JlLY 10, 1003.] 



SCIENCE. 



43 



Beeqiierel rays, some i)f which produce 

 stroug photographic effects, while others, 

 not so stroug photographically, produce in- 

 tense electrical effects. More recently it 

 has been found uot only that the rays 

 from different substances differ, but that 

 a single substance sends out rays of M'idely 

 different properties. Three types of 

 Beequerel rays have thus far been recog- 

 nized. An active substance in general 

 sends out all three kinds, but the distribu- 

 tion of the radiation among the dift'erent 

 tjT)es depends on the substance. For con- 

 venience these rays have been referred to 

 as the a, ,? and y rays. A brief statement 

 of more impoi-tant properties of each kind 

 of ray is given in the accompanying table. 



DIFFERENT TYPES OF BAYS. 

 a liai/s. 



Readily absorbed (f. g., by a thin sheet of 

 aluminium foil, or even by a few centimeters 

 of air). 



Relatively strong electrically, i. c, in making 

 gases conducting. 



Photographic effect small. 



Behavior in an electric field and in magnetic 

 field such as to indicate that these rays are 

 positively charged particles of molecular dimen- 

 sions moving at a speed of about 10° cm./sec. 



,? Raj/s. 



Quite penetrating (c. g., pass through several 

 millimeters of aluminium or glass). 



Electrical effects weak. 



Photographic effects relatively strong. 



Carry a negative charge. 



Probably consist of negatively charged part- 

 icles, much smaller than atoms, moving at a 

 speed nearly equal to the speed of light, i. e., 

 3.10'° cm./sec. Behavior in electric and magnetic 

 field consistent with this view. 



y Rays. 

 Highly penetrating. Pass through several 

 centimeters of metal. 

 Probably Roentgen rays. 



The existence of these different kinds of 

 rays may be proved ^nd their, separation 

 may be effected in a number of different 



wa.ys. The most obvious way is by means 

 of absorption. For example, a layer of 

 aluminium foil will absorb practically all 

 of the a rays, while it permits tlie /5 rays 

 to pass with scarcely any diminution in 

 intensity. To separate the (5 rays from 

 the r rays by absorption is more difficult, 

 for both of these rays are highly penetra- 

 ting. Separation may here be effected, 

 however, by passing the raj's through a 

 magnetic field, since the yS rays are de- 

 flected, while the r raj's are not. 



Let us consider first the /S rays. Prac- 

 tically all physicists now agree in regard- 

 ing these rays as consisting of very 

 .small negatively charged particles, or 

 electrons, moving at great speed. That 

 they carry a negative charge has been 

 shown by direct experiment. It is also an 

 experimental fact that these rays are de- 

 viated in a magnetic field, and in an elec- 

 tric field in the manner that would be 

 expected if they were charged particles in 

 motion. But the quantitative relations 

 are such as to indicate that if this hjT)oth- 

 esis is correct the ma.ss of each particle 

 must be much less than the mass of the 

 smallest atom, while the speed of the par- 

 ticles must be nearly the speed of light. 

 Each of these statements seems so incred- 

 ible and revolutionary^ that it is difficult 

 to accept the hypothesis, even in spite of 

 its complete agreement with experiment at 

 every point where a test can be applied. 

 I think that the difficulty in accepting this 

 hypothesis is probably greater in the minds 

 of chemists than it is with physicists. The 

 battle over the electron theory in its purely 

 physical aspects had already been fought 

 out during the development of the theorj- 

 of kathode rays.* The behavior of the /3 

 rays is so similar to that of kathode rays, 



* The development of this subject has been 

 traced by the writer in an article on ' Kathode 

 Rays and some Related Phenomena,' .Science, Vol. 

 Xli., p. 41, 1900. 



