[ 17 ] 
II. Electrostatic Capacity of Glass. 
By J. Hopkinson, D.Sc., M.A. 
Communicated by Sir William Thomson, F.R.S. 
Received May 17, — Read June 14, 1877. 
[Plate 2.] 
1. In his work on Electricity and Magnetism Professor Maxwell developes a theory 
in which electric and magnetic phenomena are explained by changes of position of the 
medium, the wave motion of which constitutes Light. He deduces with the aid of 
this theory that that velocity, which is the ratio of the electrostatic and electro- 
magnetic units of electric quantity, is identical with the velocity of light. This 
deduction may be said to be verified within the limits of error of our knowledge 
of these quantities. He further finds that the product of the electrostatic capacity 
and the magnetic permeability of a transparent substance is equal to the square 
of the refractive index for long waves. The only available experiments for testing 
this result when Professor Maxwell’s book was published* were the “ Determinations 
of Electrostatic Capacity of Solid Paraffin,” by Messrs. Gibson and Barclay (Phil. 
Trans., 1871), and the ‘Determinations of Befractive Indices of Melted Paraffin,’ by 
Dr. Gladstone. Considering the difference in physical state in the two experiments 
the result verifies the theory fairly well. The various kinds of optical flint glass 
are suitable for the purpose of making a comparison of refractive indices and specific 
inductive capacity, since each is an article pretty constant in its composition and 
physical properties, and has small conductivity and return charge. 
2. The only convenient form in which glass can be examined is a plate with plane 
parallel sides ; this plate must form the dielectric of a guard ring condenser. Four 
instruments are thus required, the guard ring condenser, an adjustable condenser 
which can be made equal to the first, a battery for giving equal and opposite charges 
to the two condensers, and an electroscope to show when the added charges of the 
condensers are nil. 
Guard Ring Condenser. — Fig. (1) represents the guard ring condenser in elevation ; 
* Since then determinations have been made by Boltzmann for paraffin, colophoninm, and snlphnr 
(Pogg. Annalen, 1874, vol. cli. pp. 482 and 581, and vol. cliii. p. 525), and for various gases (Pogg. Ann., 
1875, vol. civ. p. 403), by Silow for oil of turpentine and petroleum (Pogg. Ann,, 1875, vol. clvi. p. 389, 
and 1876, vol. clviii. p. 306), and by Schiller (Pogg. Ann., 1874, vol. clii. p. 535) and Wullner (Pogg. 
Ann., 1877, new series, vol. i. pp. 247, 361) for plate glass. 
MDCCCLXXVIII. D 
