424 



NATURE. 



[December 12, 1912 



Prof. Bragg emphasised the corpuscular nature of 

 other thing-s, e.g. of X-rays. A number of other 

 speakers also took part. 



Dr. Lindemann, in his reply, pointed out that Pier 

 and Bjerrum have shown that the molecular heat of 

 the diatomic gases rises above the value sR/2 at high 

 temperatures, and may be represented by the formula 

 5R/2 + R/(i',T), where /( ) is the same formula, as 

 that used for solids. The frequency ^i-) coincides with 

 the absorption bands in the case of som^ of the gases 

 with charged ions (HCl, H,0, NH„ &c.). rh° fact 

 that the molecular heat of hydrofjen falls as iow ns 

 3R/2 at the temperature of liquid air is far more 

 difficult to explain even on the assumption of quanta, 

 for the rotation takes place without potential energy 

 in this case, and one would naturally expect the 

 frequency to vary with the temperature, which would 

 lead to a much more gradual diminution of the mole- 

 cular heat than is actually the case. Personally he 

 was doubtful of the validity of Poincar^'s proof that 

 one must assume a discontinuity to obtain Planck's 

 formula. He considered it premature to construct a 

 model ; it is necessary first to find the conditions 

 which a model must fulfil. 



The most difticult fact to account for is the large 

 conductivity for heat of crystals at low temperatures 

 where the energy fall may be very small. Models 

 which are based upon electrons being ejected are of no 

 use, as thev do not explain why an uncharged 

 diamond atom does not start vibrating when struck by 

 an uncharged helium atom. The agreement between 

 calculation and e.xperiment can scarcely be regarded as 

 fortuitous, for there are no arbitrary constants in the 

 formulae. He did not think that the theory of free 

 electrons in metals can be retained in its present 

 shape. The calculation of Wiedemann-Franz's con- 

 stant is based on the assumption that the free elec- 

 trons have the mean kinetic energy of a monatomic 

 gas at the same temperature. Planck's radiation 

 formula would seem to lead to about one-third of this 

 value, but this would make the thermal conductivity 

 three times too small. It is noteworthy that the elec- 

 tric resistance annears to be very nearly proportional 

 to the energy content, becoming independent of the 

 temperature at low temperatures. The electric con- 

 ductivity of very pure quicksilver is 100,000 times 

 greater at the temperature of liquid helium than it 

 is at 0° C. 



If one assumes the number of free electrons to be 

 very small, but to contain the energv of a monatomic 

 gas, the electrical conductivity shows that the mean 

 free path must be very large, and one comes into 

 conflict with optical measurements, more particularly 

 with those of Hagen and Rubens. He would be very 

 loathe to accept the theory of corpuscular radiation. 

 .All arguments in its favour are valid also for light. 

 The phenomena of interference show that a quantum 

 of light might be 500,000 wave lengths or 25 cm. 

 long. What would become of it if it were cut in two? 

 He believes that Planck's second hypothesis of con- 

 tinuous absorption and discontinuous emission is able 

 to account for the chief difficulty, viz. that a compara- 

 tively fast electron may be emitted under the influence 

 of comparatively weak radiation. 



Specirosc.oi>y. 

 .\ third discussion was opened by Dr. J. W. Nichol- 

 son on series in spectra. The opener gave a general 

 account of the work which has been done in the 

 representation of spectra by formulae, and followed 

 with a review of the attempts made to obtain these 

 formula from model atoms. Ho concluded that the 

 Ritz formula cannot represent the actual facts, and 

 that Hicks's modifjcntion and Whittakcr's formula are 



NO. 2250, VOL. go] 



difficult to interpret physically. A modern theory 

 apparently must build up the atom from electrons and 

 positive electricity — the latter, from work on radio- 

 activity alone, being densely concentrated at the centre 

 of the atom. The electrons must be arranged in 

 rings to avoid excessive radiation (Schott), and the 

 atom is Saturnian. The necessary permanence of J 

 structure can be secured by allowing expansion ofl 

 electrons, or by a quantum theory, which is prefer-' 

 able. It has been shown that it is possible to explain 

 the coronal and nebular spectra by simple ring systems 

 with a quantum theory which implies a definite change 

 of energv when an electron enters or leaves an atom. 

 The spectra of such elements do not exhibit the usual 

 series, but a series in which the cube-roots of the wave 

 lengths differ by a constant amount. This is in 

 accordance with a radiation of energy in discrete 

 amounts proportional to the frequency. The difficulty 

 in explaining Balmer's series is that in dynamical 

 systems it is the square of the frequency, not the 

 frequency itself, which is a rational function of in- 

 tegers. This difficulty is absent from the model of 

 Ritz; but it is more probable that the origin of spectral 

 series is kineoiatical (as Rayleigh has suggested) 

 rather than dynamical. A process was sketched bv 

 which a series of lines 



X = X„. 



., ((? = const.) 



can be obtained for an atom with two rings of 

 electrons by simple kinematical principles. If the 

 outer ring contains only one electron the lines are 

 doublets. The infinite number of lines is due to the 

 infinite number of degrees of freedom of the asther. 



In the discussion. Prof. Kayser said that the first 

 thing is to get a theor}' — none are quite right. Ritz's 

 is approximately right, but it cannot be the true one, 

 because it leads to too large a number of components 

 in the Zeeman effect. Hicks lays too much value on 

 the accuracy of the measured wave lengths. We 

 cannot hope to get the true wave length to one- 

 thousandth Angstrom unit. For example, some lines 

 in the iron spectrum are variable, being different at 

 the positive and negative poles. Their character 

 depends upon the conditions — Fabry obtains interfer- 

 ence effects with them, while he himself cannot. In 

 mixtures some calcium lines have a different wave 

 length from those of the pure substance. 



Prof. Fowler added the name of Halm to those 

 whose formulas required recognition. With regard to 

 accuracy, half a tenth-metre error could arise from 

 nebulositv ; this can, however, be reduced by altering 

 the conditions. 



Prof. Peddie suggested the investigation of a more 

 complicated system consisting of a succession of shells 

 of alternate positive and negative character. He 

 pointed out that if you make the atom rotate you 

 must also take into account the magnetic forces. 



Lord Ravleigh said that one point that was usually 

 omitted was the difference between the vibration in 

 the atom and that received by the observer. He 

 instanced the case, in sound, of a revolving vibrating 

 cylinder. The observer receives ma.xima and minima 

 as the cylinder revolves — that is, there are two vibra- 

 tions giving rise to beats. 



Dr. Duffield discussed the pressure effect as a means 

 for the resolution of a spectrum into series. Photo- 

 graphs were exhibited showing the different behaviour 

 of spectral lines under nressure, thus facilitating their 

 grouping. Prof. Kayser said he doubted if it Is a 

 pressure effect. Dr. King had found that in the elec- 

 tric furnace he gets a double amount, which shows 

 that other conditions enter into account. He did not 

 think it possible so far to determine series by this 



