654 BELL SYSTEM TECHNICAL JOURNAL 



To be so explicit about a particle, the existence of which is deduced 

 from a set of a dozen forked alpha-particle trails which have one prong 

 too few to the fork, may seem audacious. The evidence of the trails is, 

 however, pretty strong.'^ Blackett measured with great accuracy the 

 angles between the three trails, "stem" and "prongs" of the fork; to do 

 this it was necessary to double the number of photographs, taking two 

 simultaneous pictures from different directions every time the machine 

 operated, so that by combining the two one could in effect "view" 

 every fork in three dimensions. The two prongs and the stem always 

 lie in one plane; and this is a necessary condition for conservation of 

 momentum in a process in which the entire momentum of the impinging 

 particle is shared by two and only two. If one could determine with 

 perfect accuracy the speeds of the three corpuscles reasponsible for 

 stem and prongs, one could tell whether or not the condition of con- 

 servation of momentum is obeyed, the masses of the corpuscles being 

 put equal to 4 and 17 and 1, respectively. Or in other words, if the 

 speeds of the corpuscles and their directions could be determined 

 absolutely, one could compute by well-known formulae the masses 

 which they must have, in order to assure conservation of momentum. 

 The speed of the alpha-particle is quite well-known; but for those of 

 the two others, one is forced to depend on measurements of the lengths 

 of their paths, combined with none-too-certain semi-empirical relations 

 between their ranges and their velocities. Nevertheless, it was shown 

 by Blackett that if the masses of the corpuscles responsible for the 

 prongs of each fork are 17 and 1, the lengths and directions of their 

 paths are such that so far as one can tell, momentum is conserved.^ 



Such is the present status of the art of transmutation. To the mo- 

 ment of this writing, it has proved so difficult that no one has been able 

 to succeed in it except by using alpha-particles, nor to detect his success 

 except by employing the delicate methods fit for perceiving individual 

 fast-flying electrified corpuscles. Almost any day now, the first and 

 perhaps also the second of these statements may cease to be true. Few 

 scientific campaigns have ever enlisted so numerous, determined, 

 energetic and powerful an array of talents and devices, as are now being 



' Since these photographs were taken and interpreted, evidence has been found 

 in band-spectra for the existence of an isotope of oxygen of atomic weight 17, very 

 rare by comparison with the well-known one. 



8 Urey went further, and computed the mass of the residue to seven significant 

 figures, utilizing the latest published values for the masses of alpha-particle and pro- 

 ton; he employed relativistic instead of Newtonian mechanics; in the former, the 

 expression for momentum involves rest-mass and speed in such a way that conserva- 

 tion of momentum requires definite values for each, and these he calculated from 

 Blackett's data (so, at least, I interpret his paper, but for the details I must refer the 

 reader to it). The values which he gets range from 17.00504 to 17.00135 for the 

 forked trails; these differences he believes to be real, inferring that the residual nu- 

 cleus is left in different states by the different impacts. 



