2 Johnstone and Boltwood — Relative Activity 



the ratio in the case of the actinium series, which could 

 only be explained on the assumption that actinium origi- 

 nates as a branch product and belongs to what may be 

 termed a collateral branch of the ionium-radium-polo- 

 nium family. 



The values obtained by Boltwood in the course of his 

 experiments showed that the activity of the uranium was 

 about 2-2 times that of the radium with which it was 

 in equilibrium, although at that time the range of the 

 a-particle from uranium was supposed to be about 2-7 cm., 

 which is less than that of the a-particle from radium. 

 Since the ionizing power of an a-particle is nearly pro- 

 portional to the range, and since, on the basis of the 

 disintegration theory, an equal number of a-particles are 

 emitted per second by each of two radioactive products 

 in equilibrium with one another, it was necessary to 

 assume that either the uranium atom emitted two a-par- 

 ticles simultaneously, which was improbable, or that two 

 distinct a-ray changes existed in ordinary uranium. 

 Neither of these assumptions, however, completely obvi- 

 ated the difficulty. 



The fact that uranium actually did emit twice the num- 

 ber of a-particles to be expected on theoretical grounds 

 was subsequently demonstrated by Geiger and Ruther- 

 ford 2 and by Brown 3 who counted the number of a-par- 

 ticles emitted per second from a film of pure uranium 

 oxide and a similar film of uraninite of known composi- 

 tion. Using the scintillation method, Marsden and Bar- 

 ratt 4 made a careful examination of the a-radiation from 

 uranium and concluded as a result of their experiments 

 that ordinary uranium consists of a mixture of two suc- 

 cessive a-ray products in equilibrium with one another. 

 Attempts to measure the separate ranges of the a-par- 

 ticles emitted by these two products were made by Foch 5 

 and Friedmann. 6 By the use of a better method of 

 measurement, in which the Bragg ionization curves for 

 a uranium film were compared with the corresponding 

 curves obtained with polonium and ionium, Geiger and 

 Nuttall calculated the ranges of the a-particles from ura- 

 nium to be 2-5 cm. and 2-9 cm. (at 0°C). 



2 Phil. Mag., 20, 691, 1910. 

 3 Proc. Roy. Soc, A 84, 151, 1910. 

 4 Phys. Soc. Proc, A 23, 367, 1911. 

 5 Le Radium, 8, 101, 1911. 

 °Wien. Ber., 120, 1361, 1911. 



