CONTEMPORARY ADVANCES IN PHYSICS 131 



bodies that it has already been feasible to measure by deflection- 

 methods the speeds corresponding to a large number of different ranges, 

 and plot an empirical speed-i'5-range curve which is fixed by so many 

 points of observation that there is no important uncertainty in making 

 interpolation between these. For alpha-particles of range superior to 

 8.6 cm., such as often occur among fragments of transmutation, it has 

 heretofore been necessary to extrapolate ; but very lately the empirical 

 curve has been extended onward to 11.6 cm., thanks to a powerful new 

 magnet at the Cavendish which is able to deflect the paths of alpha- 

 particles of even such rapidity.^^ With protons our knowledge of the 

 range-V5-energy relation is less extensive and less accurate, and an im- 

 provement thereof should be one of the first and most important by- 

 products of the new methods for imparting high energies to ions. For 

 charged nuclei of other elements than hydrogen and helium, relatively 

 little is assured (what is known has been found out chiefly by Blackett 

 and his school)-" ; but this lack has not as yet been much of an im- 

 pediment to the study of transmutation, except in certain cases involv- 

 ing impacts by neutrons. 



Transmutation by Impacts of Protons and Deutons 



The earliest element to be transmuted by protons in the laboratory 

 — indeed the first to be transmuted by man with any agent other 

 than the alpha-particle — was lithium. It was fortunate that Cock- 

 croft and Walton began with this element, for its behavior turned out 

 to be uniquely lucid. In most disintegrations, a single fragment is 

 detected, and there must be a massive residue which remains unseen, 

 staying hid within the substance of the bombarded target. But in 

 some at least of the transformations which occur when lithium nuclei 

 are struck by protons or deutons, there seems to be no hidden residue; 

 every fragment is observed and recognized. These are processes of 

 "nuclear chemistry" of which we fully discern both the beginning 

 and the end; and they are described by the quasi-chemical equations: 



'^Rutherford et al., Proc. Roy. Soc. 139, 617-637 (1933). The empirical curve 

 departs slightly from a third-power law (range proportional to cube of speed) and 

 the results are expressed by an empirical formula for the departure. See also G. H. 

 Briggs, Proc. Roy. Soc. 139, 638-659 (1933). 



2° See N. Feather, Proc. Roy. Soc. 141, 204 (1933) and literature there cited. 

 The observations are made upon tracks which appear in Wilson chambers when the 

 contained gas is bombarded by alpha-particles, and which are the tracks of objects 

 of atomic mass that have suffered violent impacts. It is presumed (though not 

 always proved) that these objects are solitary or "bare" nuclei, not accompanied 

 by any of the orbital electrons which attended them before the impacts. Some (but 

 not all) of the data conform to the empirical rule that the ratio of the ranges of two 

 nuclei of masses nti and m-. and of charges Z\e and Z>e, when the two have the same 

 speed, is (wi/w2)(Zi/Z2)"-. 



