CONTEMPORARY ADVANCES IN PHYSICS 143 



with paraffin, to accentuate the effect of the neutrons; it was however 

 found that the readings were not considerably lessened when the 

 paraffin coating was absent, and consequently Lauritsen infers that 

 most of the effect is due to gamma-rays proceeding from the bombarded 

 atoms. This inference is sustained by the fact that when the rays 

 responsible for the effect are caused to pass through leaden screens, 

 the ionization falls off exponentially with the thickness of the lead; 

 and the value of the exponent suggests that the energy of the photons 

 is about 1.5 MEV. One can easily think of a process whereby deutons 

 might evoke neutrons from lithium nuclei : 



iH2 + 3Li- + To = 22He^ + on' + Tu (7) 



but with protons no plausible interaction comes readily to mind. 

 Perhaps there is a two-stage process, the protons producing the 

 reaction described by equation (1), the resultant He^ nuclei striking 

 other lithium nuclei and evoking neutrons. Or perhaps the neutrons 

 and the gamma-rays alike result from the same processes as produce 

 the groups of short-range alpha-particles revealed in Fig. 8. Questions 

 of this intricate kind will probably predominate in the study of trans- 

 mutation, in the years to come; and experiments on thin films will 

 play a very important part in settling them, both because the likelihood 

 of two-stage processes will be reduced, and because it may be possible 

 to learn which isotopes are involved. 



Little indeed is definitely known about the disintegration, by 

 protons or deutons, of any other elements than lithium or boron. 

 Charged fragments have been observed proceeding, in relatively small 

 but yet appreciable number, from bombarded targets made of a great 

 variety. But in many of these cases they may be due, so far as any 

 of the observations tell, to a minute contamination of the target by 

 boron derived from the glass of the enclosing tube; and the danger 

 of this possible source of error was vividly brought out by Oliphant 

 and Rutherford, when at first they observed such fragments, but 

 ceased altogether to observe them when the original glass of their 

 tube was replaced by a special boron-free variety! Beryllium and 

 fluorine are the only elements, other than lithium and boron, of which 

 these experimenters were sure of detecting fragments; for those of 

 fluorine they were able to plot a disintegration-function and a distribu- 

 tion-in-range, which differed sufficiently in aspect from those of 

 lithium and boron to exclude the possibility that these might be 

 responsible; those of beryllium were too scanty for such tests. The 

 elements with which they got no charged fragments, or only a few 

 per minute, were the following: Fe, O, Na, Al, N, Au, Pb, Bi, Tl, U, 



