CAPACITY OF GLASS, AND OF LIQUIDS. 
369 
II. Electrostatic Capacity of Liquids/"' 
Received January 6,—Read January 27, 1881. 
The number of substances suitable for an exact test of Professor Maxwell’s 
electromagnetic theory of light is comparatively limited. Amongst solids, besides 
glass, Iceland spar, fluor spar, and quartz have been examined by Romich and 
Nowak, t giving results for specific inductive capacity much in excess of the square of 
the refractive index. On the other hand, the same observers, with Boltzmann, obtain 
for sulphur a value of the capacity in reasonable accord with theory. 
On liquids the only satisfactory experiments are those of Silow.} on turpentine and 
petroleum oil, in which the capacity is precisely equal to the square of the refractive 
index for long waves. 
Silow finally obtains for long waves and capacity— 
/f CO • 
y/K. 
Turpentine .... 
. . . 1/461 
1-468 
Petroleum I. 
. . . 1-422 
1-439 
Petroleum II. 
. . . 1-431 
1-428 
Benzol. 
. . . 1-482 
1-483 
A comparison of the whole of the substances which have been examined indicates 
the generalisation that bodies similar in chemical composition to salts, compounds of 
an acid, or acids and bases, have capacities much greater than the square of the refrac¬ 
tive index, whilst hydrocarbons, such as paraffin and turpentine, cannot be said with 
certainty to differ from theory one way or the other. It seemed desirable to test 
this conclusion by experiments on animal and vegetable oils and on other paraffins. It 
was probable that the compounds of fatty acids and glycerine would have high 
capacities. 
Samples were tested of colza oil, linseed oil, neatsfoot oil, sperm oil, olive oil, 
castor oil, turpentine, bisulphide of carbon, caoutchoucine, the paraffin actually in use 
for the electrometer lamp, and three widely different mineral oils kindly given to me 
by Mr. F. Field, F.R.S., to whom I am indebted for the boiling points given below. 
The method of experiment was very simple. The sample was first roughly tested 
for insulation. It was found that it was useless to attempt the samples of colza or 
linseed oils, of caoutchoucine, or of bisulphide of carbon, but that the rest had 
sufficient insulation for the tolerably rapid method I was able to use. 
The fluid condenser consisted of a double cylinder to contain the fluid, in which an 
* The abstract of this paper is published in the Proceedings under the title “ Dielectric Capacity of 
Liquids.” 
t Wiener Sitzb. lxx., part ii., p. 380. 
X Pogg. Ann., 156, 1875, p. 389, and 158, 1876, p. 313. 
