8 POPULAR SCIENCE MONTHLY. 



Observer. Value of Velocity. Value of e/m. 



Rutherford 2.5 X 10" cms. per sec. 6 X 10 3 electromagnetic units. 



Des Coudres 1.6 X 10° cms. per sec. 6 X 10 3 electromagnetic units. 



Now the value e/m for the hydrogen atom is 10*. On the as- 

 sumption that the a particle carries the same charge as the hydrogen 

 atom, this result shows that the apparent mass of the a particle is 

 about twice that of the hydrogen atom. If the a particle consists 

 of any known kind of matter, this result indicates that it is the atom 

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

 heavy bodies projected with great velocit}', whose mass is of the same 

 order of magnitude as the helium atom and at least 2,000 times as 

 great as the apparent mass of the /? particle or electron. 



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^ 

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

 substance from which they are projected acquires a negative charge. 

 The corresponding effect has been observed for the (3 rays. The 

 (3 particles from radium communicate a negative charge to the 

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

 emitted acquires a positive charge. This effect has been very stri- 

 kingly shown by a simple experiment of Strutt. The radium com- 

 pound, 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 enclosed 

 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 nega- 

 tively charged particles are projected through the glass tube contain- 

 ing 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 auto- 

 matic 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 

 number 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 ft 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 



