191 1.] BOLTWOOD— RADIOACTIVITY. 335 



by the impact of a given number of alpha particles, the charge car- 

 ried by each particle could be readily determined. This was found 

 to be equal to 9.3 X lo"^'' electrostatic units. It was then shown in 

 an ingenious manner that this charge was twice that carried bv an 

 electron or by a hydrogen ion, although preceding determinations of 

 the latter magnitude had indicated that its value was somewhat less 

 than one half of 9.3 X io~^'^. The recent determinations made by Mil- 

 likam of the charge on an ion have shown, however, that 4.65 X iq-^" 

 is not far from correct and have confirmed the conclusion reached 

 by Rutherford and Geiger that the charge on an alpha particle is 

 equal to twice the unit charge of the hydrogen atom in electrolysis. 



\\'ith this modification, the ratio of the charge to the mass of an 

 alpha particle indicates that the mass is equivalent to an atomic 

 weight of four. This corresponds to the mass of an atom of the 

 gaseous element helium. 



The final proof of the intimate connection of the alpha particle 

 with the helium atom was supplied by Rutherford and Rovds who 

 proved by spectroscopic methods that readily detectible amounts of 

 helium could always be obtained when large numbers of alpha par- 

 ticles were allowed to penetrate through a thin glass wall into a 

 highly evacuated receptacle or into a screen of lead, from which the 

 helium was ultimately released by fusion of the metal. 



It is a significant fact that although the alpha particles from the 

 different types of radioactive matter appear to be all of a similar 

 nature and to consist of atoms of helium bearing a double, positive 

 charge, the velocities at which they are ejected are dift'erent for the 

 dift'erent radioactive substances. The velocity of the particles emitted 

 by the atoms of any one type of matter undergoing transformation 

 is, however, always the same and is a characteristic constant for that 

 particular radio-element. Attention was first called to this impor- 

 tant relation by Bragg and Kleeman and it is undoubtedly significant 

 in its bearing on the constitution of the radio-atoms. The observed 

 velocities of the particles appear to lie between the limits of 1.5 X 10^ 

 and 2.25 X 10^ centimeters per second. 



Owing to their high velocities the alpha particles are capable of 

 passing through thin layers of ordinary matter, and can penetrate 

 into air at atmospheric pressure for distances of from somewhat less 



