328 POPULAR SCIENCE MONTHLY. 



neutral state; positive electrification, on the other hand, since it in- 

 volves the absence of corpuscles, is accompanied by a diminution in 

 mass. 



An interesting question arises as to the nature of the mass of 

 these corpuscles which we may illustrate in the following way. When 

 a charged corpuscle is moving, it produces in the region around it a 

 magnetic field whose strength is proportional to the velocity of the 

 corjjusde ; now in a magnetic field there is an amount of energy pro- 

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

 portional to the square of the velocity of the corpuscle. 



Thus jf e 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 ^ /J e^v"^ 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 %mt'^ and thus the total energy due to the moving 

 electrified corpuscle is %(m -|-/5 e2)i;2^ so that for the same velocity 

 it has the same kinetic energy as a non-electrified body whose mass is 

 greater than that of the electrified body by /3e-. Thus 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. Thus 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. I 

 have recently made some experiments which were intended to test this 

 point; the principle underlying these experiments was as follows: if 

 the mass 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 heat 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 is '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 outwards into space as a pulse confined to a thin shell 

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

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

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

 collision is to produce Eontgen 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 

 efl'ect to be smaller when the corpuscles strike against a target per- 



