CONTEMPORARY ADVANCES IN PHYSICS 403 



the equation is supported by the facts that the ranges of the two 

 groups stand to one another in the ratio computed by assuming 

 equaUty of momenta, that particles of one are found to be paired 

 with particles of the other, and that they ionize about as much as 

 alpha-particles of equal range. 



The rest-masses of the two new nuclei are estimated by putting, 

 in equations (1), (2) and (3\ the best available values for T^ (the 

 kinetic energy of the impinging deuton, that of the other H^ nucleus 

 being negligible) and Ti (the sum of the kinetic energies of both frag- 

 ments resulting from the reaction). The results are: for the rest-mass 

 of H\ 3.0151 from (1) ; for the rest-mass of He\ 3.0166 from (3). To 

 derive the latter from (2) is not so precise, the energy of neutrons 

 being harder to evaluate than that of charge-bearing particles; Oli- 

 phant, Harteck and Rutherford prefer to say merely that the result 

 is not incompatible with that from (3).^^ 



These are the fourth and fifth of the nuclei (counting the neutron 

 as one) in order of increasing mass. The departures of their masses 

 from the adjacent integer are abnormally great for light nuclei, and 

 their packing-fractions (First Part, p. 318) are the greatest yet known 

 excepting that for H-, and fall neatly by the upper branch of the curve 

 of packing-fraction vs. mass-number (Fig. 8 of the First Part). The 

 contrast between the packing-fractions 55 of Hc^ and 5 of He^ is 

 especially striking. The new nuclei are the first isobars to be dis- 

 covered of mass-number less than 40, and the first pair to be discovered 

 of which the masses are distinguishable. 



Cockcroft and Walton have studied at length the fragments emerging 

 from lithium, boron and carbon bombarded by deutons. Lithium 

 supplies a group and boron a group of protons which may result from 

 the transformation of the lighter into the heavier isotope according 

 to the schemes, 



,W + 3Li« = 3Li7 + iHi + {T, - To), (4) 



xH2 + sBi" = 5B11 + iHi + {T, - To), (5) 



but the two members of each equation (in which all the rest-masses 

 are known by deflection-experiments) do not agree very well. Carbon 

 supplies a group and boron two more groups of protons which cannot 

 be made to fit into such a scheme without postulating emission of 

 gamma-rays to achieve the balancing of masses — an emission for which, 



1^ These results are computed by assuming that the values of the rest-masses of 

 H^, H-, He^ Li'', Li" and n^ given by Aston, Bainbridge and Chadwick are e.xact, and 

 that no additional fragment (such as a ganmia-ray photon) of appreciable energy is 

 emitted at the transmutation. 



