632 



Prof. J. Jolv and Mr. A. L. Fletcher : 



a molecular formula or from a chemical analysis. In the 

 first case we multiply the square root of each atomic weight 

 by the number of such atoms present; sum the values so 

 found ; and finally divide by the number of atoms entering 

 into the formula. In the second case we deal with each 

 molecule separately, adding- the square roots of the component 

 atomic weights and dividing by the number of atoms in the 

 molecule. The number obtained is the average square root 

 of the atomic weights entering into the molecule. It is to 

 be multiplied by the percentage quantity of the molecule 

 present, as given in the analysis. The numbers obtained are 

 to be added and their sum divided by 100. 



The following are the ranges in air of the a rays of the 

 uranium, actinium, and thorium families of elements : — 



Uranium ... 



Ionium 



Radium 



Emanation... 



I. 



II. 



cm. 

 2-8 



2-1 



2-84 



3 60 



4-25 



6-35 



3-16 



iir. 



Thorium 



Radiothorimn . 

 TbX ... 



IV. 



cm. , 

 3o 



3-9 ' 



5-7 ; 



5-5 j 



5-0 



8-6 



cm. 

 35 



2-8 



3-54 



4-23 



4-83 



7 06 



3 86 



Radioactinimn . 

 ActX 



cm. 



4-8 



6-55 



5-8 

 5-50 



Emanation 



ActB 



Emanation 



Th B 



RaA 





EaC 



Th C 



RaF 









Column I. gives the limiting ionization ranges in air at 

 760 mm. pressure, and Col. II. the approximate distances in 

 air at which the ionization effect is a maximum in the case 

 of the uranium family. The values in Col. II. are deduced 

 partly by scaling directly from the Bragg and Kleeman 

 curves, partly by assuming that a deduction of 7 mm. from 

 the limiting range gives the approximate position of most 

 intense ionization. These values we shall require later. The 

 distances in Cols. III. and IV. are the limiting ionization 

 ranges in air in the case of the actinium and thorium 

 families*. 



In a pleochroic halo we may have either the uranium 

 derivatives acting or those of thorium ; or, again, a mixture 

 of both. In the first case the radius of the halo is limited 



* See Bragg and Kleeman, loc. cit. and Phil, Mag. June 1906; Allen, 

 Phvs. Rev. xxvii. 1908 ; Le Radium, Jan. 1910, p. 2 ; Levin, Phys. Zeit. 

 1906, p. 521 ; Kueera and Masek, Fkys. Zeit. 1900, p. 339; Halm, Phil. 

 Mao-. Sept. 1906, 



