OUTLINE OF RADIOACTIVITY 115 



radium bromide, thus for the first time proving the actual derivation of 

 one element from another. Mrs Curie and Mr Dewar " made confirmatory 

 experiments on radium chloride fused and sealed in a quartz glass tube. 

 Helium was also shown by Mr Debierne " to be one of the products of 

 the activity of actinium. 



It was found that a series of different but analogous radioactive sub- 

 stances results from the breaking up of the radium atom, and the constant 

 association of radium with uranium suggested that radium itself forms at 

 the expense of uranium. These facts led Messrs Eu.therford and Soddy 

 in 1902 to their disintegration theory,* according to which "it is sup- 

 posed that the atoms of radioactive bodies are unstable, and that a certain 

 fixed proportion of them become unstable every second and break up with 

 explosive violence, accompanied in general by the expulsion of particles." 

 Of these the most important are called a particles and are regarded by 

 most investigators as molecules of helium. They are expelled at tremeti- 

 dous velocities, the maximum being approximately 20,000 kilometers per 

 second, about one-fifteenth of the velocity of light. Less important are 

 the so-called ^ particles, which are electrons and have only about a four- 

 thousandth part of the mass of the a particles. They are emitted only 

 by certain of the radioactive substances and have a velocity in some cases 

 approaching that of light, but in spite of their velocity they possess far 

 less energy than the helium atoms because of their minute mass. These 

 ;8 particles are identical with those which compose cathode rays. Of 

 relatively small importance are the y rays emitted by some radioactive 

 substances. They are believed to be irregular disturbances of the ether 

 identical in character with the Eoentgen rays. 



The heating effect produced by radioactive substances is almost entirely 

 due to the dissipation of the energy of the a particles moving at the 

 immense velocities already indicated. The excess of temperature of 

 radium compounds above that of surrounding objects is a consequence of 

 tlie arrest by other portions of the mass of a particles set free within the 

 salt. The energy liberated by radioactive processes is of a wholly differ- 

 ent order of magnitude from that of ordinary chemical reactions. Thus 

 the transformation of radium emanation is accompanied by nearly 4 

 million times as much heat as is given out by an equal vohune of hydrogen 

 and oxygen combining to form water. 



The disintegration theory implies that there is a long series of products 

 whose general formula might be written IT — n He, Avhere n is a whole 



8 Comptes Rendus, vol. 138, 1004, p. 190. 

 ' Comptes Rendns, vol. 141, 1!)05, p. 383. 

 * E. Rutherford : Radioactive transformation, 1906, p. 14, 



