334 POPULAR SCIENCE MONTHLY 



a grand piano seems simple in comparison with the iron atom. But 

 spectroscopic evidence does not end here, but indicates what it is in the 

 atom which does something and how it does it. 



Ten years ago Professor Zeeman placed a sodium flame between the 

 poles of a powerful electro-magnet and examined its light by the spec- 

 troscope. He observed the most striking and peculiar effects of the 

 magnetic force on the character of the light. The time is too far gone 

 to permit a description of what the effects were, but the light sent out 

 by the flame showed exactly the characteristics which magnetic force 

 would produce, provided the light came from atoms inside which 

 minute electric charges were rapidly revolving. It was even possible 

 to compute the ratio of charge to mass for these revolving mites. The 

 ratio revealed was that previously obtained for the cathode particle. 



Hence the mechanism which enables the material atom to emit light 

 may be the same electron we met flying through the vacuum tube, now 

 revolving in an orbit about the atom center as a planet revolves about 

 the sun. Thus the chief difference between the atoms of one chemical 

 element and those of another, may lie in the number and arrangement 

 of electrons in a revolving system. 



It had long been known that hints about the internal fabric of the 

 atom would be most effectively sought with the spectroscope, but we 

 have here gained at a single bound the most amazing insight into a 

 most complex system. Here also we meet another of those astonishing 

 previsions of Faraday. He tried Zeeman's experiment over fifty years 

 ago, but was balked in his quest by the inadequacy of the instrumental 

 equipment of his day. 



The quite recent discovery of the wholly new and unsuspected prop- 

 erty of radio-activity in a group of heavy elements has done much to 

 confirm the views already expressed of the connection between elec- 

 tricity and matter, and much more, for radio-active phenomena suggest 

 for the first time that some kinds of matter are not only unstable, but 

 mutable. 



Taking radium as the most highly developed example of its class, 

 we find it, with the help of its numerous progeny, sending out three 

 distinct types of rays, which for convenience of classification have been 

 called a-, /?- and y-rays. 



a-rays closely resemble canal rays. They carry positive electric 

 charges and possess a mass or inertia comparable with that of the 

 helium or hydrogen atom. 



/?-rays appear identical with cathode rays. They consist of nega- 

 tive electrons hurled out at speeds as great as nine tenths the velocity 

 of light. 



y-rays are of the nature of X-rays — a purely ethereal phenomenon. 

 All these rays penetrate matter to varying depths, and absorption varies 

 with density as in cathode rays. 



