OcrouicR 8, 1896] 



NA TURE 



565 



to ihi> very subject, he wrote as follows : " Go on ami prosper, 

 llieri.- is nothing in the world of science half so good as an earlh- 

 «liiake hypothesis, even if it only serve to show how firm are the 

 Inundations on which wc build." I have given you the earth- 

 cjuake hypothesis ; it is for those of you who oppose my con- 

 clusions, 10 prove the firmness of your foundations. 



PHYSICS AT THE BRITISH ASSOCIATION. 



piCKIIAl'S Section A does not discuss the question of science 

 teaching in schools so often as Section B does. But the 

 many teachers of science who listened to the address of the 

 President (I'rof. J. J- Thom.son) on Thursday, must have 

 heard with pleasure the testimony of so competent an authority 

 that the teaching of physical science in schools has greatly im- 

 proved in recent years. \'ery welcome, too, was his advice as 

 to the importance of e.xperiment.il work and method in teaching, 

 and his warning as to the danger of trying to cover too much 

 ground. The Section was favoured with the presence of physicists 

 from various foreign countries, iiicUuling Profs. Kohlrausch 

 (Director of the Reichsan.stalt), Lenard (Aachen), Bjerknes 

 (Stockholm), J. E. Keeler (Pittsburg), Max Wolf (Heidelberg), 

 and Elsier and Geitel (Wolfenbiittel). The mention of Prof. 

 Lenard's name in the President's address was the signa for very 

 hearty applause. 



After the President's address, the Section adjourned from the 

 Arts Theatre to the Physics Theatre. The Report of the 

 Committee on the Establishment of a National Physical Labor- 

 atory was presented by Sir Douglas Gallon, who gave details 

 of the cost of the Reichsanslalt, where the capital e.\penditure 

 lias amounted to ^200,000, and the annual working expenses 

 are ^^ 14. 500. The Committee was appointed last year (see 

 Nature, September 26, 1895) to consider — or rather to 

 reconsider — a suggestion made by Prof. Oliver Lodge at the 

 O.xford meeting. It now proposes that the Kew Observatory 

 at Richmond be extended so as to include a nucleus for the 

 suggested National Physical Laboratory, and that the Goxern- 

 nient lie apjiroached with a request for the modest sum of about 

 /"20,ooo f.jr buildings and equipment, and ^^3000 per annum for 

 maintenance. The control of the laboratory should be vested 

 in a Council of Advice app(jiiUed by the Royal Society, either 

 alime(like the present Kew Committee) or in conjunction with 

 one or more of the chief scientific bodies in (he country ; but 

 the immediate executive and initiative power should be vested in 

 a paid chief or director of the utmost eminence attainable. The 

 functions of the institution would include an extension of certain 

 branches of work now performed by the Kew Observatory, such 

 as the verification of standards and comparisons of length, 

 weight, capacity, gravity, sound, light, &c., and variations of 

 conditions due to temperature, vibrations, or other causes, as 

 well as quality of materials. K.esearch work of the following 

 types should also be undertaken : (I) observations of phenomena, 

 the study of which must be prolonged through periods greater 

 than the average duration of life; (2) testing and verification 

 of instruments for physical investigation, and the preservation of 

 standards for reference ; (3) systematic determination of physical 

 constants and numerical data which may be useful for scientific 

 or industrial purposes. In the discussion which followed, Lord 

 Kelvin, Profs. Lodge, Ayrton, I'itzgerald, Riicker, and S. P. 

 Thompson, .and the Director of the Reichsanslalt took part. Dr. 

 Isaac Roberts read a paper, in which he dealt with the analytic 

 and synthetic methods of tracing the evolution of stellar systems. 

 Very beautiful photographs of stars and nebula; (taken with 

 about four hours' exposure) were [irojected on the screen. Prof. 

 Ci. II. Darwin read a jmper on |)eriodic orbits, which Lord 

 Kelvin characterised as a monument of skilful and painstaking 

 calculation. In the afternoon I'rof. McKendrick gave a most 

 interesting demonstration of a method of transcribing wave 

 forms from a phonograph cylinder to paper, with other experi- 

 ments illustrating his researches on the phonograph. During 

 the week a number of instruments and exhibits were on view in 

 the physical laboratory. These included the apparatus with 

 vvhich Dr. Lotlge has sought to determine whether a moving 

 body sets the ether in its neighbourhood in motion ; X-ray tubes 

 and photographs taken with them ; a large influence machine ; 

 Prof. Lodge's electrostatic model ; and Mr. Barlow's model 

 illustrating the nature of homogeneity in crystals. 



On I-'riday there was a joint discussion with Section B on 

 Rontgen rays and allied phenomena. The interest felt in 



NO. 1406. VOL. 54] 



these was evidenced by the large attendance, many members 

 having to content themselves with seats in the gallery of the 

 large lecture theatre of University College. The subject was 

 appropriately introduced by Prof. P. Lenard, who described his 

 researches on kathode rays, and his views as to their nature. 

 The separation of these rays was made possible by Hertz's 

 discovery that they can pass through thin plates or films, e.g. of 

 aluminium. Lenard's discharge tube had an aluminium window 

 at the end opposite the kathode. Aluminium is a turbid medium 

 for these rays, so that in p.issing through the window they are 

 diffused. They are almost invisible in air, which is only very 

 feintly illuminated by them ; they are also largely absorbed by 

 air, so that their intensity diminishes very quickly. If the tube 

 is continued beyond the aluminium window, and the pressure 

 of the air in this second chamber is reduced, the rays travel 

 much further. This favours the view that they are not due to 

 projected matter, but are of the nature of ether-waves. By 

 placing a second screen with a diaphragm beyond the aluminium 

 window a more distinct beam is obtained, and this is allowed to 

 fall upon a phosphorescent screen. By introducing plates of 

 metal in the path of the rays, it is found that their opacity is 

 roughly proportional to their superficial density (gm. per sq. 

 cm.). The same is true for gases ; air can be made as trans- 

 parent as hydrogen by reducing its pressure. Kathode rays 

 exhibit differences in degree analogous to those of differently 

 coloured lights ; these differences can be exhibited by varying 

 the pressure in the discharge tube, and observing the different 

 amounts of deflection produced by a magnet. By reducing the 

 pressure we get less deflectible rays ; these are the least absorbed 

 by ordinary matter, and are the easiest to investigate. X-rays 

 are of this nature ; they travel easily through air, and may be 

 regarded as kathode rays which can only be very slightly affected 

 by a magnet. They were probably present in Lenard's experi- 

 ments, but must have been very faint, for the aluminium window 

 was small and (on account of the pressure) not very thin. Prof. 

 Lenard subsequently exhibited his experiments in the physical 

 laboratory. 



In the discussion which followed. Sir George Stokes main- 

 tained the view that the rays are due to projected matter. The 

 inside of the aluminium window is bombarded by molecules of 

 gas or by particles discharged from the electrode. Why should 

 not this bombardment give rise to a corresponding projection of 

 molecules from the outside of the window ? It is not necessary to 

 suppose that they pass through the window. We have an analogue 

 in the electrolysis of copper sulphate between copper electrodes. 

 If a third (idle) electrode is introduced between them, we find that 

 copper ions are deposited on one side of it and removed from the 

 other. The absence of diffraction effects and other properties 

 favour the view that X-rays are due to sudden and non-periodic 

 disturbances. Prof. Fitzgerald congratulated Prof. Lenard on 

 his skilful investigation, and pointed out that, whereas Rontgen' s 

 experiments had soon been repeated by hundreds of observers, 

 Lenard's earher experiments were of such a difficult nature that 

 no one had since repeated them. Although Hertz held that the 

 kathode rays were due to ethereal vibrations, his own suggestion 

 that their deflection by a magnet may be analogous to the Hall 

 effect tells against this view ; for the Hall effect only occurs 

 when matter is present. Again, Hertz found support for his views 

 in his remarkable discovery that a magnet was not deflected by 

 kathode rays. He does not seem to have considered that corre- 

 sponding to the direct conduction current in the tube there 

 must be a reverse convection current outside. Would not this 

 back current neutralise the effect of the first convection current ? 

 Or the explanation may simply be that the effect upon a magnet 

 is so slight that we cannot detect it. Prof. J. J. Thomson gave 

 an account of experiments, made by himself and Mr. E. 

 Rutherford, on the laws of conduction of electricity through 

 gases exposed to the Rontgen rays. These rays convert gases 

 into conductors, and the gas retains its conducting power for some 

 time after the rays have ceased to pass. When the gas is forced 

 through a wire gauze or muslin strainer into another vessel, it 

 still conducts ; but filtering through glass wool removes the 

 conducting power, and so does bubbling through water. It is 

 remarkable that the passage of a moderate electric current 

 through the gas entirely destroys the conductivity ; even very 

 .small currents reduce it considerably. This seems to indicate 

 that the conduction is electrolytic. A theory based on this 

 assumption has been tested by quantitative measurements, and 

 the results are in satisfactory accordance with the theory. For 

 an E. M.F. of I volt per cm. the ionic velocity is between I mm. 



