4 Prof. G. Quincke on the Constant of D {electricity 
This determines K s , since K p has been ascertained by means 
of the electric balance. 
Special experiments showed that the amount of the surface- 
tension of the insulating fluids did not change during pro- 
longed action of electric forces. 
With the same fluid and at the same temperature the refrac- 
tion-index for Fraunhofer's line D was measured by means of 
a Steinheil hollow prism and an Oertling circle, permitting 
readings of 2". 
According to Maxwell*, for light- waves of infinite length 
the constant K would be equal to the square of the index of 
refraction. This relation is not confirmed by my observations 
collected in the opposite Table. On the contrary, K p is nearly 
equal to K s , and constantly greater than K. Colza oil forms 
the only exception. 
All the fluids were as pure and free from dust and contained 
as little water as possible, and insulated excellently. As, how- 
ever, the observations took some time, a portion of the fluid 
evaporated, the temperature fell, and on opening the balance- 
case some water condensed on the surface of the fluid, through 
which the numbers found for the dielectricity-constants were 
too high. This source of error was especially great in the 
case of ether. Four successive determinations with ether 
which had stood for a long time over calcined marble, and 
was filtered as quickly as possible through a fan filter into the 
glass vessel of the electric balance, gave 
K p =4-399 4-516 4-739 4-891. 
The numbers in the Table therefore refer to ether which had 
already absorbed a trace of water. 
In order to investigate the alteration of the optical proper- 
ties of fluids by electric forces, I employed condensers with 
plane electrodes, or with electrodes consisting of two concen- 
tric cylinders. The electrodes or coatings, of nickel-covered 
brass, were insulated by flint-glass rods from one another, and 
from the wider glass tubes closed by plane-parallel glass plates 
which contained the fluid as pure and free from dust as pos- 
sible. The wires, insulated by flint glass, which connected 
the electrodes with the earth or with the inner coating of a 
large battery of eight Leyden jars of flint glass, passed through 
two side tubes. The coatings of the fluid condenser and the 
Leyden battery were charged, as in the experiments with the 
electric balance, by a Holtz machine, and maintained at a con- 
stant difference of potentials, which was controlled by the 
Electricity and Magnetism, ii. § 788. 
