854 



SCIENCE 



[N. S, Vol. XLII. No. 1094 



of the deflection of the a-partieles from 

 radium in a magnetic and an electric field 

 was made in 1906 by Rutherford.^ This 

 gave a value of 5.1 X 10^ for the ratio of 

 the charge to the mass (e/m) of an a-par- 

 ticle. Since the value of e/m for the 

 hydrogen ion in the electrolysis of water is 

 nearly 10", Rutherford decided that of a 

 number of possible explanations of these 

 two differing values, the most probable one 

 was that the a-partiele consisted of an atom 

 of the element helium (atomic weight 4) 

 with a charge twice that of the electron. 

 If this assumption is introduced into the 

 last previously considered calculation of 

 the life of radium, the number for the half- 

 value period comes out 2,600 years instead 

 of 1,300 years. 



In 1908 Rutherford and Geiger" devised 

 an experiment in which the actual number 

 of a-particles emitted by a known quantity 

 of radium could be accurately counted. 

 They also accurately measured the charge 

 carried by a known number of these par- 

 ticles, and demonstrated the correctness of 

 Rutherford's earlier assumption that the 

 charge on a single particle was twice that 

 carried by a single electron. From the 

 counting experiments it was evident that 

 the number of a-partieles emitted per sec- 

 ond from one gram^ of radium was 3.57 X 

 10^°. The results of these experiments 

 also gave data from which a more accurate 

 estimate could be made of the number of 

 atoms in one gram of hydrogen, viz., 

 6.2 X 10". Using the numbers thus derived 

 the magnitude of the half-value period of 

 radium was again calculated and found to 

 be 1,690 years. 



A direct determination of the rate of dis- 

 integration of radium by measurements of 

 the decrease in radioactivity of a given 

 radium salt is not practicable from an ex- 



■5 Fhil. Mag., 11, p. 348. 



6 Proceedings of the Moyal Society, A, 81, p. 

 141; ibid.. A, 81, p. 162. 



perimental standpoint. The rate of disin- 

 tegration is so relatively slow and the ex- 

 perimental difSculties of accurately meas- 

 uring the very small yearly decrease in the 

 amount of radium present are so insur- 

 mountable that this method of attacking 

 the problem is practically excluded. There 

 is, however, a way in which a knowledge of 

 the life of radium can be obtained which 

 depends upon very different principles from 

 those involved in calculations employed by 

 Rutherford. This method was first sug- 

 gested and applied by the writer, and its 

 general principles can be briefly described 

 as follows: 



The work of Boltwood, McCoy and others 

 has conclusively demonstrated that radium 

 is a transition product in the radioactive 

 disintegration of the element uranium. The 

 sources of radium consist solely of old 

 minerals containing uranium. In these nat- 

 ural compounds the uranium has been 

 undergoing transformation for long periods 

 of time and the products of its disintegra- 

 tion have been accumulating and have been 

 retained in association with the unchanged 

 uranium in the mineral. Now the nature of 

 the successive changes occurring in radio- 

 active substances is such that, in any system 

 such as that represented by a uranium 

 mineral, after sufficient time has elapsed a 

 comparatively simple relation will exist be- 

 tween the quantities of the different genet- 

 ically connected elements present. The 

 condition finally attained is known as a 

 state of radioactive equilibrium. In this 

 state a simple expression will define the 

 relative amounts of the different, related 

 radio-elements contained in the mineral, 

 and, what is more important to our imme- 

 diate interests, a very simple relation will 

 exist between the amounts of the different 

 radio-elements undergoing transformation 

 in equal periods of time. 



The rate of change of a radio-element is, 



