2s6 



NATURE 



[May 30, 19 1 8 



vol. vii. [1916], p 157) have recently shown that the 

 same quantity h appears in connection with the impact 

 of corpuscles against any kind of target, the observa- 

 tion here being- that the highest frequency in the 

 general or white-light X-radiation emitted when cor- 

 puscles impinge upon a target is found by dividing the 

 kinetic energy of the impinging corpuscle by h. Since 



exhibited in De Brcglie's photographs here shown 

 (Figs. 6 and 7).' It will be seen from these photo- 

 graphs that the atoms of each particular substance 

 transmit the general X-radiation up to a certain critical 

 frequency and then absorb all radiations of higher fre- 

 quency than this critical value. The extraordinary 

 significance of this discovery lies in the fact that it 

 indicates that there is a 





Fig. 6. — Absorption of certa 



iun of K-radiation 



Em«fiSt<»n«{-Wan<f 



w 



^ 



black-body radiation is presumably due to the impact 

 of the free corpuscles within a metal upon the atoms, 

 it is probable that the appearance of h in black-body 

 radiation and in general X-radiation is due to the 

 same cause, so that, contrary to Planck's assumption, 

 there need not be, in either of these cases, any coin- 

 cidence between natural and impressed periods at all. 

 The hv which here appears is not a characteristic of 

 the atom, but -merely a 

 property of the aether pulse 

 which is generated by the 

 stopping of a moving elec- 

 tron. Why this aether pulse 

 should be resolvable into 

 a continuous or white-light 

 spectrum, which, however, 

 has the peculiar property of 

 being chopped off sharply 

 at a particular limiting fre- 

 quency given by hv — PD x e 

 is thus far a complete mys- 

 tery. All that we can say 

 is that experiment seems to 

 demand a sufficient modi- 

 fication of the aether-pulse 

 theory of white-light and 

 of general X-radiation to 

 take this experimental fact 

 into account. 



On the other hand, the ap- 

 pearance of h in connection 



with the absorption and emission of monochromatic 

 light (photo-electric effect and Bohr atom) seems to 

 demand some hitherto unknown type of absorbing and 

 emitting mechanism within the atom. This demand is 

 strikingly emphasised by the remarkable absorbing 

 property of matter for X-rays discovered by Barkla 

 (Phil. Mag., vo\. xvii. [1909], p. 749) and beautifully 



NO. 2535, VOL. lOl] 



tvpe of absorption which is 

 not due either to resonance 

 or to free electrons. But 

 these are the only types of 

 absorption which are recog- 

 nised in the structure of 

 modern optics. , We have 

 as yet no way of conceiv- 

 ing of this new type of 

 absorption in terms of a 

 mechanical model. 



There is one result, 

 however, which seems to 

 be definitely established 

 by all this experimental 

 work. Whether the radia- 

 tion is produced by the 

 slopping of a free elec- 

 tron as in Duane_ and 

 Hunt's experiment, and 

 presumably also in black- 

 body experiments, or by 

 the absorption and re-emis- 

 sion of energy by bound 

 electrons, as in photo- 

 electric and spectroscppic 

 w-ork, Planck's h seems to 

 be always tied up in some 

 way with the emission and absorption of energy by the 

 electron, h may, therefore, be considered as one of 

 the properties of the electron. 



The new facts in the field of radiation which have 

 been discovered through the study of the properties of 

 the electron seem, then, to require in any case a 

 fundamental revision or extension of classical theories 

 of absorption and emission of radiant energy. The 



Rbsor^-tion V 



R bsOT^tt'tovt i>^ 

 T\bSor|vii«nvn. 



By* 



W 



La 



h' 



If'! 





m jMii^ 



Atsoijitiorvin 



Absorption of uranium and thorium in region of L-radiations 



Thomson-Einstein theory throws the whole burden of 

 accounting for the new facts upon the unknown nature 

 of the aether and makes radical assumptions about its 

 structure. The loading theory leaves the aether as it 



' The'^e photographs will be found also in the August, 1917, number of the 

 Physical Rcviczv(%ei-prc<\Aent\A\ address of the president. of the Physical 

 Society). 



