536 



NATURE 



[April 7, 1898 



LETTERS ^ TO THE EDITOR 



[The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.^ 



Misleading Applications of Familiar Scientific Terms. 



May I, not as an expert in science, but as one who has made 

 some research into the conditions of lucidity, venture to thank 

 you for the protest which appears in your current number 

 against a misleading application of the familiar term " Light" ? 

 This is not of course the only instance of the kind ; but it seems 

 especially regrettable as tending, by the very success and popu- 

 larity of the Lectures reviewed, to introduce gratuitous confusion 

 into youthful minds. 



I may perhaps be pardoned for adding that I was fortunate 

 enough in my little book, " Grains of Sense," published last 

 year, to anticipate the verdict of your reviewer, and to point 

 out how much, in this and similar cases, such modes of ex- 

 pression on the part of scientific men tend on the one hand to 

 diminish our precious and too slender store of clearness of 

 thought, and on the other to hinder the progress of science 

 itself. V. Welby. 



April I. 



The Kinetic Theory and Radiant Energy, 



In the course of the discussion which took place in your 

 columns during the winter of 1894-5 o" the kinetic theory of 

 gases, emphasis was rightly laid on the difficulty of reconciling 

 the law of partition of energy among the different degrees 

 of freedom of molecules of gases with the large number of 

 such degrees of freedom indicated by their spectra, and, 

 generally, of explaining, on the kinetic theory, the relations 

 between matter and the ether required to account for 

 radiation. It was even suggested, by one writer, that the ether, 

 with its vastly larger number of degrees cf freedom, must 

 ultimately absorb all the energy of the molecules. I instanced 

 the case of a sphere moving in an infinite mass of perfect liquid 

 as exemplifying a system where no such ultimate absorption of 

 energy would take place, and pointed out that everything de- 

 pended on the laws according to which transference of energy 

 took place between the molecules and the ether. 



The object of this letter is to show that the subsequent 

 discovery of the Rontgen rays has suggested a theory of the 

 radiation of heat which may possibly throw considerable light 

 on the difficulties referred to by affording an answer to the 

 question, " If the temperature of a gas is proportional to the 

 mean translational kinetic energy of the molecules, how comes 

 it that this kinetic energy can be transferred from one set of 

 molecules to another by radiation through the ether?" 



Consider the Rontgen rays : we know, firstly, that they are 

 produced by the impact of the kathodic rays on the Crookes' 

 tube, these latter consisting not improbably of streams of 

 bombarding molecules ; secondly, that they not only have the 

 power of discharging electrified bodies, but also of modifying 

 the electrical state of gases in such a way as to enable these to 

 discharge bodies. In this modified air, to which Villari has 

 applied the somewhat barbarous name of "aria Xata" or 

 "xd. air," some kind of dissociation of the electrons must 

 necessarily have taken place. 



Arguing from analogy the idea suggests itself that the 

 encounters between molecules of a gas, no less than the kathodic 

 bombardments, may give rise to radiations, and these, too, 

 when falling on another mass of gas may modify the electrical 

 state of its molecules in such a way that their original electrical 

 state is only restored by encounters between them. 



Now taking, as a simple illustration, two oppositely electrified 

 perfectly elastic conducting spheres ; as these approach one 

 another, they acquire kinetic energy in virtue of their attraction. 

 On coming into contact they are discharged and the attraction 

 ceases, so that their kinetic energy of separation is greater 

 than that which they had previously to coming within each 

 other's influence. Again, when a charged and an uncharged 

 body impinge, the charge is distributed between them ; they 

 repel one another as they separate, and again acquire an increase 

 of kinetic energy — as in the ordinargy pith-ball experiment. 



NO. 1484, VOL. 57] 



It follows that the incidence of rays possessing the property 

 suggested above will tend to increase the temperature of a gas. 



The discharge which takes place at an encounter will, how- 

 ever, be an oscillatory one, and will lead, therefore, to further 

 generation of undulatory rays. 



Considering two masses of gas at unequal temperature, the 

 impacts in the hotter gas, being the more frequent and violent, 

 will give rise to the more copious emission of rays, and these 

 falling on the cooler gas, will produce the greater electric dis- 

 sociation resulting in the greater acquisition of kinetic energy 

 in collisions between the molecules. The feebler rays from the 

 colder gas will have less effect on the molecules of the hotter 

 one, and the kinetic energy supplied in this way will not 

 compensate for that lost by radiation. Thus the "theory of 

 exchanges " will hold good. 



A still more important consequence of such a theory is that 

 no interaction will take place between the ether and molecules 

 except where there are encounters between the latter, and, 

 moreover, the interactions which occur in an isolated mass of 

 gas will not affect the translational velocity of its centre of mass, 

 nor the angular momenta about axes through its centre of mass. 

 Thus it results that the celestial bodies go on in their course 

 experiencing no resistance whatever from the ether. 



On the other hand, the fact that light from distant stars is not 

 absorbed before it reaches the earth, no longer implies the com- 

 plete absence of matter in interstellar space. Isolated molecules 

 will absorb no energy from the ether ; and so long as the mole- 

 cules moving about in interstellar space are assumed to be so 

 few and far between that collisions practically never occur, 

 there will be nothing to impede the passage of light or heat 

 rays. It is only when such rays fall on assemblages of mole- 

 cules sufficiently dense to possess the attributes of what we call 

 matter— an, for example, when they reach our atmosphere — 

 that absorption of energy will take place. 



The phenomena of irreversibility and of degradation of 

 energy would thus, so far as the present view goes, be re- 

 stricted to material bodies, and hence the conditions necessary 

 for the existence of life on our earth may have been brought 

 about without the enormous waste of energy which would be 

 required in the absence of some such theory. 



A photo-voltaic theory of photographic action formed the 

 subject of exhaustive experimental investigation at the hands 

 of Herr Luggin last year, and photo-voltaic theories of vision 

 have also been proposed. It would thus seem that the analogy 

 between the action of heat rays, visible-light rays, ultra-violet 

 rays and Rontgen rays may be complete. The question still 

 remains, how are ethereal waves able to affect the electric state 

 of assemblages of molecules? But since Rontgen-ray physicists 

 have proved that they do this, the question has to be faced 

 in any case. It is now rendered no more difficult, and, on 

 the other hand, our theories of the relations between ether and 

 matter are simplified by referring radiation of heat to the same 

 phenomenon. G. H. Bryan. 



Note on Mr. Wood's Method of Illustrating Planetary 

 Orbits. 



I fear that Mr. Wood's beautiful method of illustrating 

 planetary orbits by means of a bicycle ball rolling on a glass 

 plate about the pole of an electro-magnet (Nature, April 29, 

 1897), has rather fallen into disrepute in the minds of many 

 physicists since its criticism by Mr. Anderson in Nature, May 

 13, 1897. Mr. Anderson there states that the law of attraction 

 in such a case would be that of the inverse fifth power of the 

 distance. This could only be true if the ball were of very soft 

 iron. A bicycle ball is far from this, and becomes strongly mag- 

 netised after brief use in the experiment, behaving like a per- 

 manent magnet of great coercive force. Under these conditions 

 the attraction between the pole and the ball will vary approxi- 

 mately as the inverse third power. There is also another factor 

 to be considered. If the true pole lies below the glass plate, 

 only a certain component of the total force is active in producing 

 the attraction towards the centre of motion. To determine what 

 the law of variation of this component will be, I have had one of 

 my students take a number of series of observations on the 

 attraction of a bicycle ball along a plane perpendicular to the 

 axis of a magnet. 



In the experiments the magnet was horizontal, and the bicycle 

 ball with its magnetic axis vertical was fastened to one end of a 

 strip of spring brass, the other end of which was clamped fast in 



