard measured the distance these rays would have to travel 

 before the intensity of the rays fell to one-half their original value. 

 The results are given in the following table : — 



Substance. 



Hydrogen (3 mm. press.) 



(760) .. .. 

 Air (0-760 mm. press.) 



SO2 



Collodion 



Glass 



Aluminium 



Silver 



Gold 



Coefficient of 

 Absorption. 



0-00149 



0-476 



3-42 



8-51 

 3,310 

 7,810 

 7,150 



32,200 



53,600 



Density. 



0-000000368 

 0-0000484 

 0-00123 

 0-00271 



Absorption 

 Density 



4040 

 5640 

 2780 

 3110 

 3010 

 3160 

 2650 

 3070 

 2880 



We see that though the densities and the coefficient of absorption 

 vary enormously, yet the ratio of the two varies very little, and the 

 results justify, I think, Lenard's conclusion that the distance through 

 which these rays travel only depends on the density of the substance 

 — that is, the mass of matter per unit volume, and not upon the 

 nature of the matter. 



These numbers raise a question which I have not yet touched 

 upon, and that is the size of the carriers of the electric charge. Are 

 they or are they not the dimensions of ordinary matter ? 



We see from Lenard's table that a cathode ray can travel through 

 air at atmospheric pressure a distance of about half a centimetre 

 before the brightness of the phosphorescence falls to about one-half 

 of its original value. Now the mean free path of the molecule of air 

 at this pressure is about 10-^ cm., and if a molecule of air were pro- 

 jected it would lose half its momentum in a space comparable with 

 the mean free path. Even if we suppose that it is not the same molecule 

 that is carried, the eflFect of the obliquity of the collisions would 

 reduce the momentum to one-half in a short multiple of that path. 



Thus, from Lenard's experiments on the absorption of the rays 

 outside the tube, it follows on the hypothesis that the cathode rays 



