of a particles from various Radioactive Substances. 



617 



and that on account of scattering and possibly other causes 

 many are stopped at a somewhat shorter distance. One of 

 us * has shown that the velocity of expulsion of the a par- 

 ticles is proportional to the cube root of its maximum range. 

 Thus, knowing the velocity of one product the velocities of the 

 others can be deduced. Comparison is most suitably made 

 with Ra C, for which the initial velocity has been measured 

 by Rutherford f and was found to be 2'06 x 10 9 cm./sec. The 

 initial velocities of expulsion calculated in this way are added 

 in the table. 



Substance. 



Range 15° C. 



Range 0° C. 



Initial Velocity 

 of Expulsion. 



Uranium 



2-72 cm. 

 3-00 „ 

 330 „ 

 3-77 „ 

 2-72 „ 

 3-87 „ 



2-58 cm. 

 2-84 „ 

 313 „ 

 3-58 „ 

 2-58 „ 

 367 „ 



151 x 10 9 cm./sec. 

 1-56 XlO 9 

 1-61 Xl0 9 „ 

 1-68 XlO 9 „ 

 1-51 XlO 9 „ 

 1-70 XlO 9 „ 



Ionium 



Radium 



Polonium 



Thorium 



Radiothorium ... 



We have already mentioned the earlier estimates of the 

 ranges of uranium and thorium. The range of ionium has 

 been measured by Boltwood J, that of radium by Bragg and 

 Kleeman§, that of polonium by Kucera and Masek||, and by 

 Levin U, and that of radiothorium by Halm**. Nearly all 

 the figures given in the table above are in fair agreement 

 with those obtained by previous observers. In some cases, 

 however, pressure and temperature of the air are not men- 

 tioned, so that it is difficult to make a comparison of value 

 between the present and older determinations. 



In connexion with the redetermination of the ranges of 

 the a particles from a number of products, given above and 

 in a previous paper by one of us|1\ we have reconsidered the 

 possibility of a relation between the period o£ the active 

 substances and the ranges of the a particles emitted by them. 



* H. Geiger, Proc. Roy. Soc. A. lxxxiii. p. 505 (1910). 



f E. Rutherford, Phil. Mag. xii. p. 348 (1906). 



\ B. B. Boltwood, Amer. Journ. Sc. xxv. p. 365 (1908). 



§ W. H. Bragg and R. D. Kleenian, Phil. Mag. x. p. 318 (1905). 



|| Kucera and Masek, Phys. Zeitschr. vii. p. 337 (1906). 



5j M. Levin, Phys. Zeitschr. vii. p. 519 (1906). 



** O. Hahn, Phys. Zeitschr. vii. p. 456 (1906). 



ft H. Geiger, Phil. Mag. xxii. p. 201 (1911). 



