326 



NATURE 



[August 4, 1892 



motion of matter and motion of the medium is a vital one, and 

 we shall probably not make any serious advances until experi- 

 ment has found a new opening. But vi'e must expect many 

 negative results before some clue is discovered. Nor can we 

 attach much importance to negative results unless they are made 

 by some one in whose care and judgment we place full reliance. 

 We should all the more, therefore, recognize the courage and 

 perseverance of those who spend their valuable time in such in- 

 vestigations as Prof. Lodge has recently undertaken. That 

 ultimately some relation will be found between moving matter 

 and electrical action there is no reasonable doubt. 



One of the most hopeful openings for new investigations has 

 always been found in the pursuing of a theory to its logical con- 

 clusions, and there is one result of the electromagnetic theory 

 of light which has not, in my opinion, received the share of 

 attention which it deserves. 



When sound passes through air it is propagated more quickly 

 with the wind than against it, and we may easily find the 

 velocity relative to the earth by combining the ordinary sound 

 velocity with the velocity of the wind. Similarly, when any 

 waves pass through a medium moving with uniform velocity, 

 the waves being due to internal stresses in the medium, we may 

 treat of the velocity of the waves independently of that of the 

 medium, and say that the wave-velocity in the direction of 

 motion of the medium, and relative to a fixed body, is the sum 

 of the wave-velocity, calculated on the supposition that the 

 medium is at rest and the velocity of the medium. Prof. J. J. 

 Thomson,^ applying Maxwell's equations, has arrived at a 

 different result for electromagnetic waves, and has come to the 

 conclusion that in order to get the velocity of light along a stream 

 of flowing water we have to add to the velocity of light only half 

 the velocity of water. The following considerations suggest 

 themselves to me with respect to this result. Maxwell's theory is 

 founded on certain observed effects, which all depend on the 

 relative motion of matter. A result such as the one referred to 

 implies actions depending on absolute motion, and appears there- 

 fore to point to something which has been introduced into the 

 equations for which there is no experimental evidence. The 

 only assumption clearly put down by Maxwell is that electro- 

 magnetic actions are transmitted through the medium, and it is 

 possible that that assumption necessarily carries Prof. J. J. 

 Thomson's result with it. If a careful examination of the 

 subject should show that this is the case, we are brought face to 

 face with a serious difficulty. It is said, with justice, to be one 

 of the great advantages of Maxwell's theory that it does away 

 with action at a distance ; but what do we gain if we 

 replace action at a distance by something infinitely more difficult 

 to conceive, namely, internal stresses of a medium depending 

 on the velocity of the medium through space ? I can only see 

 one loophole through which to escape, namely, that Maxwell's 

 medium is not homogeneous, but consists of two parts, and that 

 if we speak of the medium as moving, we mean the motion of 

 one of these parts relative to the other. 



While we may hope to obtain important results from an in- 

 vestigation of the relation between what we call electricity and 

 the medium, we must not lose sight of another avenue, namely, 

 the relation between electricity and chemical effects. The pas- 

 sage, of electricity through ga-^es presents us with a complicated 

 problem to which a number of physicists have given their at- 

 tention of late years. There seems no reasonable doubt that 

 electricity in a gas is conveyed by the diffusion of particles con- 

 veying high charges, probably identical with those carried by 

 the electrolytic ion. The fact that this convection is a process 

 of diffusion with comparatively small velocity is shown by the 

 experimental result that the path of the discharge is affected by 

 any bodily motion of the gas which conveys the current. Even 

 the convection currents due to the heat produced by the dis- 

 charge itself are sufficient to deflect the luminous column which 

 marks the passage of the current. 



The most puzzling fact, however, connected with the discharge 

 of electricity through gases consists in the absence of symmetry 

 at the positive and negative poles. There must be some differ- 

 ence between a positively and negatively charged atom which 

 seems of fundamental importance in the relation between matter 

 and what we call electricity. A discussion of the various 

 phenomena attending the discharge of electricity through gases 

 seems to me to point to a conclusion which may possibly prove 

 a step in the right direction. 



A surface of separation between bodies having different con- 

 1 Phil. Mag., vol. ix. p. 284 (1880). 



NO. n88, VOL. 46] 



ductivities becomes electrified by the passage of a current, while 

 at the surface between two chemically distinct bodies we have, 

 according to Helmholtz, a sheet covered at the two sides with 

 opposite electricities. These surface electrifications are not 

 merely imaginary layers invented to satisfy mathematical surface 

 conditions. They can be proved to be realities. Thus, when 

 one electrolyte floats on another, the specific resistances being 

 different, we often observe secondary chemical effects due to the 

 action of the ions which carry the surface electrification. 



If the passage of electricity from the solid to the gas involves 

 some work done, we must expect a double sheet of electricity 

 at the boundary, the gas in contact with the kathode becoming 

 positively, and that in contact with the anode negatively, elec- 

 trified. A priori we can form no idea how a layer of gas, the 

 atoms of which carry charges, will behave. The ordinary proof 

 that all electrification must be confined to the surface implies 

 that all forces act according to the law of the inverse square, 

 but where we have also to consider molecular forces, I see no 

 reason why the electrification at a surface may not stretch across 

 a layer having a thickness comparable with the mean free path 

 of the molecule. It is here that there seems to be the funda- 

 mental difference between positive and negative electricity. A 

 negaiive electrification of the gas, like that of a solid or a 

 liquid, seems always confined to the surface, and no one has 

 ever observed a volume electrification of negative electricity. 

 The case is different for the positively electrified part of the gas. 

 Wherever from other considerations we should expect a posi- 

 tively electrified surface sheet, we always get a layer of finite 

 thickness. The result implies a different law of impact between 

 positively and negatively electrified ions, but I see no inherent 

 improbability in this. That the kathode let into a gas is sur- 

 rounded by a positively electrified layer of finite thickness ex- 

 tending outwards must be considered as an established fact, and 

 several of the characteristic features of the discharge are 

 explained bv it. The laige fall of potential at the kathode can 

 also be explained on the view which I have put forward, for in 

 order to keep up the discharge there must be a sufficient normal 

 force at the surface, and if this force is not_ confined to the sur- 

 face, but necessarily stretches across a finite layer, the fall of 

 potential must be multiplied a great number of times. Similarly 

 Goldstein has shown that some of the phenomena of the kath- 

 ode are observed at every place at which the positive current 

 flows from a wide to a narrow part of a column of gas. At 

 such places we should expect a positive surface electrification^ 

 and here, again, the whole appearance tends to show that we 

 are dealing with a positive volume electrification. No corre- 

 sponding phenomena are observed when the current passes from 

 the narrow to the wide part. 



The fact that in all cases experimented upon positive volunie 

 electrifications are observed but never similar negative electri- 

 fications is surely of significance. 



Some of the results recently brought to light by investigations 

 on the discharge of electricity have interesting cosmical appli- 

 cations. Thus it is found that such a discharge through any 

 part of a vessel containing a gas converts the whole gas into a 

 conductor.^ The dissociation which we imagine to take place 

 in a liquid before electrolytic conduction takes place must be 

 artificially produced in a gas by the discharge itself. _We may 

 imitate in gases which have thus been rendered conductive many 

 of the phenomena hitherto restricted to liquids : thus I hope to 

 bring to the notice of this meeting cases of primary and 

 secondary cells in which the electrolyte is a gas. There are 

 other ways in which a gas can be put into that sensitive state in 

 which we may treat it as a conductor, and we have every reason 

 to suppose that the upper regions of our atmosphere are in this 

 state. The principal part of the daily variation of the magnetic 

 needle is due to causes lying outside the surface of the earth, 

 and is in all probability only an electro-magnetic effect due to 

 that bodily motion in our atmosphere which shows itself in the 

 diurnal changes of the barometer. A favourite idea of the late 

 Prof. Balfour Stewart will thus probably be confirmed. The 

 difference in the diurnal range between times of maximum and 

 times of minimum sun-spots is accounted for by the fact that 

 the atmosphere is a better conductor at times of maximuoi sun- 

 spots. 



The mention of sun-spots raises a point not altogether new to 

 this section. Careful observation of celestial phenomena may 



I An experiment by Hittorf (IVied. Ann. vi!.. p. 614) .suggested the 

 probability of this fiCt, which was proved independently by Airhenius and 

 myself. 



