8 DISCOVERY AND NATURE OF RADIOACTIVITY 



X Rays from Radium ; Gamma Rays : It was Villard •^''' ^" who 

 discovered that radium, besides emitting a and ^ rays, is the source 

 also of a non-deviable ray, analogous to very penetrating or " hard " 

 X rays. Becquerel,'" Strutt,^^^ and Eve ^^ also identified the very 

 penetrating, non-deviable rays. Now a type of X ray arises whenever 

 a ^ particle is either started or stopped, and Strutt considered that 

 the non-deviable rays from radium arise secondarily by the self-bom- 

 bardment of the radium by the /? particles. Ashworth,^ on the other 

 hand, supported the theory that these rays were not of a secondary 

 nature, but resulted directly from the disintegration of the radium- 

 atom. Finally Rutherford '"^ showed that this " hard " type of X ray 

 (a narrow electro-magnetic pulse) arises from radium by atomic dis- 

 integration, while a " soft " type of X ray (a broad electro-magnetic 

 pulse) arise at the points where the /3 particles strike another body. 

 The non-deviable rays were named by Rutherford y rays. 



The Emanation : In addition to the giving off of three types of 

 rays as above indicated, it was found by Rutherford '"^ that a radio- 

 active gas diffuses from thorium, and in the same year Dorn ^" deter- 

 mined the same fact to be true of radium. To this heavy, radio- 

 active gas Rutherford gave the term emanation* Its gaseous nature 

 was confirmed by Rutherford and Brooks.*^"- ^^^ The emanation was 

 found to condense at — 150° C.,^" and in 1904 its spectrum was 

 mapped by Ramsay and Collie. ^'^ Ramsay,''^ also, gave further evi- 

 dence that the emanation is a gas by showing that it obeys Boyle's 

 law. About three fourths of the activity of radium, according to 

 Rutherford,^"' is due to the emanation. 



The Nature of Radioactivity: In brief, then, we know that 

 certain elements of very high atomic weight are giving rise, spon- 

 taneously, to three types of mvisible radiation, as follows : 



I. A stream of positively charged particles, with slight pene- 

 trating power, with a mass twice that of an atom of hydrogen, and 

 moving with about one tenth the velocity of light. Streams of these 

 particles constitute the a. rays. The u. rays consist of " veritable 

 atoms of matter projected at a speed, on an average, of 6,000 miles 



* Sir George Stokes proposed to Crookes^^ a systematic nomenclature in radiology 

 as follows: "Ray — A disturbance propagated in the ether. Jet — A discharge of 

 electrons. Emanation — To include both Rays and Jets." The distinction between 

 the first two terms has obvious advantages, and is used by Crookes in the paper above 

 cited. The term emanation, however, is now firmly established as referring to the 

 radioactive gas. 



