650 Dr. H. Geiger and Mr. J. M. Nuttall on Ranges of 



Curve Y. was first subtracted which represented the ioniza- 

 tion produced by the a particles o£ the active deposit of 

 thorium. This latter curve was obtained by a separate ex- 

 periment, the results of which are represented by the crosses 

 of Curve Y. The difference curve, shown in Curve II., 

 represents the ionization curve of the « particles from 

 thorium X, thorium emanation, and thorium A. It was 

 now necessary to analyse this latter curve into its three 

 parts, due to the three types of a particles named above. 

 This could be done by subtracting from Curve II. the ioni- 

 zation curve of a simple a ray product taken under exactly 

 the same conditions. The accurate shape of the ionization 

 curve of a simple a ray product could be taken from the 

 long-range a particles of thorium C 2 from either of Curves I. 

 or V. It had, however, to be borne in mind that the number 

 of a particles from thorium C 2 is not the same as that from 

 the other products. As Marsden and Barratt * have shown, 

 thorium Ci emits 35 a particles and thorium C 2 65 a particles 

 for a hundred disintegrating atoms of thorium emanation. 

 Moreover, on account of the fact that the period of thorium B 

 is of the same order of magnitude as that of thorium X, there 

 does not exist true radioactive equilibrium between these 

 substances. Thorium B and the succeeding products will be 

 present in excess of their true equilibrium values. From a 

 simple calculation it is easy to show that the excess of 

 thorium B over thorium X amounts to about 12 per cent.| 

 On the other hand, thorium X, thorium emanation, and 

 thorium A are for practical purposes in true radioactive 

 equilibrium, i. e., they emit the same number of a particles 

 per second. By multiplying the ordinates of the end portion 



of Curve Y. by the two factors -—- and r^ an ionization 



J no 100 



curve is obtained which, except for the range, must be 

 identical with the curve for the separate a particles from 

 thorium X, thorium emanation, or thorium A. This curve 

 was subtracted twice from Curve II., making the ends of the 

 ranges coincide in each case. The result of the first sub- 

 traction is shown in Curve III., giving the range of the 

 thorium emanation. After the second subtraction, Curve IY. 

 is obtained, which gives the range of the a particle from 

 thorium X. Hence, finally, the horizontal distances from 

 the origin at which the Curves II., III., IY. cut the axis of 

 abscissae give the ranges of the a particles from thorium A, 

 thorium emanation, and thorium X respectively. The ranges 



* E. Marsden and T. Barratt, Proc. Phys. Soc. xxiv. p. 50 (1911). 

 t iiutherford and Chad wick, Troc. Phys. Soc. xxiv. p. 141 (1912). 



