CA.THQDE RAYS. 



165 



produced by it is not bright, so that I will show instead the other prop- 

 erty of the cathode rays — that of carrying with them a negative charge. 

 I will x)lace this cylinder in front of the hole, connect it with the elec- 

 trometer, turn on the rays, and you will see the cylinder gets a nega- 

 tive charge. Indeed, this charge is large enough to produce the well- 

 known negative figures when the rays fall on a piece of ebonite which 

 is afterwards dusted with a mixture of red lead and sulphur. 



From the experiments with the closed cylinder we have seen that 

 when the negative rays come ur) to a surface even as thick as a milli- 

 meter the opposite side of that surface acts like a cathode and gives 

 off the cathodic rays, and from this point of view we can understand 

 the very interesting result of Lenard that the magnetic deflection of 

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

 comx^osition of the gas outside the tube, tliough 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 

 momentum acquired by the particles outside would be the same, and, 

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

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

 of affairs inside the tube. 



The investigation by Lenard on the absorption of these rays shows 

 that there is more in his exjieriment 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-millimeter pressure) 



Hydrogen (760) 



Air (0.760-millimeter pressure) . . . 



so, 



Collodion 



Glass 



Aluminum 



Silver 



Gold 



Coefficieut 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 



1.1 



2.47 



2.70 

 10.5 

 19.3 



Absorption 

 density. 



4,040 

 5,640 

 2,780 

 3,110 

 3,010 

 3,160 

 2,650 

 3,070 

 2,880 



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 u]3on 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? 



