Negatively Electrified Corpuscles^ by Hot Bodies. 259 



X 2 X 2 x 10~ 8 would be very small; so that the maximum 

 velocity acquired by the corpuscles would be very small com- 

 pared with the velocity of light. If, however, the sun gave 

 out Hertzian waves of considerable wave-length, these would 

 communicate to the corpuscles velocities comparable with the 

 velocity of light, so that the sun would appear to repel the 

 corpuscles with great vigour. Thus, for example, if a comet 

 by near approach to the sun got raised to such a high tempe- 

 rature that the corpuscles began to come off, these would be 

 repelled if any Hertzian waves came from the sun, and appear 

 behind the comet as a luminous tail. 



I now pass on to consider another result of the emission of 

 these negatively electrified corpuscles : we may regard these 

 corpuscles coming out of the metal as evidence for the exist- 

 ence in the metal itself of streams of corpuscles which move 

 freely between the molecules of the metal. Some of these 

 moving at more than a certain speed are able to escape from 

 the attraction of the metal, and produce the stream of nega- 

 tive electricity coming from the metal. These corpuscles 

 moving through the metal constitute streams of cathode-rays, 

 and when they come into collision with the molecules will 

 give rise to pulses of electric and magnetic force analogous 

 to those produced by the stoppage of cathode-rays in a 

 vacuum-tube ; inasmuch, however, as the velocity of the 

 corpuscles in a hot body is small compared with that of 

 cathode-rays in a vacuum-tube, the pulses produced by the 

 corpuscles will be very " soft " compared with the Rontgen 

 ravs produced in a vacuum-tube, i. e. the pulses produced in 

 the hot body are very much thicker than those produced in a 

 vacuum-tube. A succession of sufficiently broad pulses would, 

 however, on the electromagnetic theory of light, produce a 

 continuous spectrum of the kind given out by a hot body. 

 Part of the radiation from a hot metal might arise in this 

 way; and this part would have the characteristic property of 

 radiation from a solid of increasing very rapidly with the 

 temperature. For we may regard the corpuscles in the metal 

 as analogous to the molecules of a liquid, and the escape of 

 the corpuscles from the metal as analogous to the evaporation 

 of the liquid. The corpuscles are supposed to be attracted 

 by the metal ; so that it is only those escape from the surface 

 which start from near the surface and move so rapidly that 

 their velocity is sufficient to carry them beyond the region of 

 the attraction of the metal. Thus suppose that c is the distance 

 at which the attraction of the metal on the corpuscles is 

 appreciable — c is analogous to the range of molecular attraction 

 in Laplace's Theory of Capillarity — and consider a layer of 



S 2 



