12 DISCOVERY AND NATURE OF RADIOACTIVITY 



parent of radium. All the a rays given off by the same product be- 

 have alike, but the particles from the different products are quantita- 

 tively unlike, though qualitatively similar. 



Phosphorescence : It was the association of phosphorescence 

 with X rays that led to the discovery of radioactivity. As might 

 be expected, radium is phosphorescent. The source of this light is 

 not definitely known, but experiments of Lord and Lady Huggins^" 

 seem to indicate that the luminosity is not due to the /9 rays. 



Measurement of Activity : Eve"^ has shown that the activity 

 of radium is a function of the amount present ; therefore the purer 

 the radium salt the greater the activity, weight for weight. The de- 

 gree of activity is stated in terms of that of uranium as a standard. 

 Thus, to indicate that a given preparation is of 10,000 activity 

 (10,000 X ), means that it is ten thousand times as active as an equal 

 weight of uranium. The purest radium bromide so far obtained has 

 an activity of 1,800,000. The activity of a given radium prepara- 

 tion enclosed in a sealed glass tube shows no signs of decreasing 

 with time. The intensity of activity, according to Rutherford,^"® 

 does not vary with the concentration of the salt. In his experi- 

 ments, " a distribution of the radiating matter over a thousand times 

 its original volume has no appreciable influence on its original 

 activity." 



Amount of Energy Evolved : The results of Curie and La- 

 borde,'^® confirmed by Runge and Precht,'** indicate that one gram of 

 radium emits energy at the rate of 100 gram-calories per hour, or 

 2,400 per day. Rutherford"^ has pointed out that this is nearly as 

 much energy as is required to dissociate one gram of water (3,900 

 gram-calories). Rutherford"^ has calculated that the total energy 

 of radiation during the disintegration of one gram of radium is 

 1.6 X 10^ gram-calories, and the energy radiated may not represent 

 all of the energy involved in the change. The energy of the trans- 

 formation is at least twenty thousand times, and may be a million 

 times as great as the energy of any molecular change, such, for 

 example, as that involved in the union of hydrogen and oxygen to 

 form water. Ramsay ^^ states that the emanation given off by one 

 gram of radium evolved 75 calories in one hour. 



In replying to expresssions of doubt as to the validity of the 

 foundations of the theory of atomic disintegration, Soddy ^^^ empha- 

 sized the fact that it is firmly grounded on experimental evidence, 



