396 



SCIENCE. 



[N. S. Vol. IV. No. 90. 



connected with, a gold leaf electroscope to 

 the bombardment of the cathode rays, and 

 found that the disc received a slight 2^ositiv6 

 charge ; with this arrangement, however, 

 the charged particles had to give up their 

 charges to the disc if the gold leaves of the 

 electroscope were to be affected, and we 

 know that it is extremely dif&cult, if not 

 impossible, to get electricity out of a charged 

 gas merely by bringing the gas in contact 

 with a metal. Lord Kelvin's electric 

 strainers are an example of this. It is a 

 feature of Perrin's experiment that since it 

 acts by induction, the indications of the 

 electroscope are independent of the commu- 

 nication of the charges of electricity from 

 the gas to the cylinder, and since the cathode 

 rays fall on the inside of the cylinder, the 

 electroscope would not be affected, even if 

 there were such an effect as is produced 

 when ultra-violet light falls upon the sur- 

 face of an electro-negative metal when the 

 metal acquires a positive charge. Since 

 any such process cannot affect the total 

 amount of electricity inside the cylinder, it 

 will not affect the gold leaves of the electro- 

 scope ; in fact, Perrin's experiments prove 

 that the cathode rays carry a charge of neg- 

 ative electricity. 



The other view held as to the constitu- 

 tion of the cathode rays is that they are 

 waves in the ether. It would seem diffi- 

 cult to account for the result of Perrin's 

 experiment on this view, and also I think 

 very diflS.cult to account for the magnetic 

 deflection of the rays. Let us take the 

 case of a uniform magnetic field, the ex- 

 periments which have been made on the 

 magnetic deflection of these rays seem 

 to make it clear that in a magnetic field 

 which is sensibly uniform, the path of 

 these rays is curved; now if these rays 

 were due to ether waves, the curvature of 

 the path would show that the velocity of 

 propagation of these waves varied from 

 point to point of the path. That is, the 



velocity of propagation of these waves is 

 not only affected by the magnetic field, it 

 is affected differently at different parts 

 of the field. But in a uniform field what 

 is there to differentiate one part from 

 another, so as to account for the varia- 

 bility of the velocity of wave propagation in 

 such a field ? This could not be accounted 

 for by supposing that the velocity of 

 this wave motion depended on the strength 

 of the magnetic field, or that the magnetic 

 field, by distorting the shape of the boun- 

 dary of the negative dark space, changed 

 the direction of the wave front, and so 

 produced a deflection of the rays. The 

 chief reason for supposing that the ca- 

 thode rays are a species of wave motion is 

 afforded by Lenard's discovery, that when 

 the cathode rays in a vacuum tube fall on a 

 thin aluminium window in the tube, rays 

 having similar properties are observed on 

 the side of the window outside the tube ; 

 this is readily explained on the hypothesis 

 that the rays are a species of wave motion 

 to which the window is partially transpar- 

 ent, while it is not very likely that "parti- 

 cles of the gas in the tube could force their 

 way through a piece of metal. This dis- 

 covery of Lenard's does not, however, seem 

 to me incompatible with the view that the 

 cathode rays are due to negatively charged 

 particles moving with high velocities. The 

 space outside Lenard's tube must have been 

 traversed by Hontgen rays, these would 

 put the surrounding gas in a state in which 

 a current would be readily started in the 

 gas if any electro-motive force acted upon 

 it. JS'ow, though the metal window in Le- 

 nard's experiments was connected with the 

 earth, and would, therefore, screen off from 

 the outside of the tube any effect arising 

 from slow electrostatic changes in the tube, 

 it does not follow that it would be able to 

 screen off the electrostatic effect of charged 

 particles moving to and from the tube with 

 very great rapidity. For in order to screen 



