MR. J. E. H. GORDON ON MEASUREMENTS OF ELECTRICAL CONSTANTS. 445 
Conclusion. 
It will be seen from tlie foregoing table that the results of the experiments on 
certain dielectrics, notably two out of the four specimens of optical glass,* and the 
paraffin agree tolerably closely with the formula deduced from Professor Clerk Max¬ 
well’s theory. For other dielectrics the formula does not even approximately hold 
good. I think we may fairly conclude that the square of the refractive index is a 
term in the expression of the specific inductive capacity, and that in some dielectrics 
it is by far the most important term. In other dielectrics it is overshadowed by some 
other term whose nature is at present unknown. 
Pending the discovery of this unknown term, I propose the following method of 
eliminating it in a particular case :— 
Let two plates of Iceland spar be constructed with their faces, perpendicular and 
parallel, to the axis of the crystal respectively. Let the specific inductive capacities 
be determined, and then if Professor Clerk Maxwell’s views of the nature of an 
electromagnetic disturbance in the ether are correct, I think we ought to have 
{ Ratio of specific inductive capacities 
along and across axis. 
f Ratio of refractive indices ~1 
t 
along and across axis. 
1 
To me it seems possible that this equation may be unaffected by the existence of the 
unknown term, whatever it may be.f 
I am proposing to continue this research, using 1000 Leclanche cells instead of 
secondary current ; 500 of these are already erected, and a rapid reverser and chrono¬ 
graph have been constructed. 
Since the appearance of Messrs. Ayrton and Perry’s paper on the viscosity of 
dielectrics,| it has become of interest to determine whether there is any change in the 
specific inductive capacity when the duration of the electrification is varied from -yq- 0 -th 
second to several days. 
It is impossible to use the coil for this work, as any variation in the speed of the 
break (or reverser) produces large changes in the electromotive force of secondary 
currents. 
It is also important to determine whether difference of electromotive force produces 
any difference in specific inductive capacity. The agreement of my experiments on 
* When newly cast. See note to page 438 (September 25, 1879). 
f Note added June 17, 1879.-—After many unsuccessful attempts to obtain plates of Iceland spar of the 
size required for my induction balance at anything like a reasonable price, I have at last, at the suggestion 
of Professor Coexu, decided to have a minature induction balance constructed, for tbe determination of 
the specific inductive capacity of spar and other expensive substances. This instrument is now almost 
finished; I hope a description of it, and of experiments performed with it, will form the subject of a 
future paper. 
1 4 Proc. Roy. Soc.,’ 1877-8, vol. xxvii., p. 238. 
