640 BELL SYSTEM TECHNICAL JOURNAL 



Evidently it was the first of these possible causes of confusion which 

 Rutherford feared the most, for in setting up his screen in such a place, 

 he shielded it by absorbers sufficient to stop the primary particles, and 

 counted the flashes which appeared in spite of this obstruction. The 

 datum therefore is a count of corpuscles having ranges greater than 

 seven cm. With a few of the lightest elements, the critical range is 

 somewhat lower; for an alpha-particle, when deflected through 90° by 

 a close approach to a nucleus not many times more massive than itself, 

 loses an appreciable part of its speed in the deflection. 



The Cavendish school examined many elements — including all of 

 the nineteen lightest, the first nineteen of the periodic table — in their 

 quest for transmutation. Under the bombardment of seven-centi- 

 meter alpha-rays, most of these nineteen emitted corpuscles which 

 satisfied their strict criterion. The first four (hydrogen, helium, 

 lithium and beryllium) did not; neither did the sixth nor the eighth 

 (carbon and oxygen); all of the others, beginning with boron the fifth, 

 and ending with potassium the nineteenth, appeared transmutable. 

 No element beyond potassium ejected corpuscles with a range great 

 enough to exceed those of the two other kinds, which Rutherford and 

 his school were so anxious to exclude. 



One is never long satisfied with the assertion that a certain eflfect 

 does occur in certain cases, does not occur in others. Invariably the 

 questions follow: in the cases where it happens, how much does it 

 happen? in the cases where it is not observed, what is the least amount 

 of it which could have been observed? 



As a rule it is much more difficult to answer these questions, than 

 merely to establish that with given means of observation either the 

 effect is found or it is not. The study of transmutation is no exception 

 to this rule. No one has ever set up a screen which surrounded the 

 bombarded substance on every side, and therefore no one has counted 

 the corpuscles which go off in all directions, nor even in a moderately 

 great fraction of all directions. Screens have been set up in various 

 directions from the piece of matter suffering transmutation, and the 

 data, far from encouraging us to assume that the protons are fired off 

 at random, indicate instead that more of them go off at inclinations of 

 less than 90° to the beam (prolonged in the forward direction) of the 

 alpha-rays, than at inclinations more than 90°,— more go "forward" 

 than "backward." The distribution-in-angle, however, requires 

 much further research. 



In answering the questions, Rutherford, Chadwick and Ellis say 

 inter alia that when one million alpha-particles of seven-centimeter 



