658 



SCIENCE 



[N. S. Vol. XLIV. No. 1141 



rays are a form of radiant energy similar 

 to X-rays. 



The beta rays consist of negatively 

 charged particles which are identical with 

 the particles of the cathode rays or elec- 

 trons. The beta particles are thrown off 

 with enormous velocities, almost equal in 

 some cases to the velocity of light. These 

 high velocities have made it possible to de- 

 termine more definitely the nature of the 

 mass of the electron. Kaufmann and 

 Bucherer have both determined the veloci- 

 ties and masses of beta particles moving 

 with different velocities. They found that 

 the mass increase is equal to the calculated 

 increase in the mass of a moving electric 

 charge. 



These results indicate clearly that the 

 mass of the electron is entirely electromag- 

 netic in its nature, and that the electron is 

 in reality a disembodied electric charge. 

 There is considerable experimental evi- 

 dence to show that all electric charges are 

 made up of some integral multiple of this 

 charge, and that all electric currents are 

 due to some kind of movements of the elec- 

 trons. If we substitute the values of e and 

 m in the equation m = %(e 2 /a), we obtain 

 2 X 10" 13 centimeters as the value of a 

 which represents the radius of the sphere 

 of action of the electron. This value prob- 

 ably does not exceed %oooo °f the diameter 

 of the atom. 



The alpha rays from radioactive sub- 

 stances also consist of particles, but of an 

 entirely different nature from that of the 

 beta particles. They are slightly deflected 

 by a magnet, and in the opposite direction 

 from that of the beta particles, thus show- 

 ing them to be positively charged. The 

 ratio of the charge to the mass, that is e/m, 

 has been determined and found to be about 

 4,820, which is one half the value of e/m 

 for the hydrogen ion. The charge carried 

 by the alpha particle is equal to about 

 9.3 X 10 -10 electrostatic units, which is 



twice the charge of the hydrogen ion. 

 From the foregoing relations, it is evident 

 that the mass of the alpha particle is equal 

 to four times that of the hydrogen ion, or 

 approximately equal to the mass of the 

 helium atom. Rutherford and Royd deter- 

 mined experimentally that, when the charge 

 is neutralized by the surrounding matter, 

 the alpha particle becomes a neutral atom 

 of helium. 



The counting of the alpha particles has 

 confirmed in a remarkable manner previ- 

 ous estimates of the number of atoms and 

 molecules in a given quantity of matter. 

 This difficult experiment was performed by 

 Rutherford and Geiger, 2 who constructed 

 an apparatus which would automatically 

 magnify several thousand times the elec- 

 trical effect of individual alpha particles. 

 They found that one gram of radium emits 

 3.4 X10 10 , or 34 billion alpha particles 

 per second, and that one gram of radium 

 in equilibrium with its products emits 

 1.36 X 10 u alpha particles per second. 



Various methods have been employed to 

 calculate the number of atoms and mol- 

 ecules in one cubic centimeter of gas. Prob- 

 ably the most reliable estimates are those 

 based on Millikan's 3 determination of the 

 magnitude of the atomic charge. As al- 

 ready indicated, this value is 1.59 X 10" 20 

 electromagnetic units. It is well known 

 that one electromagnetic unit of charge lib- 

 erates 1.1657 cubic centimeters of hydrogen 

 gas, at standard conditions of temperature 

 and pressure. Knowing the amount of 

 charge or current required to set free one 

 atom, and that required to set free a 

 known volume of hydrogen, it is a simple 

 matter to calculate the number of atoms in 

 one cubic centimeter. In this way it has 

 been estimated that one cubic centimeter 

 of hydrogen under standard conditions con- 



2 Pros. Boy. Soc, A, 81, p. 141, 1908. 



3 Phys. Bev., 32, p. 349, 1911. 



