TBANSACTIONS OF SECTION A. 681 



lu either case we look upon the field as the seat of energy distributed per unit 

 of volume according to Maxwell's law. The total energy is obtained by integration 

 throughout the field. 



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

 integral over the boundary, together vdth a volume integral through the space ; 

 and the form of these integrals shows us that we may look upon the effects, dealing 

 for the present 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 imaginary matter, then, in the ordi- 

 nary theory we give the name of Electricity. 



An electrified conducting sphere, according to these analogies, is not a body 

 charged with a quantity of something called electricity, but a surface at which 

 there is a discontinuity 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 electrical 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 electri- 

 fied ; the energy of the system is that which wouJd arise from the presence over 

 their surfaces of attracting and repelling matter, attracting or repelling 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 your time with it in this crude form. The words of 

 the French sarrtw^, quoted by Poincar^, 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 electror 

 magnetic 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. 



The following Reports and Papers were read : — 



1. Interim Report of the Committee on a National Physical Laboratory. 



See Reports, p. 120. 



Interim Report of the Committee on Electro-optics. 

 See Repoi'ts, p. 121, 



3. Report of the Committee on Solar Radiation. — See Reports, p. 144. 



4. Report of the Committee for Comparing and Reducing Magnetic 

 Observations. — See Reports, p. 120. 



5. Report of the Committee in connection with the Magnetic Work of the 

 Falmouth Observatory. — See Reports, p. 121. 



