236 BODIES SMALLER THAN ATOMS 



portional to the square of the strength, and thus, in this case, propor- 

 tional to the square of the velocity of the corpuscle. 



Thus, if - is the electric charge on the corpuscle and v its velocity, 

 there will be in the region round the corpuscle an amount of energy 

 equal to \ l> <'■>•"■ where is a constant which .depends upon the shape 

 and size of the corpuscle. Again, if m is the mass of the corpuscle its 

 kinetic energy is \ mv 2 , and thus the total energy due to the moving 

 electrified corpuscle is \{m | ■'-)<•'■. so that for the same velocity it 

 has the same kinetic energy as a nonelectrified body whose mass is 

 greater than that of the electrified body by /J, ''. Tims, a charged body 

 possesses in virtue of its charge, as I showed twenty years ago, an 

 apparent mass apart from that arising from the ordinary matter in 

 the body. Thiis. in the case of these corpuscles, part of their mass is 

 undoubtedly due to their electrification, and the question arises whether 

 or not the whole of their mass can be accounted for in this way. 1 

 have recently made some experiments which were intended to test this 

 point: the principle underlying these experiments was as follows: If 

 the ma-- of the corpuscle is the ordinary "mechanical" mass, then, if 

 a rapidly moving corpuscle is brought to rest by colliding with a solid 

 obstacle, its kinetic energy being resident in the corpuscle will be 

 spent in heating up the molecules of the obstacle in the neighborhood 

 of the place of collision, and we should expect the mechanical equiva- 

 lent of the he;it produced in the obstacle to be equal to the kinetic 

 energy of the corpuscle. If, on the other hand, the mass of the cor- 

 puscle i.N •■ electrical," then the kinetic energy is not in the corpuscle 

 itself, but in the medium around it. and. when the corpuscle is stopped, 

 the energy travels outward into space a- a pulse confined to a thin shell 

 traveling with the velocity of light. I suggested some time ago that 

 this pulse forms the Rontgen rays which are produced when the cor- 

 puscles strike against an obstacle. On this view, the first effect of the 

 collision is to produce Rontgen rays, and thus, unless the obstacle 

 against which the corpuscle strikes absorbs all these rays, the energy 

 of the heat developed in the obstacle will be less than the energy of 

 the corpuscle. Thus, on the view that the mass of the corpuscle is 

 wholly or mainly electrical in its origin, we should expect the heating 

 effect to be smaller when the corpuscles strike against a target per- 

 meable by the Rontgen rays given out by the tube in which the cor- 

 puscles are produced than when they strike against a target opaque to 

 these rays. I have tested the beating effects produced in permeable 

 and opaque targets, but bave never been able to get evidence of any 

 considerable difference bet ween the two cases. The differences actually 

 observed were small compared with the total effect and were some- 

 times in one direction and sometimes in the opposite. The experi- 

 ments, therefore, tell against the view that the whole of the mass of a 

 corpuscle is due to its electrical charge. The idea that mass in gen- 



