June ii, 1891] 



NA TURE 



^39 



Elias MetschnikofT, Director of the Paris Pasteur Institute, Prof. 

 W. H. Flower, C.B., F.R.S., and Mr. W. E. II. Lecky. 



Delegates from the seventh International Congress of Hygiene 

 and De uography will be received by the Vice-Chancellor in the 

 Senate House on Saturday, Aui^ust 15. 



The Museums Association hold their annual meeting in July 

 in the buildings lately erected for the departments of Anatomy 

 and Physiology. 



Prof. Foster is appointed by the University a Member of 

 Council of the Marine Biological Association. 



The reference to the Syndicate on the question of Greek in 

 the Previous Examination has been enlarged to include Latin 

 also, and will be decided on by the Senate early in the October 

 term. An animated discussion on the question took place in 

 the Arts School, in which the claims of modern (non-classical) 

 education for consideration by the University were strongly put 

 forward by men of the highest classical distinction. 



Mr. J, N. Keynes, the Secretary for the Local Examinations, 

 has been approved for the degree of Doctor in Science. 



An election to an Isaac N-wton Studentship will take place in 

 October. The value is £,ZQO a year for three years from April 

 15, 1891. Candidates are to send their names and testimonials 

 to the Vice-Chancellor between October i and 10. 



It is proposed to affiliate the University to the University of 

 Adelaide, South Australia. 



The General Board of Studies propose that Dr. Ruhemann, 

 hitherto Assistant to Prof. Devvar, shall be appointed a Univer- 

 sity Lecturer in Organic Chemistry. 



A room in the new Physiological Laboratory is to be set aside 

 for Psychophysics, and a grant of £ijo for instruments is 

 recommended by the General Board. 



The Annual Report of the University Observatory contains a 

 good record of work done and in progress. Prof. Adams is to 

 he congratulated on the satisfactory way in which, notwith- 

 standing his long and severe illness, the Observatory has been 

 conducted. 



SOCIETIES AND ACADEMIES. 

 London. 



Royal Society, May 14. —"On the Theory of Electro- 

 dynamics." By J. Larmor, Fellow of St. John's College, 

 Cam iridge. Comaiunicated by Prof. J. J. Thomson, F. R.S. 



The electrical ideas of Clerk Maxwell, which were cultivated 

 partly in relation to mechanical models of electrodynamic action, 

 led him to the general principle that electrical currents always 

 flow round complete circuits. 



When this principle of circuital currents is postulated, the 

 theory of electrodynamics is reduced to the Ampere-Neumann 

 theory of complete circuits, of which the truth has been fully 

 established. It leads, as shown by Maxwell, to the propagation 

 of electrical action in dielectric media by waves of transverse 

 electric displacement, which have the intimaterelations to waves 

 of light that are now well known. 



The problem of determining how far these remarkable con- 

 clusions will still hold good when a more general view of the 

 nature of dielectric polarization is assumed was considered by 

 von Helmholtz in a series of memoirs. 



The most general conception of the polarization of a medium 

 which has been formed is the Poisson theory of magnetization. 

 The magnetized element, whether actually produced by the 

 orientation of polar molecules, or otherwise, may be mathe- 

 matically considered to be formed by the displacement of a 

 quantity of ideal magnetic matter from its negative to its posi- 

 tive pole, thereby producing defect at the one end, and excess 

 at the other end. The element is defined magnetically by its 

 moment, which is the product of the displaced quantity and the 

 distance through which it is displaced. The displacement per 

 unit volume, measured by this product, is equal to the magnetic 

 moment per unit volume, whether the magnetized molecules fill 

 up the whole of that volume or are a system of discrete particles 

 with unoccupied space between them. 



In the electric analogue we replace ideal magnetic matter by 

 ideal electric matter ; the displacement thus measured consti- 

 tutes the electric displacement, and its rate of change per unit 

 time represents the displacement current in the dielectric. We 

 have to consider whether a displacement current of this type suf- 

 fices to make all electric currents circuital ; and it will be suffi- 

 cient and convenient to examine the case of a condenser which 



NO. IT 28, VOL. 44] 



is charged through a wire connecting its two plates. In the 

 first pl.ice, this notion of electric displacement leads to the same 

 distribution of potential between the plates as the ordinary one, 

 adopted by Maxwell ; for in the theory of induced magnetism 

 there occurs a vector quantity of circuital character, the mag- 

 netic induction of Maxwell, of which the components are 

 -n[dWldx), -fji(JV/(fy), -fiidV/dz), and which, therefore, 

 eads to the characteristic equation of the potential 



dA'' Ux) "" dyV-dy) "■ -dzV-d.)=°' 

 corresponding to the one given above. If the displacement in 

 the dielectric is -K{dV/dx), -K^dV/dy), - K{dVfdz), thsu 

 fi = I + 4ir/c. 



The displacement in a unit cube may, of course, be considered 

 as a displacement across the opposite faces of the cube. 



Now, considering the case of a plane condenser, let F be the 

 electric force in the dielectric between the plates ; then the 

 displacement is /cF. Let a be the surface density of the charge 

 conducted to a plate ; then the eflfective electrification along 

 that plate will be of surface density a' = a - kF ; therefore, 

 by Coulomb's principle, 



so that 



47r(7 

 4Tr((r 



'F); 



a- = ^-F = kF + ' F. 

 4ir 4ir 



Thus the current is not circuital, but there is an excess of the 

 surface density conducted to the surface over the displacement 

 current from the surface, which is equal to F/4ir. 



The specific inductive capacity, as determined by static experi- 

 ments on capacity, is here measured by /u, the coefficient in the 

 expression for cr. 



In addition to this discontinuity at the face of a condenser 

 plate, the induction in the mass of the dielectric will not be 

 circuital unless the electric force is itself circuital, which it is 

 not in the general form of the electrodynamic theory. 



The most general type of electrodynamic relations which is 

 consistent with the established theory of complete circuits, is 

 discussed on the basis of von Helmholtz's work, but with 

 avoidance of certain restricting conditions introduced by him, 

 the chief conclusion being as follows : 



In a complete circuit the one thing essential to the established 

 theory is that the electric force integrated round the circuit should 

 be equal to the time rate of change of the magnetic induction 

 through it, and, therefore, have an ascertainable value, though 

 its distribution round the circuit is a subject of hypothesis. The 

 , conclusion that waves of transverse displacement will be propa- 

 gated in a dielectric with velocity Ki will hold good if we 

 assume any form whatever for the electric force which does not 

 violate this one relation, and also assume an electrostatic 

 polarization of the medium, equal at each point to the electric 

 force multi|>lied by a con-tant Kj/4ir. 



The increased generality which can be imparted to the theory 

 merely leads to various modes of propagation of a condensa- 

 tional wave. 



If K2 denote the specific inductive capacity of the medium, 

 measured in static units, this polarization constant Kj is equal 

 to K2 - X ; and the velocity of the transverse waves is the 

 ratio of the electric units of quantity in a medium of unit induc- 

 tive capacity multiplied by the static value of Kj"*. The cor- 

 respondence of the refractive index for the simpler media with 

 Ka" , as well as direct measures of the relative velocities of 

 electric waves in other media, give for the value of this velocity 

 the same ratio multiplied by K," . These values can be recon- 

 ciled only by the limiting form of the theory of polarization 

 which is equivalent to Maxwell's theory. 



May 28. — "On the Anatomy and Physiology of Protoptents 

 annectens." By W. N. Parker, Ph.D., F.Z.S., Professor of 

 Biology in University College, Cardiff. Communicated by 

 W. H. Flower, F.R.S. 



The work which has resulted in the present paper was begun 

 in Freiburg in the summer of 1888, when the author was for- 

 tunate enough, owing to the generosiiy of Prof. Wiedersheim, 

 to obtain a number of fre>h specimens lor examination. As so 

 many interesting points presented themselves at an early stage 



