280 CATHODE RAYS AND RONTGKN KAYb. 



rays. This fact Avas determined about the same time by Birkeland 

 and Jean Perrin. 



B}^ exceedingly clever experiments it has been possible to measure 

 the velocit}^ of propagation of the cathode rays, which is, according to 

 the emission theory, the true velocit}^ of the projectile thrown off b}^ 

 the electrode. This velocity is enormous and, moreover, varies greatly 

 according to the circumstances of its production. It ma}'^ be 200 kms. 

 a second, which is the lowest limit, and may reach 50,000 kms., which 

 seems to be the highest limit, or one-sixth the velocity of light. 



We can scarcely point out the principles by which this calculation 

 has been made. It is founded upon the experimental measurement of 

 the magnetic deflection exerted by a known magnet and by the elec- 

 tric deflection excited by an electric current having an intensit}" equally 

 known. It is very clear that these deflections depend upon the velocity 

 and the mass of the cathode projectiles. In short, it is evident that 

 the magnet or the current will deflect the cathode ray more if it travels 

 with a feeble velocit}'' and less if the velocitv is great. 



It is possible, moreover, to diminish this velocitj' in order to give 

 greater accuracy to the methods. Lenard made use for this purpose 

 not only of the rays produced in the Crookes tube but also of those the 

 existence of which had been discovered by Gustave Le Bon and which 

 result from the action of light on metals. 



The velocity of the cathode ray is prodigious and can produce 

 mechanical effects surpassing the imagination, if you consider that the 

 mass of the projectile is infinitely small and the projectile itself but 

 the fragment of an atom. Jean Perrin has calculated one of the 

 eff'ects, the calorific effect which will be produced by the blows of 

 an appreciable proportion of these projectiles. The quantity of 

 heat which a kilogram of this matter would generate, when suddenly 

 arrested b}' an obstacle in its course, would be sufficient to raise instantly 

 to the boiling point the water of a lake 1,000 hectares in extent and 5 

 meters in depth. 



The measurement of the cathode velocity brings to bear a final argu- 

 ment in favor of the ballistic or materialistic theory. If the cathode 

 were the result of certain vibrations of the ether, instead of resulting 

 from the projection of matter, it would not be possible to comprehend 

 that such a disturbance should be propagated with a variable velocit}' 

 of 200 kms., since the same medium transmits the solar disturbance 

 with a uniform velocit}^ of 300,000 kms. 



No matter from what side we study this question the advantage 

 always remains with the theory of material emission. In this discus- 

 sion which has been renewed in our time between the two systems of 

 emission and of undulations, this time it is the first that carries off the 

 palm. 



The cathode ra}' nui}' ))e considered, then, as formed of a row of 



