Relative Activities of Radio- Products of Thorium. 35 3 



elapsed for D (period 3*1 minutes) to have accumulated in 

 sufficient amount not to influence the results. 



In one experiment in which 43 activity measurements 

 were made, the mean value of the period was found to be 

 6089 minutes, with an average deviation from the mean of 

 '20 minutes for the individual measurements. The last 

 three measurements of this series, made after the activity 

 had fallen to about 5 per cent, of the initial value, indicated 

 periods of 60*77, 60*94, and 60*86 minutes ; thus showing 

 that the sample was free from appreciable amounts of B. 

 Two similar series of experiments, each made with a separate 

 preparation of 0, gave mean periods of 60*89 and 60*7 L 

 minutes. The most probable value, as indicated by our 

 experiments, is 60*81 minutes, which is a little greater than 

 the value found by Lerch, 60*4. We have used the period 

 60*8 minutes in our calculations of the ratios as and y. 



V. The Range of Thorium X. 



Prior to the appearance of the recent paper of Marsden 

 and Barratt *, our knowledge of the existence of two alpha- 

 ray products in the excited activity of thorium rested largely 

 on the fact discovered by Hahn, that alpha rays of ranges 8*6 

 and 5*0 cm. respectively were found by Bragg's method- 

 This fact did not seem to be in accord with the value 

 x = 0*427 if one assumes that each alpha-ray product pro- 

 duces per second the same number of alpha rays as does 

 the equilibrium quantity of ThX. We therefore constructed 

 a range apparatus of the type described by Greiger and 

 Nuttall, using instead of a quadrant electrometer a gold-leaf 

 electroscope. The active source was a circular aluminium 

 disk 5 mm. in diameter, which had been exposed, on negative 

 potential, to a source of thorium emanation until it had an 

 alpha activity about fifteen times the uranium standard. It 

 was then removed from the emanation and allowed to stand 

 over night so that its decay would occur exponentially with 

 the period of B. The activities (corrected for decay during- 

 the experiment) at various pressures between 88*21 and 

 21*82 cm. of mercury gave graphs which coincide essentially 

 with those published by Marsden and Barratt, and show the 

 existence of two alpha rays of range S'6 and 4*8 cm. 



The difficulty apparently caused by the proof of the 

 existence of two alpha-ray products in the excited activity 

 has been removed, however, by Marsden and Barratt's 

 discovery that for each 100 alpha particles of Em there are- 



* Proc. Lond. Phys. Soc. xxiv. p. 50 (1912). 



