November ii, 1920] 



NATURE 



357 



Physics at the British Association. 



THE programme of Section A included papers of 

 wide and varied interest, ranging over the sub- 

 jects of pure mathematics, experimental physics, geo- 

 physics, and astronomy. A great deal ol the time of 

 the Section was absorbed in atomic problems, and 

 it was in relation to these that much interesting 

 discussion arose. 



Dr. Aston gave a concise and comprehensive account 

 of his work on isotopes, starting from his original dis- 

 covtry of the complex nature of neon and chlorine, 

 and he spoke of his early attempts to separate the 

 components of these gases. He described liis very 



iegant modification of Sir J. J. Thomson's method 

 uf [K)sitive-ray analysis, by which it has now become 

 possible to obtain mass spectra of the rare gases and 

 many other elements with high dispersion of the com- 

 ponent lines. He shosved how the spectra of various 

 elements could be analysed into groups of lines due 

 to the individual isotopes, and the results interpreted 

 h\ examining the spectra of different orders produced 

 by atoms carrying multiple charges. In this way 

 it has been possible to eliminate uncertainties arising 

 from radiations consisting of compound molecules 

 and to determine the number of components due to 

 ■ .ich element. Thus it was shown that chlorine con- 



isted of three isotopic components, krypton of as 

 many as six, and xenon of five, corresponding with 



itomic weights represented by whole numbers, taking 

 oxygen of atomic weight i6 as standard. Hydrogen 

 alone gave an atomic weis;ht of 1008, differing from 

 an integral value, and this discrepancy could be ex- 

 plained by considering that the spectrum of hydrogen 

 was due to a hydrogen atom from which an electron 

 had been withdrawn, and which from theoretical con- 

 siderations should have a mass differing by the 

 observed amount from that of the^ hydrogen atom. 

 The results thus show that the elements may be con- 

 sidered as being composed of these hydrogen nuclei, 

 or " protons " as Sir Ernest Rutherford would have 

 us call fhem, and we thus return to Prout's concef)- 

 tion of the constitution of matter, modified onlv by 

 the recent discoveries and ideas of modern physics. 



Sir Ernest Rutherford follo.wed with an account of 

 his researches on the structure of the atom, starting; 

 from the point of view of radio-activity. When 

 o-partirles pass through matttT thev arc scat- 

 tered, and when they pass sufficiently near to 

 the atomic nucleus they mav even be turned back 

 upon themselves. In such cases, at any rate with 

 the lighter elements, the forces involved arc 90 

 enormous that the nucleus may suffer disruption and 

 charged hydrogen atoms, or protons," be torn 

 from the nucleus. Hydrogen atoms travelling with 

 high velocities thus appear, and can be detected by 

 their scintillations produced on n fluorescent screen. 

 These have been found when nitrogen is bombardied 

 with a-parficles, as have elementary projected particles 

 of atomic weight •?, probably an isotopic form of 

 tvlium. The inve.tigation has been extended to other 

 I lements, and it would appear that the nuclei of 

 atoms of the lighter elements ran be regarded as 

 nude up of suitable combinations of hydrogen arxl 

 this new isotope of helium with electrons. In flie 

 heavier elements It would seem that a condensation 

 (K-curs bv which is formed the ordinan- helium atom 

 of mass 4. Models were shown illusfr.'iting the 

 possible constitution of some of the lighter nuclei, but 

 the complete elucidation of this sucgesiive and in- 

 t'enioi!'" II".. '^f tbdiight must await further exp«'ri- 

 ment' 



vo 



r,. Tofi] 



The discussion of the origin of spectra directed 

 attention to other scarcely less important aspects of 

 atomic phenomena. Prof. Fowler opened the discus- 

 sion with a masterly description of the known pheno- 

 mena of spectroscopy, referring to the latest results 

 obtained in examining and classifying the different 

 spectral series of the elements, leaving the considera- 

 tion of the theories which have been devised to 

 explain the observations to Prof. Nicholson, wiio 

 described Bohr's well-known tlieory of atomic radia- 

 tion. This simple view is, however, insufficient to ex- 

 plain the complicated structure of the lines composing 

 the series, and Prof. Nicholson outlined the extension 

 of Bohr's theory recently developed bv Sommerfekl, in 

 which the electronic orbits are considered as elliptical 

 instead of circular. By this extension the relations 

 obtained by Bohr are modified so as to explain the 

 structure of the components of the various spectral 

 series, and the predictions of theory have been strik- 

 ingly verified by the work of Paschen, whose observa- 

 tions also indicate that the Zeenian and Stark effects 

 are of the magnitude to be expected by theory. Prof. 

 VV. L. Bragg directed attention to the difikulty of 

 reconciling the above theory with X-ray observations 

 on crystals and with the chemical evidence leading to 

 Langmuir and Lewis's theory of atoms containing 

 stationary electrons; and Dr. Oxley in a separate 

 paper pointed out the bearing on the question of the 

 magnetic properties of the atom — a subject which has 

 hitherto not received the attention it deserves. From 

 the magnetic evidence Dr. Oxley postulates a binding 

 of the atoms in the hydrogen molecule by a rotating 

 electron system — a complication which, it is to be 

 hoped, will find some simpler substitute. 



The subject of relativity was represented by two 

 papers, one by Mr. Evershed and the other by Sir 

 Oliver Lodge. The former paper was concerned' with 

 the observations made during the last seventeen years 

 sX the Kodaikanal Observatory on the shift of the 

 Fraunhofer lines in the solar spectrum. The con- 

 clusion is reached that the general shift of the lines at 

 the centre of the sun's disc and at the limb is not due 

 to pressure, and it is suggested that the increase of 

 shift in passing from the centre of the disc to the 

 limb may be explained bv a constant shift towards 

 red over the disc, which is partly compensated bv a 

 shift towards violet, due to a movement of ascent 

 radial to the sun. \ comparison w.is drawn between 

 solar phenomena and the results of observations in the 

 electric arc, and the experiments of Royds wen- 

 quoted as showing that the vapour density in the sun 

 is probably less than is found at the centre of an 

 iron arc. Mr. St. John's measurements on band lines 

 were discussed and compared with observations made 

 at Kodaikanal, which give values at the sim's limb 

 nearly in agreement with the Einstein theory. ThiTe 

 are, however, difficulties involved in fully interpreting 

 the results on this theory, and the .alternative hypo- 

 thesis that motion is the sole cause was considered. 

 This view demands an earth effect and a general 

 recession of the iron vapour from the earth. Mr. 

 Evershed described his ingenious experiments in which 

 the displacements were observed bv examining the 

 light reflected from Venus. When the angle Venus- 

 sun-earth is about qo° such observations should l)e 

 crucial, for in this case we should be observing the 

 sun at right angles to the supposed movement. The 

 results are regnrde<l as being favourable to the motion 

 hvpothesis. hut it cannf)t he c on«.iderrd that fhe^- are 

 as vet decisive. 



