;io 



NA TURE 



[July 



1S9; 



Omori. (See p. 275.) — Notes on the state of Etna, by Prof. A. 

 Ricco. — Xoteson Italian earthquakes (January, February, 1895), 

 by Dr. M. Baratta. These are inserted as an apj^endix to each 

 number, and form a catalogue of all earthquakes, tremors and 

 pulsations recorded at the Italian gemlynamic and meteorological 

 observatories, Aic. They are a ctmtinuation of the valuable 

 SuppUmcnti \o the Annali ai the Ufficio Centrale di Mcteoro- 

 Ic^^ e Geodinamica. 



SOCIETIES AND ACADEMIES. 



Lo.NDOX. 

 Royal Society, June 20. — "ADjaiamical Theory of the 

 Electric and Luminiferous Medium. I'art II. : Theory of 

 Electrons." By Joseph Larmor, F. R.S. 



In a previous paper on this subject,' it has been shown that 

 by means of a rotationally elastic a;ther, which otherwise 

 behaves as a (lerfect fluid, a concrete realisation of MacCullagh's 

 optical theory can be obtained, and that the same medium 

 affords a complete representation of electromotive phenomena in 

 the theory of electricity. The ponderomotive electric forcives 

 were, on the other hand, deduced from the principle of energy, 

 as the work of the surplus energ)' in the field, the motions of the 

 bodies in the field being thus supposed slow comp.ired with 

 radiation. It was seen that in order to obtain the correct sign 

 for the elect rodynaniic forcives between current systems, we are 

 prcxluded from taking a current to be simply a vortex ring in the 

 fluid a;ther ; but that this difficulty is removed by taking a 

 current to be produced by the convection of electrons or ele- 

 mentary electric charges through the free a.'ther, thus making 

 the current effectively a vortex of a type whose strength can be 

 altered by induction from neighbouring currents. .An electron 

 occurs naturally in the theory as a centre or nucleus of rotational 

 strain, which can have a permanent existence in the rotationally 

 elastic ather, in the same sense as a vortex ring can have a per- 

 manent existence in the ordinary perfect fluid of theoretical 

 hydrodynamics. 



In the present pai^er a further development of the theory of 

 electrons is made. .As a preliminary, the consequences as re- 

 gards ponderomotive forces, of treating .an element of current i5i 

 as a separate dynamical entity, which were indicated in the 

 previous paper, are here more fully considered. It is maintained 

 that a hy|vnhesis of this kind would lead to an internal stress in 

 a conductor carrying a current, in addition to the forcive of 

 Ampere which acts on e.ach element of the conductor at right 

 angles to its length. Though this stress is self-e<iuilibraling as 

 regards the c« inductor as a whole, yet when the conductor is a 

 liquid, such as niercur)', it will involve a change of fluid pres- 

 sure which ought to Iw of the .same order of magnitude as the 

 amperean forcive, and therefore capable of detection whenever 

 the latter is easily observed. Experiments made by I'rofs. Filz- 

 (ierald and Lodge on this subject have yielded purely negative 

 results, so that there is ground for the conclusion that the 

 ordinary current-element i5i cannot be legitimately employed in 

 framing a dynamical theory. 



This result is entirely confirmed when we work out the pro- 

 perties of the field of currents, considered as produced by the 

 convection of electrons. It is shown that an intrinsic singu- 

 larity in the a;ther, of the form of an electron e, moving with 

 velocity (j, >', i) relative to the quiescent mass of .-ether, is 

 subject to a force e (I', <^, R), given by equations of the form 



V = ci- hy - dVjdt - dVjdx ; 



in which (a, *, c) is the velocity of flow of the a;ther where the 

 electron is situated, and is etjual to the curl of (F, ("i, II) in such 

 way ihal the latter is .Maxwell's vector pHential given by the 

 forniuhe of the type 



F 



=/"^^A«i 



.li 



!-)^r, 



dz dy/r 



and where y is the electrostatic potential due to the electrons in 

 the field, so that y = c'Xr'r, where c is the velocity of radia- 

 tiim. Thi-t- equations are proved to hold good, no", merely if 

 the motions of the electrons are slow comjiared with radiation, 

 as in the previous pa|>cr, but quite irrespective of how nearly 

 the)- approach that limiting value ; thus the phenomena of 

 radiation itself are includeil in ihe analysis. 



An clement of volume of an unelectrificd material medium 

 contains as many positive electrons as negative. This force 



> Primed in atniract in Natvuk, %\\x. pp. 360, 380. 



NO. 1343, VOL. 52] 



(P, Q, R) tends to produce electric separation in the element by 

 moving them in opposite directions, leading to an electric 

 current in the case of a conductor whose electrons are in part 

 free, and to electric polaris;>tion in the case of a dielectric 

 who.se electrons are paired into polar molecules. In the former 

 case, the rate at w hich this force works on a current of electrons 

 («', v' , K''), is Vu' + Q;' + Ric' : it therefore is identical with 

 the electric force as ordinarily defined in the elementar)' 

 theory of steady currents. In the case of a dielectric it repre- 

 sents the ordinary electric force pnxlucing polarisation. So 

 long as a current is prevented from flowing, the ponderomotive 

 force acting on the element of volume of the metlium is the one 

 of electrostatic origin due to such polaris;ition as the element 

 may possess, for as the element is unelectrificd it contains as 

 many positive electrons as negative. But if a current is flowing, 

 the first two terms of (P, Q, R), instead of cancelling for the 

 positive and negative electrons, become additive, as change of 

 sign of the electron is accompanied by change of sign of its 

 velocity ; so that there is an electrodynamic force on the 

 element of volume, 



(X, Y, Z) = (t'V - 7f 'A, w'a-ii'c, u'b-i''a), 



where, however, («', v\ w') is the true current composed of 

 moving electrons, not the total circuital current (//, v, w) of Max- 

 well, which includes the rotational displacement of the free 

 ivther in .addition to the drift of the electrons. 



The electric force ( P, (,), R) as thus deduced .agrees with the 

 form obtained originally by .Maxwell from the direct considera- 

 tion of his concrete model of the electric field, with idle wheels 

 to represent electrification. It has been pointed out by von 

 Helmholtz and others, that the .abstract dynamical analysis given 

 in his Treatise does not really lead to these e<iuations when all 

 the terms are retained ; this later analysis proceeds, in fact, by 

 the use of current-elements, which form an imperfect represen- 

 tation, in that they give no account of the genesis of the current 

 by electric separation in the element of volume of the 

 conductor. 



The ponderomotive force (X, Y, Z) is at right angles to the 

 direction of the true current, and is precisely that of .\mpere 

 in the ordinary cases where the difference between the true 

 current and the total current is inappreciable. It difl'ers Irom 

 Maxwell's result in involving true current instead of total 

 current ; that is, the forcive tends to move an element of 

 a material body, but there is no such forcive tending to move 

 an element of the free xther itself. In this respect it (lifters also 

 from the hypothesis underlying von Helmholtz's recent treatment 

 of the relations of moving matter to ;vther 



The theory is applied ( 1 ) to the determination of the electric 

 and magnetic stresses in material media and of the mechanical 

 i)ressure caused by radiation, (2) to optical propagation, includ- 

 mg detailed theories of dispersion and metallic reflexion, 

 including also the influence of motion of the material medium. 

 It is shown that if electrons are accepted as the basis 

 of nmterial atoms, the latter topic is fully elucidated : also 

 th<at the theory is not at a loss when explanations of the 

 phenomena of inertia, gravitation and spectra are demanded. 



June 20. — "An Inquiry into the Nature of the \'esicating 

 Constituent of Croton Oil." By Wyndham K. Dunslan, 

 F. U.S., and Miss L. E. Boole. 



The vesicating constituent, or more strictly, the pustule- 

 producing constituent of croton oil, has been the sul>jecl of 

 mvcstigation by numerous chemists and pharnlacologi^ts during 

 the ]>ast forty years. According to the researches of liuchheim, 

 and more recently of Robert and Hirscheydt, the vesicating 

 action is due to an acid closely allied to oleic acid, which h.is 

 been given the name of crotonoleic acid. This substance is now 

 prepared on a large scale in (lerm-iny fur medical use, lieing 

 extracted from croton oil by the method devised by Kohert ind 

 Mirscheydt. This consists in sajionifying with liariuni hyilrovide 

 that part of croton oil which readily dissulvos in strong alcolml. 

 The resulting barium salts are waslieil with water, (hen dried, 

 and repeatedly extr.acted with ether, which dissolves the liariuni 

 salts of oleic an<l cmtonoleic .acids. These sails are separated 

 by means of ether, which disscilves only the barium crotomileate, 

 and this, when decomposed with dilute sulphuric acid and 

 extracted with ether, furnishes the crotMUdleic acid as a viscid ciil. 

 .Since very little is known about this ,icid, even ilscomposilion 

 being undeterinineil, the aullicirs prepared il willi Ihe oiijecl of 

 studying its properties and, if possible, of determining the con. 

 .stitulion since no fatty acid of known constitution exhibits the 

 property of vesicatmg. Starting with the crotonoleic acid 



