CONTEMPORARY ADVANCES IN PHYSICS 



651 



the bombarding alpha-particles are kept within certain limited inter- 

 vals — remains thus far unique. Nobody else has reported a similar 

 observation. 



As for the substances for which the distribution-in-range curve of 

 the protons has been traced, for at least one speed of impinging alpha- 

 particles and one value of the angle d, they number seven. Of the 

 German studies of boron and aluminium I have already written; 



Fig. 10 — Transmutation of a nitrogen atom attended by capture of the impinging 

 alpha-particle (P. S. Blackett). 



Chadwick and his colleagues also studied both, employing the alpha- 

 particles of polonium with their pristine and with reduced speeds, and 

 observing two groups with the former element, three (when the fastest 

 particles were used) with the latter. With fluorine the Cambridge 

 physicists observed three groups, with phosphorus one, with nitrogen 

 a single group remarkably sharply defined. Sodium gave them a curve 

 sloping smoothly downward without horizontal or even flattish seg- 

 ments. Lithium, carbon, oxygen, magnesium, silicon yielded under 



