CONTEMPORARY ADVANCES IN PHYSICS 153 



Table, we get 23.002 for the left-hand member and 23.0043 for the 

 right-hand member. The agreement is within the uncertainty of the 

 data; so also would it have been, had {Ti — To) been ignored. Its im- 

 portance is perhaps enhanced by the fact that it is ex post facto: the 

 mass of Ne^^ was inaccurately known at the time of the experiments of 

 Chadwick and Constable, and there was ostensibly a disagreement. 



I repeat that it is not proved that transmutation occurs in every case 

 by capture; and an isolated value of {Ti — To), such as one often sees 

 computed from a single observation on a particular group evoked by a 

 particular beam of alpha-particles, is not necessarily valid. 



Transmutation with production of neutrons 

 This mode of transmutation has been proved, according to the 

 Cavendish school and the Joliots, for the elements Li, Be, B, F, Ne, Na, 

 Mg, and Al. The outstanding cases are those of beryllium and boron, 

 with lithium and fluorine following after. Negative results have been 

 reported by the Joliots for H, C, O, N, P and Ca, and there is no record 

 of a positive result for He. Positive results have been reported for 

 quite a number of elements both light and heavy by the Vienna school. 

 There is nothing which can properly be called a distribution-in-range 

 curve for neutrons; but there is something which is potentially as use- 

 ful — the integral distribution-in-range curve of the protons emanating 

 from a thin layer of matter rich In hydrogen, placed between the source 

 of the neutrons and the detector. If one can measure the speed of a 

 proton recoiling in a known direction from the impact of a neutron, 

 one can deduce the speed of the neutron ; in particular, if one can meas- 

 ure the speeds of the protons projected straight forward by central im- 

 pacts of the oncoming neutrons, one may consider their speeds as 

 practically the same as those of the neutrons themselves. ^^ It is thus a 

 proper procedure to obtain the integral distribution-in-range curve of 

 the protons projected forward, and convert it into a distribution-in- 

 energy curve which is that of the protons and the neutrons alike. It 

 has however not been an easy procedure, on account of the sparseness 

 of the available sources of neutrons and hence of the streams of recoil- 

 ing protons. Chadwick has published a solitary curve of this sort, 

 relating to the neutrons from beryllium ejected by the alpha-particles 

 of polonium; and Dunning has obtained a curve displaying good 

 plateaux and steps, relating to the neutrons from beryllium ejected by 

 yet faster alpha-particles.^* Steps and plateaux, as heretofore, signify 

 groups of protons and consequently groups of neutrons. Feather has 



33 Cf. Part I, page 300. 



'■' To be published in the article mentioned in P"ootnote 27, and by Dr. Dunning 

 himself. 



