DISCOVERY AND NATURE OF RADIOACTIVITY 1 3 



that no fact has been adduced that does not conform to it, and adds 

 that it is impossible to form " from words or reading the least idea of 

 the really startling character of some of the new discoveries." 



Effect of External Conditions : As might be inferred from 

 the fact that radioactivity is neither a molecular nor an atomic, but a 

 sub-atomic change, external conditions have no effect upon it. It is 

 not affected by temperatures ranging from 200" C. to that of liquid 

 air, nor by variations in atmospheric pressure. The emanation, of 

 course, being a gas, diffuses from the radioactive body more slowly 

 under increased pressure and lower temperature, and the rate of 

 escape is greater in moist air, and when the radioactive salt is in 

 solution, but its radioactivity is not thereby affected in the least.* 



Excited Radioactivity : It has been found that bodies under 

 certain conditions of exposure to radioactive substances become 

 themselves radioactive. Radioactivity thus produced is called ex- 

 cited radioactivity. Rutherford ^"^^ demonstrated that the presence of 

 the emanation is necessary in order to produce excited radioactivity, 

 and later (1903) showed that the excited radioactivity is due to a 

 deposit of radioactive matter (called the "active deposit") from the 

 emanation of either thorium or radium. 



Origin of Radium : The observation of Lord and Lady Hug- 

 gins ^^ of the gradual appearance of the spectrum lines of helium in 

 the spectrum of radium is the first demonstration in the history of 

 science of the origin of one chemical element from another. Ramsay 

 and Soddy ^^ in the same month (July, 1903) announced their obser- 

 vation that gases occluded by 20 mg. of radium bromide contain 

 helium, and soon thereafter they ^^ observed the spectrum lines of 

 helium gradually appear in the spectrum of the radium emanation 

 after it has stood for four days, thus confirming the results of Lord 

 and Lady Huggins. Further confirmation of this transformation was 

 published by Dewar and Curie, ^■' and by Himstedt and Meyer."- 



The observation of the actual transmutation of one element into 

 another suggests the question as to the origin of radium. In 1903 

 Rutherford and Soddy ^-^ suggested that radium is a disintegration 

 product of one of the other radioactive substances found in pitch- 

 blende. Soddy ^^^ in 1904 expressed his belief that uranium is the 

 source of radium, and in 1905 obtained'^" experimental evidence that 



♦Makower" states that the activity of the radium emanation measured by the rays 

 it gives off can be changed by high temperature, but this has not yet been confirmed. 



