A CENTURY 'S PROGRESS IN PHYSICS 359 



eyes a continual transformation of one element into 

 another, a change, by the way, which appears to be in no 

 slightest degree either hastened or delayed by changes in 

 temperature (H. L. Bronson, 20, 60, 1905) or external 

 electrical condition of the radioactive element. Uranium 

 is the progenitor of a long line of descendants, of which 

 radium was supposed for some time to be the first mem- 

 ber. Boltwood (25, 365, 1908) of Yale, however, showed 

 that the slow growth of radium in uranium solutions is 

 incompatible with this assumption, and soon isolated an 

 intermediate product which he named ionium. Radium 

 itself disintegrates into a gas known as radium emana- 

 tion, which in turn gives rise to a succession of other 

 products. Analyses by Boltwood (23, 77, 1907) of radio- 

 active minerals from the same locality show such a con- 

 stant ratio between the amounts of uranium and lead 

 present that it is natural to conclude that lead is the end 

 product of the series. This hypothesis is confirmed by 

 the fact that the oldest rocks show relatively the greatest 

 amounts of this element. 



In addition to the Ionium-Radium series two others 

 have been discovered. Of these Boltwood's (25, 269, 1908) 

 investigations seem to indicate that the one which starts 

 with actinium is a collateral branch of the radium series 

 and comes from the same parent uranium. The other 

 begins with thorium and comprises ten members. As 

 yet the end products of the actinium and thorium series 

 have not been identified, although there is some reason 

 for believing that an isotope of lead may be the final 

 member of the latter. 



As the amount of a radioactive element which disin- 

 tegrates in a given time is proportional to the total mass 

 present, an infinite time would be required for the sub- 

 stance to be completely transformed. Hence the life of 

 such an element is measured by the half value period, or 

 time taken for half the initial mass to disintegrate. 

 This time varies widely for different radioactive sub- 

 stances, ranging from a small fraction of a second for 

 actinium A to five billion years for uranium. Bolt- 

 wood's (25, 493, 1908) original determination of the life 

 of radium from the rate of its growth in a solution con- 

 taining ionium gave 2000 years as its result, although 



