DR. J. HOPKINSON ON ELECTROSTATIC CAPACITY OF GLASS. 
23 
S!=50 on removing glass equal to ft (+) 3 ‘72, so glass 12' 70 and air 0'55 is 
equivalent to air 2 ’475. 
K=6'55. 
Mean=6'57. 
An attempt was made to determine K for a piece of plate glass ; the considerable 
final conductivity of the glass caused no serious inconvenience, but the very great 
development of that polarization on which residual charge depends produced a 
condenser in which the capacity seemed to increase very rapidly indeed during a 
second or so after making connexions ; this effect could not be entirely separated 
from the instantaneous capacity, a value K = 7 was obtained, but it was quite 
certain that a considerable part of this took time to develope. 
5. The repetition of the experiment in each case gives some notion of the probable 
error of the preceding experiments. Something must be added for the uncertainty 
of the contact reading. It will perhaps not be rash to assume the results to be true 
within 2 per cent. 
Since the magnetic permeability cannot be supposed to be much less than unity, it 
follows that these experiments by no means verify the theoretical result obtained 
by Professor Maxwell, but it should not be inferred that his theory in its more 
general characters is disproved. 
If the electrostatic capacities be divided by the density, we find the following 
quotients : — 
/i (index of 
p K — refraction for 
P line D) 
Light flint 3'2 6‘85 2T4 L574 
Double extra dense . . . 4’5 10T 2'2.5 1 - 710 
Dense flint 3’66 7*4 2‘02 1'622 
Very light flint .... 2'87 6‘57 2‘29 1'541 
Thus 
— is not vastly different from a 
constant quantity. 
Messrs. Gibson and 
Barclay find K for paraffin P97 7 ; taking the density of paraffin as 0 - 93, we have 
the quotient 2T3. This empirical result cannot of course be generally true, or the 
capacity of a substance of small density would be less than unity. 
