98 THE EVOLUTION OF MATTER 



far the most energetic and important of all, the 

 second capable of penetrating perhaps &th of an inch 

 of glass or aluminium without being totally stopped, 

 and the third reduced to half their original intensity 

 by about inch of lead, though not absolutely com- 

 pletely stopped even by 20 inches. The y-rays are 

 far the most penetrating rays known and are really 

 X-rays, but far more penetrating than any that can 

 be artificially produced. They are light waves of 

 wave-length thousands of times shorter than those 

 of visible light, and are probably a secondary pheno- 

 menon accompanying the expulsion of the /3-rays. 

 The /3-rays, or /3-particles, are electrons the atoms 

 of negative electricity divorced from matter, recog- 

 nised as such by Sir J. J. Thomson in 1897, but 

 previously well-known in the phenomena of the 

 Crookes' tube. They travel at a speed varying from 

 a third up to nearly the velocity of light itself, which 

 is very much greater than any that can be produced 

 artificially. The a-rays, or a-particles, are atoms of 

 matter, carrying two atomic charges of positive 

 electricity just twice the charge of positive electricity 

 that the /3-particles carry of negative electricity and 

 travelling with a velocity varying from ^Vth to Ath 

 that of light, about a hundred times faster than 

 matter had ever been known to travel previously. 

 Their mass is several thousand times as great as 

 that of the /3-particle, and in spite of their feeble 

 penetrative power, and, at first sight, less showy 

 qualities, over 90 per cent, of the energy evolved in 

 the change of an atom is emitted in the form of these 

 a-particles. Much of Rutherford's finest work has 

 been in connection with these a-particles. 



The early measurements of the mass of the atom 

 constituting the a-particle left a choice as to its 

 nature, whether it was an atom of helium or of 

 hydrogen, but strong indirect evidence of a very 



