On Reflexion and Refraction of Elastic Waves. 567 



cloud in dust-free air on positive ions than on negative ones 

 when the ions are produced by Rontgen rays. 



Though only a small fraction of the mass of an atom can be 

 detached by any known process, it does not follow that the 

 part left behind does not contain more corpuscles which could 

 be detached by more powerful means than we have hitherto 

 been able to use. For it is evident that it will require a 

 greater expenditure of energy to tear two corpuscles from 

 one atom than to tear two corpuscles one from each of two 

 separate atoms; for when one corpuscle has been torn off from 

 an atom the atom is positively electrified, and it will be more 

 difficult to tear off a second negatively electrified corpuscle 

 from this positively electrified atom, than it was to tear the 

 first from the originally neutral atom. A reason for believing 

 that there are many more corpuscles in the atom than the one 

 or two that can be torn off, is afforded by the Zeeman effect. 

 The ratio of the mass to the charge, as determined by this 

 effect, is of the same order as that we have deduced from our 

 measurements on the free corpuscles; and the charges carried 

 by the moving particles, by which the Zeeman effect is ex- 

 plained, are all negatively electrified. Now, if there were 

 only one or two of these corpuscles in the atom, we should 

 expect that only one or two lines in the spectrum w r ould show 

 the Zeeman effect ; for even if the coordinates fixing the 

 position of the moving corpuscles were not " principal co- 

 ordinates/' though there might be a secondary effect on the 

 periods of the other oscillations due to their connexion with 

 these coordinates, yet we should expect this secondary effect 

 to be of quite a different order from the primary one. As, 

 however, there are a considerable number of lines in the 

 spectrum which show Zeeman effects comparable in intensity, 

 we conclude that there are a considerable number of cor- 

 puscles in the atom of the substance giving this spectrum. 



I have much pleasure in thanking my assistant Mr. E. 

 Everett for the help he has given me in making the experi- 

 ments described in this paper. 



LIX. On Reflexion and Refraction of Elastic Waves. 



University of Edinburgh, 



24th Oct., 1899, 



To the Editors of the Philosophical Magazine. 



Gentlemen, — The accompanying letter from Prof. T. Gray 



contains important corrections of some numbers I gave on 



page 68 of my paper on " Reflexion and Refraction of Elastic 



Waves" (Phil. Mag. July 1899). As explained in the 



footnote on page 67, I got part of the information from 



manuscript notes in Professor Milne's possession. Apparently 



I misunderstood them aud deduced inaccurate conclusions from 



