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must have traversed this chamber without happening to strike any 

 nucleus). See now the contrast with the present time, as illustrated 

 by Fig. 10 which shows an expansion-chamber traversed by the 



Fig. 10 — Tracks of protons recoiling from impacts of a dense stream of neutrons. 

 (F. N. D. Kurie, University of California.) 



neutrons released when deuterons from a high-voltage machine bom- 

 bard a target.'^ The machine in question was the famous cyclotron 

 of E. O. Lawrence; there is no more striking illustration of the powers 



7 This was another reaction than the one just mentioned, the target being of 

 beryllium. 



