Spectrum of Penetrating y Rays from Radium B and C. 271 



Some experiments were made to try to obtain transmission 

 photographs with pure radium deposited on a nickel wire 

 •as a source. The photographic effect, however, was far too 

 faint for measurement. It seems not unlikely that the 

 radiations in the neighbourhood of 1° 10', 1° 24', and 1° 40', 

 are to be ascribed to both radium B and radium C, although 

 the actual frequencies of the corresponding lines may be 

 distinct in the two cases. 



Discussion of Spectra. 



It will be seen that the wave-lengths of the penetrating 

 *y rays from radium B and radium C are much shorter than 

 any previously determined. Moseley has determined the 

 '" K " spectra of silver and found the wave-length of the 

 strong line "56 x 10~ 8 cm. The wave-length of the most pene- 

 trating 7 ray observed is 0*7 x 10~ 9 , or eight times shorter. 

 When the great penetrating power of the radiations from 

 radium C — half absorbed in 6 cm. of aluminium — is con- 

 sidered, and the shortness of its wave-length, it is surprising 

 that the architecture of the crystal is sufficiently definite to 

 resolve such short waves. This is especially the case when 

 we consider that owing to the heat agitation of the atoms, 

 the distance between the atoms must be continually varying 

 over a range comparable with the wave-length of the 

 radiation. One photograph was taken with the crystal 

 immersed in liquid air, but no obvious improvement in 

 definition was observed. 



The appearance of these high frequency vibrations from 

 radium B and radium C is accompanied by the expulsion of 

 very high speed ft particles from the atom. It does not, 

 however, follow that it will be necessary to bombard the 

 material with such very high speed ft rays to excite the 

 ^corresponding radiation. If we may assume, as seems 

 probable, that Planck's relation E = Av holds for the energy 

 of the ft particle required to excite radiation of frequency v 1 

 it can be deduced that the electron to excite this radiation in 

 radium must fall freely through a difference of potential 

 of 180,000 volts, which is equivalent to a velocity of about 

 07 that of light. This is much smaller than the velocity of 

 the swift ft particles from radium B or C, and is not beyond 

 the range of possible experiment. With the tube recently 

 designed by Coolidge * there should be no inherent difficulty 

 in exciting the corresponding radiation in a heavy element 

 like platinum or uranium. 



* W. D. Coolidge, < Physical Review,' Dec. 1913, p. 409. 



