CONTEMPORARY ADVANCES IN PHYSICS 125 



traversed ("air-equivalent" of the mica, p. 127 infra) }^ The point 

 is, that exactly the same curve was obtained when a beam of alpha- 

 particles was projected through the same thicknesses of mica into the 

 same chamber. Mere similarity in the shape of the curves would 

 prove nothing, for this is the shape obtained with all kinds of charged 

 particles, electrons and protons and more massive charged nuclei; in 

 particular, every such curve rises from zero to a maximum and there- 

 after descends continually as the energy of the particles is raised 

 indefinitely upward from the least value sufficient for ionization. ^^ 

 However, the ordinates of the curve of Fig. 7 are equal to those of the 

 alpha-particle curve, and about four times as great as would have been 

 observed with protons; and this it is which proves the fragments to 

 be alpha-particles. Almost as good a proof could be made by two 

 measurements: by measuring the range of the fragments and the total 

 ionization produced by any fragment in a chamber deep enough to 

 swallow it up, and comparing the latter datum with the ionization 

 produced in the same chamber by an alpha-particle of equal range. 

 This proof, or some other substantially like it, has been adduced in 

 certain cases. When alpha-particles are the agents of the transmuta- 

 tion, the same test has proved in several cases that the fragments are 

 protons. In some cases the test has not yet been applied. 



I have already had to speak of interposing mica in the path of the 

 fragments, in order to learn something about them. This is a pro- 

 cedure with which it is necessary to be familiar. It would be very 

 pleasant indeed to be able to apply electric and magnetic deflecting 

 fields to a narrow stream of fragments all flying in the same direction, 

 for one could then spread it out into a velocity-spectrum, and not 

 only identify the corpuscles perfectly but also determine their distribu- 

 tion-in-range, which as we shall presently see is of the first importance. 

 This has not yet been done, partly (I presume) because of the high 

 fieldstrengths that would be needed, chiefly because the available 

 streams of particles are too scanty. It will be a happy day when at 

 last we get streams of fragments so intense that they can be dispersed 

 into a velocity-spectrum which will appear imprinted on a photographic 

 film, as has been feasible for years with beta-rays. For the time being 

 we must be content with curves such as many figures in this article 

 display, Figs. 8 and 9 and 1 1 for example. 



^^ The quantity plotted as ordinate is obtained from such records as those of Fig. 6, 

 in which every fragment produces a vertical line. Cockcroft and Walton observed 

 many such lines for each thickness of mica, and ascertained in each case the most 

 frequently-occurring value of line-length. 



^^ "Electrical Phenomena in Gases," pp. 40-44, 70-71. Such a curve as that of 

 fig. 7 is sometimes called a "Bragg curve." 



