Want of Symmetry slwwn by Secondary X-Rays. 855 



with pure hydrogen under a pressure nearly equal to the 

 atmospheric pressure, and in which is a voltaic arc o£ high 

 voltage between two metallic electrodes. In all the cases, 

 taking minute precautions for avoiding the impurities that 

 may come from the walls or from the electrodes, and using 

 hydrogen as pure as possible, I have obtained the two spectra 

 always together. Particularly I have also seen that if I cool 

 with liquid air the wall of one of these last tubes, the relative 

 intensities of the two spectra remain unchanged. These 

 experiments, made in tubes at high pressure, do not seem to 

 be exposed to the criticisms that may be urged against the 

 use of vacuum-tubes, because the possible impurities can then 

 be neglected. 



The new work of Mr. Hogley confirms, then, the con- 

 clusion of my former researches, which is the following : — The 

 secondary spectrum of hydrogen is due to hydrogen itself, 

 and not to impurities, as has been believed a long time. 

 The stellar spectrum is supposed to be due to a relatively 

 simpler vibrating system, for instance to the atom of hydrogen, 

 while the secondary spectrum might be attributed to the 

 molecule of hydrogen, more complicated than the atom. 



LXXXI. On a Want of Symmetry shown by Secondary 

 X-Rays. By W. H. Bragg, M. A ., F.R.S., Elder Professor 

 of Mathematics and Physics in the University of Adelaide, 

 and J. L. GlassuN *. 



[From " Transactions of the Royal Society of South Australia," 

 vol. xxxii., 1908.] 



ON the assumption that the Rontgen rays consist of aether 

 pulses it has been shown by J. J. Thomson (' Conduct, 

 of Electr. through Gases/' p. 323) that it is possible to 

 account for the existence of secondary Rontgen rays by 

 assuming that the primary pulses set in motion electrons 

 over which they pass, and cause them to become new centres 

 of radiation. If the electron easily follows the guiding force 

 of the primary pulse, then the secondary radiation resembles 

 the primary in quality. But if the electron is hampered by 

 attachments to other portions of the atom to which it belongs, 

 then the new pulse has not the same quality as the old ; the 

 time of motion of the electron is dragged out, and the pulse 

 produced is softer. 



Now, if an electron becomes in this way a centre of 

 radiation the intensity of the secondary effect must be 



* Communicated bv the Phvsical Society : read April 23, ] 909. 



3M2 



