Boltwood — Life of Radium. 49^ 



Art. LI. — On, the Life of Radium; by Bertram B. 



• BoLTWOOD. 



[Contributions from the Sloane Physical Laboratory of Yale University.] 



The theory proposed by Rutherford and Soddy to explain 

 the behavior of radio-active substances assumes that the phe- 

 nomena of radio-activity are accompanied by the disintegration 

 of the atoms of the radio-elements and the production of 

 atoms of other elements having distinctive physical and chem- 

 ical properties. On the basis of this theory it is necessary to 

 conclude that in any salt of radium a certain proportion of the 

 total number of radium atoms present are constantly being 

 transformed into atoms of other substances. 



A number of estimates of the approximate order of magni- 

 tude of the change occurring in radium have been made by 

 Rutherford. From the heating effect observed in radium salts 

 he at first calculated that the half-value period of radium, i.e., 

 the time required for exactly half of a given quantity of radium 

 to disintegrate into other substances, was about 800 years,* 

 but very shortly afterward he decided upon 1500 yearsf as a 

 more probable value. 



Later, he was able to determine;}: the quantity of positive 

 electricity communicated to an insulated conductor by the a 

 particles from a known weight of radium bromide in the form 

 of a thin film and in this way obtained further data on which 

 a calculation of the life of radium could be based. Thus, for 

 example, when the film contained 0'484 mg of radium bromide, 

 the current due to the charge carried by the a particles was 

 found to be 9*8 XlO -13 ampere. This corresponded to 

 2*03 XlO -9 ampere per gram of radium bromide and, since at 

 most not more than half of the a particles escaped (the rest 

 being absorbed in the plate supporting the film), the total 

 charge carried per second by the a particles from one gram of 

 radium bromide was equal to 4*07 XlO -9 ampere. Assuming 

 that the charge carried by each a particle was the same as the 

 charge on an ion, viz., 1*13 XlO -19 coulomb, the total number 

 of a particles emitted per second by one gram of radium 

 bromide was given as 3*6xl0 10 . Taking the composition of 

 the radium salt used to be RaBr„, containing 58 per cent of 

 pure radium, it followed that the total number of a par- 

 ticles expelled per second from one gram of radium was 

 6*2 XlO 10 . This number was for radium itself, that is for 

 radium free from other products (Em, Ra A, etc.). If it is 



* Bakerian Lecture, Phil. Trans. Roy. Soc, cciv, 169, 1904. 

 + Radio-activity, 1st edition, p. 333, 1904. 

 {Phil. Mag., x, 193, 1905. 



Am. Jour. Sci.— Fourth Series, Vol. XXV, No. 150.— June, 1908. 

 33 



