CONTEMPORARY ADVANCES IN PHYSICS 653 



the personal inspection of the myriads of photographs, to locate those 

 few which display "forked trails." 



These forked trails were first studied by Blackett of Cambridge, 

 bombarding nitrogen with very fast a-particles of 8.6 cm. range. 

 Most of the few which he found are signs merely of elastic impacts: the 

 alpha-particle has rebounded from an atom-nucleus leaving it intact, 

 as one elastic ball rebounds from another; one of the two tines of the 

 fork is the path of the recoiling nucleus, the other that of the rebound- 

 ing alpha-particle. Nevertheless, among the trails of two hundred and 

 seventy thousand alpha-rays of 8.6 cm. range,^ Blackett found eight 

 which were bifurcated in an evidently different way. Not, as he had 

 expected, that there were three prongs to the fork instead of two. One 

 would anticipate a long thin track for the proton (long because of its 

 great range, thin because it produces fewer ions and therefore fewer 

 droplets per unit length of its path), a short thick one for the alpha-ray 

 after its impact, another short thick one for the recoiling residue of the 

 nucleus. Actually in these eight cases there was a long thin track, 

 undoubtedly that of the proton; and one, but only one, short heavy 

 track. Harkins and two of his pupils, Shadduck first and later Schuh, 

 made a similar search; chance was not so gracious to them as to 

 Blackett; in the first research two forked trails were detected (not 

 counting those resulting from elastic impact) among two hundred and 

 fifty thousand; in the second, the same small number among an equal 

 multitude. 



This lack of a third prong to the fork probably means that the a-par- 

 ticle coalesces with the nucleus which it has just bereft of a proton, the 

 solitary short track being the path of the resultant lately. This must 

 be a nucleus of charge + 8e; for the charge of the nitrogen nucleus is 

 -j- le, and to it has been added the charge + 2e of the alpha-particle, 

 and from it has been deducted the charge -\- e oi the proton. [As 

 usual, e here stands for the magnitude of the fundamental electric 

 charge, 4.77-10"^'' electrostatic unit.] Further, it must have a mass 

 approximately equal to 17, on the familiar chemical scale on which 

 the oxygen atom has mass 16; for the masses of nitrogen nucleus, alpha- 

 particle and proton are approximately 14, 4 and 1 upon this scale. 

 The ordinary atoms of oxygen have nuclear charge + 8e and mass 16. 

 This new particle thus has the nuclear charge of an oxygen atom, not, 

 however, its mass. It is consequently an "isotope" of ordinary 

 oxygen. 



^Actually, there were somewhat more than half as many additional trails due to 

 a-rays of shorter range (5 cm.). The calculations mentioned in the next paragraph 

 but one indicate, and practically prove, that all of the transmutations were performed 

 by the faster rays. 



