202 SCIENCE AND METHOD. 



of light, while Hertz held to the undulatory theory. 

 The facts seemed to be in favour of Crookes. 



It was recognized in the first place that the cathode 

 rays carry with them a negative electric charge : they 

 are deviated by a magnetic and by an electric field, 

 and these deviations are precisely what would be pro- 

 duced by these same fields upon projectiles animated 

 with a very great velocity, and highly charged with 

 negative electricity. These two deviations depend 

 upon two quantities ; the velocity on the one hand, 

 and the proportion of the projectile's electric charge to 

 its mass on the other. We cannot know the absolute 

 value of this mass, nor that of the charge, but only 

 their proportion. It is clear in fact, that if we double 

 both the charge and the mass, without changing the 

 velocity, we shall double the force that tends to deviate 

 the projectile ; but as its mass is similarly doubled, 

 the observable acceleration and deviation will not be 

 changed. Observation of the two deviations will 

 accordingly furnish us with two equations for deter- 

 mining these two unknown quantities. We find a 

 velocity of 6,000 to 20,000 miles a second. As for 

 the proportion of the charge to the mass, it is very 

 great ; it may be compared with the corresponding 

 proportion in the case of a hydrogen ion in electro- 

 lysis, and we find then that a cathode projectile 

 carries with it about a thousand times as much 

 electricity as an equal mass of hydrogen in an 

 electrol}'te. 



In order to confirm these views, we should require a 

 direct measure of this velocity, that could then be 

 compared with the velocity so calculated. Some old 

 experiments of Sir J. J. Thomson's had given results 



