356 Prof. McCoy and Dr. Viol : Chemical Properties and 



similar relationships in the thorium series have heretofore 

 been unsatisfactory. There were reasons why this might be 

 so. In the first place the activity ratios were not known 

 with sufficient accuracy, as we pointed out in Section II. of 

 this paper ; secondly, Boltwood's assumption might not be 

 correct; and finally, as is now known, the components of 

 C are not successive, C 1 giving 65 per cent, and C 2 35 per 

 cent, as many alpha particles per second as the equilibrium 

 amount of the emanation. We have made careful determina- 

 tions cf two activity ratios, x and y (Sections II. and III.), 

 by means of which we can compare the activities of the 

 respective substances with their ranges. The work of Bragg * T 

 and Halm "j", Rutherford ±, Geio-er 5, Tavlor II and others has 

 thrown much light on the nature of the ionization curve of 

 an alpha particle. It is now quite certain that ionization 

 is not uniform along the path of an alpha ray, but that it 

 increases markedly toward the end of the range. It appears,, 

 however, that during the last two or three cm. of their ranges, 

 the alpha particles of the various radio-bodies all behave 

 exactly alike in respect to the number of ions produced in 

 each corresponding element of the remainder of the path. 

 This conclusion is wholly consistent with the fact that all 

 alpha particles are atoms of helium, and differ from one body 

 to another only in initial velocity. Bragg H concluded that the- 

 ionization of an alpha particle was giA^en by the expression 



a 

 l== VK-Zh-1'33' 



where a is a constant, II is the range, and Z is the distance the- 

 particle has already travelled ; R — Z is then the remainder 

 of the range to be traversed. More recent investigations- 

 by Geiger ** have shown that 



-VeS, (13 > 



where i is the ionization at any part of the path, and where 

 R and Z have the same significance as before and a' is a 

 constant. If we represent the total ionization of an alpha 



* Phil. Mag. [6] xi. p. G17 (1906) ; xiii. p. 333 (1907). 



t Phijs. Zeitschr. vii. p. 412 (1906). 



X Phil. Mag. [6] xii. p. 134 (11)06). 



§ Proc. lloy. Soc. A. lxxxiii. p. 505 (1910). 



|| Phil. Mag. [6] xxiii. p. 670 (1912). 



11 Loc. at. ** Loc. at. 



