CONSTITUTION OF MATTER BECQUEREL. 279 



In December, 1895, M. Jean Perrin succeeded in a fundamental 

 experiment. He demonstrated that the cathode rays carried nega- 

 tive electricity, and he charged with negative electricity an insulated 

 cylinder placed in a grounded metallic vessel. 1 



Let us consider now some other properties of cathode rays. If 

 they traverse an electric field — that is to say, if they are passed 

 between two electrified metallic plates, one positive and one nega- 

 tive, a pencil of these rays describes a parabola, as would be ex- 

 pected of a stream of corpuscles attracted by the positive plate and 

 repelled by the negative plate. 



If submitted to the influence of a magnet (magnetic field) the 

 pencil is curved and describes a helix around the lines of force. 



These two deviations, electric and magnetic, allow, as Sir J. J. 

 Thomson has shown, the measurement of the velocity of propaga- 

 tion of the corpuscles, as well as the relation between the electric 

 charge carried by a corpuscle and the mass of the corpuscle. Other 

 methods have also led to the estimation of the same quantities with 

 the following result : The velocity of the cathode corpuscles varies, 

 according to the conditions of the experiment, between 30.000 and 

 100,000 kilometers per second. The ratio of the mass to the charge 

 is 2,000 times as small as that of the hydrogen ion in electrolysis. 

 This ratio is always the same whatever the electrodes or the rarified 

 gas in the tube happen to be. Here is a first important experimental 

 result. • 



At the same time that these researches were being carried on, experi- 

 ments were being made on radioactive bodies, whose study has led to 

 conclusions of at least equal importance. 



You all know that certain bodies possess the property, discovered 

 in February, 1896, by Henri Becquerel, of spontaneously giving out 

 radiations of various sorts without any energy being furnished to 

 them. 



The electrified particles emanating from these bodies ionize the 

 air — that is to say, by tearing electrified corpuscles away from the 

 molecules of gas and surrounding these corpuscles with neutral 

 molecules, they cause the formation of electrical nuclei which render 

 the air electrically conductive.- It is on account of this fact that 

 these rays discharge electrified bodies. 



Of the three varieties of rays which emanate from radioactive 

 bodies, one group, the /? rays, are charged with negative electricity 

 and are formed of corpuscles identical with the cathode corpuscles, 

 which may be determined by measuring their deviation in an electric 

 field and in a magnetic field. Henri Becquerel has shown that 



1 Experiment : Deviation by a magnet of a sheaf of cathode rays determined by an 

 opening made in a screen placed a few centimeters in front of the cathode. 



2 Experiments : (1) Discharge of an electroscope by the radiations emanating from a 

 radium salt. (2) Increase of the discharge distance of a spark in the vicinity of radium. 



