CONTEMPORARY ADVANCES IN PHYSICS 133 



It is the verification of these predictions which gives us such great 

 confidence that we have recognized the processes which really happen. 



I have already said how Cockcroft and Walton proved that the 

 fragments, when lithium is bombarded by protons, are alpha-particles. 

 The integral distribution-in-range curve of these fragments, obtained 

 by Oliphant Kinsey and Rutherford with the apparatus of Fig. 2 and 

 proton-currents running up to SOixa, appears in Fig. 8; a.nd that for 

 the fragments created when deutons are used instead of protons 

 appears in Fig. 9. In both of these one cannot but be struck by the 

 beautiful long horizontal plateaux, and the sharpness of the steps 

 which end them on the right. The groups of fragments of which 

 these steps are the signs have ranges stated by the observers as 8.4 and 

 13.2 cm respectively, with uncertainties of ± 0.2 cm. (These figures 

 are evidently taken from the bottom of the step, probably because it 

 is assumed that under ideal conditions of narrow beam and thin 

 bombarded film — the actual beam had a divergence of about 15° and 

 the actual target was thick — the step would rise vertically from the 

 point whence it actually begins to rise obliquely.) The corresponding 

 energy-values are estimated as 8.6 and 11.5 MEV (millions of electron- 

 volts) respectively; and as Tq, the energy of the impinging protons, 

 is at most two-tenths of a million, these values may be compared 

 directly with the halves of the numbers in equations (3) and (4). 

 Meanwhile at Berkeley, Lewis Livingston and Lawrence were driving 

 deutons with an energy of 1.33 MEV— no longer negligible — against 

 lithium, and observing fragments with a range of 14.8 cm., corre- 

 sponding to an energy of 12.5 MEV; and this is to be compared with 

 half of 23.7 millions on the right-hand side of equation (4). 



The agreement in the case of protons impinging on lithium is 

 admirable, and well within the uncertainty of the data. The agree- 

 ments in the cases of deutons impinging on lithium are ostensibly 

 not so good, but this is not so serious as it seems at first glance, because 

 of the required extrapolation of the range-2^5-energy curve of alpha- 

 particles (page 131), and because it is not always the "symmetrical 

 case" which occurs. For the present there is no compelling reason to 

 suppose that equation (2) is contradicted by the data. 



A further point susceptible of test: if the processes described by 

 equations (1) and (2) are actual, the alpha-particles of the stated 

 ranges must be shot ofif in pairs, the two members of each pair flying 

 off in almost opposite directions — in directions which would be exactly 

 opposite were it not for the original momentum of the proton, but 

 which because of that momentum must make with one another an 

 angle slightly (and calculably) less than 180°. Cockcroft and Walton 



