514 Prof. W. H. Bragg on the Influence of the 



the range of the RaC particles by 1'13 and of the Ra particles 

 by *93, the difference being *2 cm. This means that when 

 the effects of gold are compared with those of air, we obtain 

 a larger difference than when they are compared with those 

 of aluminium; from which we should expect that the stopping- 

 power of the latter metal varies to some extent with the speed 

 of the a particle. 



To sum up, it is clear that the stopping-power of a metal 

 increases with the speed of the a particle, and the more so 

 the greater the atomic weight of the metal. The effects are 

 small, and not easy to measure accurately ; so that, although 

 there is not a complete agreement between the results obtained 

 by different methods, yet it is possible that the inconsistencies 

 may disappear with more perfect measurements. 



It was observed by Madame Curie that when a pair of 

 metal sheets was used as one of the walls of the ionization 

 chamber, the ionization current was varied by the inversion 

 of the sheets. Rutherford (' Radioactivity/ p. 188) ascribed 

 this to differences in the secondary radiation from the various 

 metals. Kucera and Masek have rejected Rutherford's ex- 

 planation, and supposed the effect to be due to a differential 

 scattering of the a rays (Phys. Zeit. vii. 19. pp. 650-654). 

 But it is clear that an effect of this kind ought to occur if 

 the range of the « particle after passing through the sheets 

 is altered by the inversion ; and before ascribing it to secon- 

 dary radiation or scattering of the a rays it must be made 

 clear that it cannot be completely explained in this direct 

 way. ^ 



It is not easy to calculate the change that should be ex- 

 pected in Madame Curie's form of the experiment. For 

 when a radioactive plate is placed at a little distance from an 

 aperture of about the same size as the plate, geometrical 

 difficulties are introduced into the calculation, since it is 

 necessary to find the solid angles subtended by the aperture 

 at various points of the plate. The difficulties largely dis- 

 appear if the metal sheets are laid directly on the radioactive 

 surface, and the ionization chamber is large enough in 

 every way to allow the a particles to complete their paths 

 therein. 



I have carried out an experiment under these conditions, 

 using a deposit of RaC on a small piece of foil, a sheet of Al 

 equivalent to 3*35 cm. of air, and the 28 gold-foiis already 

 referred to. The metals were inverted at regular intervals of 

 four minutes^ and the successive readings were : — 



