September 14, 1893J 



NATURE 



477 



Still such a theory is not mechanical. Electric force acts on 

 matter charged with electricity, and the ratio of the force to the 

 charge can be measured in mechanical units. A fundamental 

 conception in Maxwell's theory is electric displacement, and 

 this is proportional to the electric force. Moreover, its con- 

 vergence measures the quantity of electricity present per unit 

 volume ; hut we have no certain mechanical conception of 

 electric displacement or quantity of electricity, we have no 

 satisfactory mechanical theory of the electromagnetic field. The 

 first edition of the "Electricity and Magnetism" appeared 

 twenty years ajjo. In it Maxwell says : " It must be carefully 

 borne in mind that we have made only one step in the theory of 

 the action of the uiedinm. We have supposed it to be in a 

 state of stress, but we have not in any way accounted for this 

 stress or explained how it is maintained. This step, however, 

 appears to me to be an important one, as it explains by the 

 action of consecutive par'.s of the medium phenomena which 

 were formerly supposed to be explicable only by direct action 

 at a distance. I have not been able to make the next step, 

 namely, to ace mnt by mechanical considerations for these 

 stresses in the dielectric." And these words are true still. 



But, for all this, I think it may be useful to press the theory 

 of the quasi-labile ether as far as it will go, and endeavour to 

 see what ihe consequences must be. 



The analogy between the equations of the electromagnetic 

 field and ih ise of an elastic solid has been discussed by many 

 writers. In a most interesting paper on the theory of dimen- 

 sions, read recently before the Physical Society, Mr. Williams 

 has called attention to the fact that two only of these analogies 

 have throughout a simple mechanical interpretation. These 

 two have been developed at some length by Mr. Heaviside in 

 111-, paper in ihe Electrician for January 23, 1891. To one of 

 them Lord Kelvin had previously called attention ("Collected 

 I'.ipers," vol. iii. p. 450.) 



Starting with a quasi-labile ether, then, we may suppose that 

 u., the mai^nelic permeability of the medium, is 4irp,' where p is 

 the density, and that K, the inductive capacity, is l/4irB, B 

 I'sing the rigidity, or the quasi- rigidity conferred by the 

 rotation. 



The kinetic energy of such a medium is A p (^- + tf \- f-), 



.vhere j ri. ^ are the components of the displacemt-nt. Let us 



' iitify tins with the electromagnet energy (o" -f j8^ -t- 7-)8ir, 



■5, y being comooneiits of the magnetic force, so that 

 {, 3 = 7), 7 = f. Then the components of the electric 

 )lacement, assuming them to be zero initially, are given by 



4ir \ify dz) 



hat is, the electric displacement 5) multiplied liy 4ir is equal to 

 he rotation in the medium. Denote this by n 

 The potential energy due to the strain is 



A Bn^, or ii6w^BS)'', 



nJ on substituting for B this becomes 



2 K ' 



ich is Maxwell's expression for the electrostatic energy of the 

 1. 



Thus so far, but no farther, the analogy is complete ; the 



inetic energy of the medium measures the magnetic energy, 



e potential energy measures the electrostatic energy. The 



^ses in the eiher, however, are not those given by Max- 



il's theory. 



in the other form of the analogy we are to take the inductive 



pacity as 47rp and the magnetic permeability as l/4!rB. The 



i^locity measures the electric force, and the rotation the 



iagnetic force, so that electrostatic energy is kinetic, and 



'agnetic energy potential. Such an arrangement is not so 



V to grasp as the other. Optical experiments, however, 



vv Ui that in all probability it is p, and not B, which varies, 



le from our electrical measurements we know that K is 



lable and ^ constant ; hence this is a reason for adopting the 



<nd form. 



In either case we look upon the field as the seat of energy 

 i.lributed per unit of volu iie ace irding to Maxwell's law. 

 le total energy is obtained by integration throughout the 

 hi. 



If we adopted Mr. HeavUide';^ rational system of unit-s the 477 would 

 M.pear. 



Now we can transform this integral by Green's theorem to a 

 surface integral over the boundary, together with a volume in- 

 tegral through the space ; and the form of these integrals shows 

 us that we may look upon the effects, dealing for the pi-esent 

 with electrostatics only, as due to the attractions and repulsions 

 of a certain imaginary matter distributed according to a definite 

 law over the boundary and throughout the space. To this im- 

 aginary matter, then, in the ordinary theory we give the name 

 of Electricity. 



Thus an electrified conducting sphere, according to these 

 analogies, is not a body charged with a quantity of something 

 we call electricity, but a surface at which there is a discontinu- 

 ing in the rotation impressed upon the medium, or in the flow 

 across the surface ; for in the conductor a viscous resistance to 

 the motion takes the place of rigidity. No permanent strain 

 can be set up. 



From this standpoint we consider electiical force as one of 

 the manifestations of some action between ether and matter. 

 There are certain means by which we can strain the ether : the 

 friction of two dissimilar materials, the chemical action in a cell 

 are two ; and when, adopting the first analogy, this straining is 

 of such a nature as to produce a rotational twist in the ether, 

 the bodies round are said to be electrified ; the energy of the 

 system is that which would arise from the presence over their 

 surfaces of attracting and repelling matter, attracting or repel- 

 ling according to the inverse square law. We falsely assign this 

 energy to such attractions instead of to the strains and stresses 

 in the ether. 



Such a theory has many difficulties. It is far from being 

 proved ; perhaps I have erred in trespassing on ytmr time with 

 it in tiis crude form. The words of the Fiench savant, quoted 

 by Poincare, will apply to it: "I can understand all Maxwell 

 except what he means by a charged body." It is not, of course, 

 the only hypothesis which might be formed to explain the facts, 

 perhaps not even the most probable. For many points the vortex 

 sponge theory is its superior. Still I feel confident that in time 

 we shall come to see that the phenomena of the electro-mag- 

 netic field may be represented by some such mechanism as has 

 been outlined, and that confidence must be my excuse for 

 having ventured to call your attention to the subject. 



SECTION B. 



CHEMISTRY. 



Open'ing Address by Prof. E.merson Reynolds, M.D., 

 Sc.D., F.R.S., President of the Section. 



At the Nottingham Meeting of the British Association in 

 1866, Dr. H. Bence Jones addressed the Section over which 

 I have now the honour to preside on the place of Chemical 

 Science in Medical Education. Without dwelling on this topic 

 to-day, it is an agreeable duty to acknowledge the foresight of 

 my predecessor as to the direction of medical progress. Twenty- 

 seven years ago the methods of inquiry and instruction in 

 medicine were essentially based on the formal lines of the last 

 generation. Dr. Bence Jones saw that modern methods of 

 research in chemistry — and in the experimental sciences gener- 

 ally — must profoundly influence medicine, and he urged the 

 need of fuller training of medical students in those sciences. 



The anticipated influence is now operative as a powerful 

 factor in the general progress of medicine and medical educa- 

 tion ; but much remains to be desired in regard to the chemical 

 portion of that education. In the later stages of it, undue im- 

 portance is still attached to the knowledge of substances rather 

 than of principles ; of products instead of the broad characters 

 of the chemical changes in which they are formed. Without 

 this higher class of instruction it is unreasonable to expect an 

 intelligent perception of complex physiological and pathological 

 processes which are chemical in character, or much real appre- 

 ciation of modern pharmacological research. I have little 

 doul t, however, that the need for this fuller chemical education 

 will soon be so strongly feit that the necessary reform will cotne 

 from within a profession which has given ample proof in recent 

 yeais of its zeal in the cause of scientific progress. 



In our own branch of science the work of the year has been 

 subi'anlial In character, if almost unmaiked by discoveries of 

 popular interest. We may probably place in the latter category 

 the measure of success which the skill of M lissan has enabled 

 him to attain in the artificial production of the diamond form of 

 carbon, apparently in minute crystals similar to those recognised 



HO. 1246 VOL. 48] 



