160 PHYSICS OF THE ELECTRON 



of hydrogen or of helium. The a particles thus consist of heavy 

 bodies projected with great velocity, whose mass is of the same order 

 of magnitude as the helium atom and at least 2000 times as great as 

 the apparent mass of the /? particle. 



If the a particles carry a positive charge, it is to be expected that the 

 particles, falling on a body of sufficient thickness to absorb them, 

 should under suitable conditions give it a positive charge, while the 

 substance from which they are projected should acquire a negative 

 charge. The corresponding effect has been observed for the /? rays. 

 The /? particles from radium communicate a negative charge to the 

 body on which they fall, while the radium from which they are emit- 

 ted acquires a positive charge. This effect has been very strikingly 

 shown by a simple experiment of Strutt. The radium compound , sealed 

 in a small glass tube, the outer surface of which is made conducting, 

 is insulated by a quartz rod. A simple gold-leaf electroscope is 

 attached to the bottom of the glass tube, in order to indicate the 

 presence of a charge. The whole apparatus is inclosed in a glass 

 vessel, which is exhausted to a high vacuum, in order to reduce the 

 loss of charge in consequence of the ionization of the gas by the rays. 

 Using a few milligrams of radium bromide, the gold leaf diverges to 

 its full extent in a few minutes and shows a positive charge. The 

 explanation is simple. A large proportion of the negatively charged 

 particles are projected through the glass tube containing the radium, 

 and a positive charge is left behind. By allowing the gold leaf, when 

 extended, to touch a conductor connected to earth, the gradual 

 divergence of the leaves and their collapse becomes automatic, and 

 will continue, if not indefinitely, at any rate for as long a time as the 

 radium lasts. 



When the radium is exposed under similar conditions, but un- 

 screened in order to allow the a particles to escape, no such charging 

 action is observed. This is not due to the equality between the num- 

 ber of positively and negatively charged particles expelled from the 

 radium, for no effect is observed when the radium is temporarily 

 freed from its power of emitting /? rays by driving off the emanation 

 by heat. The writer recently attempted to detect the charge carried 

 by the a rays from radium by allowing them to fall on an insulated 

 plate in a vacuum, but no appreciable charging was observed. The 

 /? rays were temporarily got rid of by heating the radium in order to 

 drive off its emanation. There was found to be a strong surface ion- 

 ization set up at the surface from which the rays emerged and the 

 surface on which they impinged. The presence of this ionization causes 

 the upper plate to lose rapidly a charge communicated to it. Although 

 this action would mask to some extent the effect to be looked for, a 

 measurable effect should have been obtained under the experimental 

 conditions, if the a rays were expelled with a positive charge; but not 



