852 



SCIENCE 



[2Sr. S, Vol. XLII. No. 1094 



coneentration, to permit its being obtained 

 in reasonable quantities in an isolated and 

 purified condition. For this reason radium 

 is considered and accepted as a standard 

 or typical radioactive substance, and its 

 physical and chemical properties, including 

 the value of its atomic weight, are known 

 with a considerable degree of precision. 

 For some time in the future, therefore, 

 radium will occupy this position of relative 

 importance and will serve as the basis for 

 calculation and comparison with other 

 radio-elements possessing less striking chem- 

 ical individualitj^ 



An accurate knowledge of the life of 

 radium is also important in the field of 

 geology, because of a method which is 

 available for estimating the geological antiq- 

 uity of some of the older rocks and minerals. 

 This method is dependent on the determina- 

 tion of the progress of the radioactive dis- 

 integration which has taken place in those 

 minerals containing appreciable propor- 

 tions of uranium. For the accurate calcu- 

 lation of these important magnitudes an 

 exact knowledge of the rate of disintegra- 

 tion of radium is essential. 



It is possible, moreover, to obtain an esti- 

 mate of the probable life of radium by a 

 calculation involving as its basis a number 

 of other important physical constants. 

 These con.stants will be referred to more 

 specifically later. If a knowledge of the 

 life of radium can be ari-ived at by experi- 

 mental methods not directly involving these 

 constants, then, if the results given by the 

 different methods are iu good agreement, 

 there is good reason for assuming that the 

 accepted values for these constants are not 

 very different from the true values. 



The disintegration of radioactive sub- 

 stances is of such a character that the trans- 

 formation of the substance into other ele- 

 ments can be expressed by a law in which 

 the rate of transformation is an exponential 



function of the time. The rate of trans- 

 formation is independent of the amount of 

 material undergoing disintegration and is 

 independent of the temperature, the pres- 

 sure or of any other external condition to 

 which we can subject the radioactive sub- 

 stance. It proceeds in such a manner that 

 if half of the material present is trans- 

 formed in a given period of time, half the 

 remaining quantity wiU be transformed in 

 a subsequent time of equal duration, and 

 half the amount still left will undergo 

 change in the third equal interval. This 

 will continue indefinitely until the amount 

 remaining will be too small to merit con- 

 sideration. Since under these conditions 

 some of the atoms of the radio-element will 

 have an inappreciably short life, whUe 

 others will have an inconceivably long one, 

 it is impossible to attach any special signif- 

 icance to the term "life of " a radio-element 

 except under certain definite restrictions. 

 The life of a radio-element may therefore 

 be somewhat dogmatically expressed in 

 terms of the time required for exactly one 

 half of it to be transformed into other sub- 

 stances. This constant is known as its 

 "half -value period," and it is the half- 

 value period of radium which particularly 

 concerns us at the present moment. 



The first estimate of the probable life of 

 radium was published by Sir Ernest 

 Rutherford in the first edition of his text- 

 book "Radio-activity (Cambridge, 1904). 

 In the disintegration theory proposed by 

 Rutherford and Soddy the assumption was 

 made that the expulsion of a-particles by 

 radium and other radio-active substances 

 was coincident with the changes taking 

 place on the disruption of the atoms, 

 namely, that the appearance of the a-par- 

 ticles was indicative of the simultaneous 

 breaking up of the atoms of the radio- 

 element. Rutherford further postulated 

 the theory that each a-particle had its 



