RADIO-ACTIVITY 197 



from radio-active minerals has an atomic weigfht 

 appreciably different from that of other lead. To 

 such elements with different atomic weiofhts but 

 identical chemical properties, he gave the name of 

 isotopes. We have seen in Chapter VI. how Aston 

 has discovered many other isotopes by a quite 

 different method. 



It is now time to put together the complete 

 pedigree of the radium family as investigated by 

 our radio-active genealogists. No place is found 

 for thorium and its derivatives. They seem to 

 form a separate and independent radio-active 

 family. Another radio-active family of some ten 

 generations is that of actinium. It is probable 

 that this is a collateral branch of the radium family 

 derived from uranium Y. 



Such is the theory of radio-activity indicated 

 by the remarkable series of investigations that 

 have followed Becquerel's original discovery. We 

 are led to refer the energy liberated to transforma- 

 tions in the chemical atoms, and to recognise 

 clearly, what has long been suspected, that the 

 store of energy in the atoms themselves enor- 

 mously transcends the energy involved in ordinary 

 physical or chemical changes, in which the atoms 

 suffer no alteration. This internal atomic energy, 

 then, must be looked on as the source of the heat 

 detected experimentally by Curie in the neighbour- 

 hood of a radium compound. Its immediate cause 

 may be, partly at least, the internal bombardment 

 of the a particles, which, shot off by the radium 

 and the emanation stored in it, are for the most 

 part absorbed by the substance itself Rutherford 

 has traced the increase of the heat effect in radium 



