Production of Fluorescent Rontgen Radiation. 657 



The results founded on the accumulation of radio-active 

 debris are open to the objection that addition or subtraction 

 of the measured substances may have occurred over the long 

 period of time involved. The results obtained from the 

 halo are less likely to be affected by this source of error. It 

 is of interest to find, therefore, that the results obtained from 

 it are not very discordant with some of the higher results 

 obtained by the lead ratio. There may be significance in 

 this rough agreement, or errors of a nature as yet unknown 

 may affect both determinations. We are not in a position to 

 say. But it is certain that if the higher values of Geological 

 Time so found are reliable, the discrepancy with estimates of 

 the age of the ocean, based on the now well-ascertained facts 

 of solvent denudation, raises difficulties which at present seem 

 inexplicable Discussion of hypotheses whereby the relia- 

 bility of the one method or the other may be called in 

 question is not within the scope of this paper. 



LXIV. On the Production of Fluorescent Rontgen Radiation. 

 To the Editors of the i Philosophical Magazine.' 



Gentlemen, — 



IN the current number of this magazine Mr. J. Crosby 

 Chapman attacks an hypothesis of mine that an inter- 

 mediary cathode ray conveys the energy from the primary 

 X-ray to the homogeneous secondary ray. 



Mr. Chapman finds by experiment that the cathode rays 

 which emerge from a gold foil under the influence of Sn 

 X-rays lose 52 per cent, of their energy in passing through 

 a second gold leaf, the energy being measured by their power 

 of ionizing air. With this I agree ; in fact I have done the 

 experiment myself. He thence concludes that the cathode 

 rays excited in a gold foil by hard primary X-rays ought to 

 emerge from the foil — or a certain fraction of them should 

 emerge — carrying a large percentage of the original power 

 possessed by those cathode rays of exciting homogeneous 

 secondary rays in gold. Clearly the conclusion does not 

 immediately follow. It seems to me that arguments of this 

 kind are always liable to one defect at least, viz., that the 

 absorption coefficient is taken as a definition of the quality 

 of the radiation. Perhaps homogeneous X-rays are suffi- 

 ciently defined by an absorption coefficient, but certainly the 

 same cannot be said of primary X-rays or of cathode rays. 



When, therefore, Mr. Chapman concludes finally that 

 homogeneous or fluorescent radiation cannot be to the 



Phil. Mag. S. 6. Vol. 25. JSo. 148. April 1913. 2 Y 



