CONTEMPOR.ARY ADVANCES IN PHYSICS 147 



by Pollard. Phosphorus and sodium have been studied only by Chad- 

 wick Constable &. Pollard, who find for the former a single group, for 

 the latter a smoothly-descending int«tral curve which may betoken 

 total absence of groups, or may be resolved, by some future and closer 

 approach to the ideal curve, into a close succession of bends and corners. 

 The four remaining elements — B, F, Mg, Al — show at least three groups 

 apiece, and indeed Chadwick and Constable deduce four pairs of 

 groups for aluminium and three for fluorine. To illustrate the degree 

 of concurrence between different observers, I quote the values for the 

 groups of aluminium — that is to say, values of the ranges of the protons 

 belonging to these groups, ejected forward by 5.3-MEV alpha-particles 

 — from the four authorities. Pose gives 28.5, 49.6, and 61.2 (cm of air- 

 equivalent) ; Steudel, 33, 49, 63; M. de Broglie and Leprince-Ringuet, 

 30, 50, 60; Chadwick and Constable give 22, 26.5, 30.5, 34, 49, 55, 61, 

 66. More detailed comparisons had best be left to those who have 

 practice in this field. 



While nearly all of the data have been obtained by other methods 

 than that of the expansion-chamber, a few beautiful pictures have 

 been taken in which there appears the track of an alpha-particle 

 passing through nitrogen, and this track is seen to end at a fork.^^ 

 One of the tines of the fork is a long thin track, apparently that of a 

 proton; there is only one other, and this is short and thick. It is 

 inferred that these reveal the only fragments which there are, and that, 

 in the usual though somewhat objectionable phrase, the alpha-particle 

 has fused with the residual nucleus. The process is then expressed 

 by the equation : 



tN^^ + 2He^ + To = sQi^ + iRi + T„ (8) 



the symbols being chosen according to the same principles as in 

 equation (1). It is commonly assumed, though in no other case with 

 such good evidence, that this happens in most if not in all cases, so 

 that when a nucleus of atomic number Z and mass-number A is 

 transmuted by an alpha-particle, the process often is : 



zM-" + 2He^ + To = z+iAH+3 + iH^ + Ti, (9) 



with an obvious symbolism. This is called "disintegration with 

 capture" (though it is the case in which the objection to the name 

 "disintegration," page 117, is gravest). The other conceivable case of 

 "disintegration without capture" would be described thus: 



zM-^ -f 2He^ + To = z-iM-^-i + iHi -f aHe^ + T,. (10) 



^'' "Transmutation," Figs. 10 and 11. 



