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SCIENCE 



[N. S. Vol. XXX. No. 765 



this tube will be anchored and withdrawn 

 as a whole from the pencil of light incident 

 on the condenser. If the energy required 

 to charge up the condenser with a unit of 

 electricity is greater than the energy in the 

 incident parcel, the tube will not be an- 

 chored and the light will pass over the 

 condenser and escape from it. These prin- 

 ciples that radiation is made up of units, 

 and that it requires a unit possessing a 

 definite amount of energy to excite radia- 

 tion in a body on which it falls, perhaps 

 receive their best illustration in the re- 

 markable laws governing secondary Ront- 

 gen radiation, recently discovered by Pro- 

 fessor Barkla. Professor Barkla has found 

 that each of the different chemical ele- 

 ments, when exposed to Rontgen rays, emit 

 a definite type of secondary radiation what- 

 ever may have been the type of primary, 

 thus lead emits one type, copper another, 

 and so on; but these radiations are not 

 excited at all if the primary radiation is 

 of a softer type than the specific radiation 

 emitted by the substance; thus the sec- 

 ondary radiation from lead being harder 

 than that from copper; if copper is ex- 

 posed to the secondary radiation from lead 

 the copper will radiate, but lead will not 

 radiate when exposed to copper. Thus, if 

 we suppose that the energy in a unit of 

 hard Rontgen rays is greater than that in 

 one of soft, Barkla 's results are strikingly 

 analogous to those which would follow on 

 the unit theory of light. 



Though we have, I think, strong reasons 

 for thinking that the energy in the light 

 waves of definite wave-length is done up 

 into bundles, and that these bundles, when 

 emitted, all possess the same amount of 

 energy, I do not think there is any reason 

 for supposing that in any casual specimen 

 of light of this wave-length, which may 

 subsequent to its emission have been many 

 times refracted or reflected, the bundles pos- 



sess any definite amount of energy. For 

 consider what must happen when a bundle 

 is incident on a suf ace such as glass, when 

 part of it is reflected and part transmitted. 

 The bundle is divided into two portions, in 

 each of which the energy is less than the 

 incident bundle, and since these portions 

 diverge and may ultimately be many thou- 

 sands of miles apart, it would seem mean- 

 ingless to still regard them as forming one 

 unit. Thus the energy in the bundles of 

 light, after they have suffered partial re- 

 flection, will not be the same as in the 

 bundles when they were emitted. The 

 study of the dimensions of these bundles, 

 for example, the angle they subtend at the 

 luminous source, is an interesting subject 

 for investigation; experiments on inter- 

 ference between rays of light emerging in 

 different directions from the luminous 

 source would probably throw light on this 

 point. 



I now pass to a very brief consideration 

 of one of the most important and interest- 

 ing advances ever made in physics, and in 

 which Canada, as the place of the labors of 

 Professors Rutherford and Soddy, has 

 taken a conspicuous part. I mean the dis- 

 covery and investigation of radioactivity. 

 Radioactivity was brought to light by the 

 Rontgen rays. One of the many remark- 

 able properties of these rays is to excite 

 phosphorescence in certain substances, in- 

 cluding the salts of uranium, when they 

 fall upon them. Since Rontgen rays 

 produce phosphorescence, it occurred to 

 Becquerel to try whether phosphores- 

 cence would produce Rontgen rays. He 

 took some uranium salts which had been 

 made to phosphoresce by exposure, not to 

 Rontgen rays but to sunlight, tested them, 

 and found that they gave out rays possess- 

 ing properties similar to Rontgen rays. 

 Further investigation showed, however, 

 that to get these rays it was not necessary 



