280 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1910. 



as long as their velocity does not approach too closel^y the velocity 

 of light, the value of the ratio of the charge to the mass of each 

 corpuscle is just the same as that of the cathode rays. 



Again, these same corpuscles are given out by incandescent metals, 

 and liberated by the action of ultra-violet light or of the X rays 

 on metals. In all these manifestations of the phenomenon the same 

 ratio of charge to mass is found. 



I can not describe here the remarkable methods due to the work of 

 Messrs. J. J. Thomson, Townsend, and H. A. Wilson, which have 

 made it possible to measure the charge of a cathode corpuscle; I 

 will only indicate the principle on which they are based. When 

 ions are formed in the air — Ave now call all electrified centers ions — 

 they condense water vapor out of a supersaturated atmosphere 

 and each electrified particle forms the nucleus of a particle of con- 

 densed water.^ The velocity of the fall of these particles allows the 

 calculation of their size, and by evaluating the total quantity of con- 

 densed water the number of particles can thus be determined and 

 therefore the number of ions. Furthermore the quantity of elec- 

 tricity carried down by the mist formed can be measured and from 

 that the charge of each ion can be calculated, since their number is 

 known. 



The result is a fundamental one; cathode corpuscles and gaseous 

 ions carry the same charge as an atom of hydrogen in electrolysis, 

 and yet their mass is two thousand times as small as that of an atom 

 of hydrogen. 



As a necessary conclusion, therefore, it follows that the cathode 

 corpuscles^ the {3 'rays, carry an atom of negative electricity and 

 ■possess a mass two thousand times as small as the lightest of h-nown 

 material atoms. 



We have just seen how our knowledge of negative electrons is a 

 result of the study of electric phenomena and of radioactivity. In 

 an entirely different branch of physics, however, in the field of 

 optics, the theory of electrons has found an extremely remarkable 

 confirmation. 



The theory of Fresnel and the results derived from the experi- 

 ments of Foucault and Young have established the fact that light 

 is a vibratory movement, and that consequently there exists a 

 medium of such a nature as to transmit the luminous waves. This 

 medium has been called ether. It is known by the properties of the 

 movements Avhich are capable of being produced and propagated in 

 it; it exists everywhere — in the interior of matter as Avell as in spaces 

 free from matter, such as a A^acuum. 



^ Experiment : A spark is discharged in the inside of a tube to produce ions ; as soon 

 as the air is cooled by expansion so that it is supersaturated, a heavy mist forms about 

 the ions. 



