NATURE 



{Feb. 14, .1889 



The fact that, in the kinetic theory of gases, Boltz- 

 mann's extension of Clerk Maxwell's theorem regarding 

 the distribution of energy in a system of colliding spheres 

 has led to results which are inconsistent with either the 

 observed value of the ratio of the specific heats of a gas, 

 or the complexity of the radiation spectra of gases, has 

 induced Prof. Tait to investigate the question afresh from 

 the very commencement. His results are contained in 

 papers " On the Foundations of the Kinetic Theory of 

 Gases," published in the Transactions. His great aim is 

 the simplification of the mathematical treatment of the 

 subject, and he has '"purposely gone into very minute 

 detail in order that no step taken, however slight, might 

 have the chance of escaping criticism, or might have the 

 appearance of an attempt to gloss over a difficulty." 

 Abstracts of the papers have appeared in the Philosophical 

 Magazine. The subjects treated in the first two papers 

 are as follows : one set of equal spheres ; mean free 

 path among equal spheres ; number of collisions per 

 particle per second ; Clerk Maxwell's theorem ; rate of 

 equalization of average energy per particle in two mixed 

 systems ; mean path in a mixture of two systems ; 

 pressure in a system of colliding particles ; effect of 

 external potential ; pressure in a mixture of two sets 

 of spheres ; viscosity ; thermal conductivity ; diffusion. 

 On the suggestion of Prof. Tait, Prof W. Burnside applied 

 a method used by the former in his first paper to the 

 problem — Given a very great number of smooth elastic 

 spheres, equal and like in all respects, whose centres of 

 figure and centres of inertia do not coincide, and the sum 

 of whose volumes is but a small fraction of the space in 

 which they move, it is required to find the ultimate 

 distribution of energy among the various degrees of 

 freedom when by collisions the system has attained a 

 " special state." Prof Burnside's result is given in a paper 

 ." On the Partition of Energy between the Translafory and 

 'Rotational Motions of a Set of Non-Homogeneous Elas- 

 tic Spheres." The various steps of the investigation are 

 .given in detail, and the assumptions are clearly specified. 

 The conclusion is that the average energies of rotation of 

 a sphere about each of the three principal axes are equal, 

 and the whole average energy of rotation of a sphere is 

 .twice the average energy of translation. 



Sir W. Thomson communicated sixteen papers to the 

 Royal Society during the period under consideration. 

 •These include a remarkable series of investigations of 

 various cases of fluid motion. Most of the papers have 

 already appeared in the Philosophical Magazine or else- 

 where ; only four are given in detail in the Proceedings. 

 These are " On a New Form of Portable Spring Balance 

 for the Measurement of Terrestrial Gravity," "Cnthe 

 Front and Rear of a Free Procession of Waves in Deep 

 Water," " On the Equilibrum of a Gas under its own 

 Gravitation only," and " On Stability of Fluid Motion : 

 Rectilinear Motion of \'iscous Fluid between two Parallel 

 Planes." From results obtained in the latter paper. Sir 

 William considers it probable " that the steady motion is 

 stable for any viscosity, however small ; and that the 

 practical unsteadiness pointed out by Stokes forty years 

 ago, and so admirably investigated experimentally five or 

 six years ago by Osborne Reynolds, is to be explained by 

 limits of stability becoming narrower and narrower the 

 smaller is the viscosity." 



The following extract will give an idea of the scope of 

 a paper by Lord Rayleigh, '• On the Colours of Thin 

 Plates ";^" The theory set forth so completely in our 

 treatises tells us indeed how the composition of the light 

 reflected depends upon the thickness of the plate, but 

 what will be Us colour cannot, in most cases, be foretold 

 without information of an entirely different kind, dealing 

 with the chromatic relations of the spectral colours them- 

 selves. This part of the subject belongs to physiological 

 optics, as depending upon the special properties of the 

 eye. The first attempt to deal with it is due to Newton, 



who invented the chromatic diagram, but his representa- 

 tion of the spectrum is arbitrary, and but a rough ap- 

 proximation to the truth. It is to Maxwell that M'e owe 

 the first systematic examination of the chromatic relations 

 of the spectrum, and his results give the means of predict- 

 ing the colour of any mixed light of known composition. 

 Almost from the time of first reading Maxwell's splendid 

 memoir, I have had the wish to undertake the task of 

 calculation from his data the entire series of colours of 

 thin plates, and of exhibiting them on Newton's diagram. 

 The results are here presented, and it is hoped may interest 

 many who feel the fascination of the subject, and will be 

 pleased to see a more complete theory of this celebrated 

 series of colours." 



A little note by the (then) Astronomer-Royal for Scot- 

 land on Brewster's line Y in the infra-red' of the solar 

 spectrum is illustrated by an exceedingly instructive plate. 

 The line Y is one of three discovered by Brewster in a 

 portion of the spectrum usually invisible. As the Astro- 

 nomer-Royal well remarks, " It was Brewster's eye that 

 looked ; so no wonder that he saw with it more than any 

 of his predecessors, and most of his successors as well." 

 Prof Smyth proves that Y is a true solar line, being more 

 distinct with a high than with a low sun. In the Royal 

 (London) Society's publications the opposite had been 

 asserted ; and ultimately, though perhaps unintentionally, 

 the line was altogether omitted from the spectrum^or, 

 rather, it was misnamed Z. It is to be hoped that Prof. 

 Smyth's paper will have the effect of removing all the 

 ambiguity, and the consequent confusion, on this point. 

 In this note, short as it is, we have evidence of the un- 

 looked for rewards which often await the scientific obser- 

 ver. While seeking for direct confirmation of M. Henri 

 Becquerel's dscovery that the line Y is due to sodium,. 

 Prof Smyth discovered three new air-lines much farther 

 in the infra-red than any previously discovered. 



Prof. Piazzi-Smyth has also a paper on " Micrometrica) 

 Measures of Gaseous Spectra under High Dispersion." 

 The paper is accompanied by maps of the spectra in- 

 vestigated on the scale of 40 feet to the visible spectrum. 

 The prism arrangement gave a dispersion of 60' from A 

 to H, and the magnifying powers of the telescope varied 

 from 12 to 36 with a further magnifying power of 5 in 

 the recording apparatus. There was thus in effect a 

 possible dispersion of 9000^. The gases dealt with are 

 CH, CO, H, O, and N. The peculiar arrangement of the 

 leading hnes (usually two) and train of linelets in each of 

 the five bands of the CH spectrum is fully shown in the 

 diagram and described in the text. The low-temperature 

 (simple spark) spectrum of hydrogen is given, over 1600 

 lines being recorded. Prof Smyth has found that three of 

 the four principal oxygen lines are triple, and he has dis- 

 covered three other such triplets. The remarkably methodi- 

 cal spacing of the lines in these triplets and of the triplets 

 themselves is noted : but a more striking example of 

 regular spacing is furnished by the lines in the green CO 

 band. The map of this band is given on the scale of 

 120 feet to the visible spectrum. Prof A. S. Herschel has 

 made out two series of lines (single and double respec- 

 tively) following the same law of arithmetical progression. 

 The full interpretation of such " natural writing ' may 

 possibly never be obtained until we know the nature of 

 molecules and atoms : but, on the other hand, a further 

 investigation of spectra such as we have here may throw 

 some light on molecular and atomic structure. So mag- 

 nificent are the maps that accompany Prof. .Smyth's 

 paper, that one is rather inclined to speak of his contribu- 

 tion to the. Transactions as a series of maps of spectra 

 with explanatory te.xt. But besides explanations the paper 

 contains a full record of the experiments, and also tells a 

 tale of high war — the combatants being the author on the 

 one hand, and the London Royal Society, the London 

 Royal Astronomical Society, et hoc genus oiiinc, on the 

 other. Vacuum-tubes containing nominally CO, but alsa 



