of Edinburgh, Session 1881-82. 497 
experimental test ; indeed, it is on this account mainly that I have 
ventured to bring it forward. 
Let C denote the capacity per square centimetre of two parallel 
plates at a distance s apart. Then, since the surface density is 
given by 
we get by (4) 
1 
s dx 
kT 
( 8 ). 
Hence, Y being the spark jiotential corresponding to the same 
distance s, since by hypothesis R is always the same, being in fact 
= constant. 
This reciprocal relation between the capacity and spark potential 
shows that the capacity of the plates for very small distances apart, 
ought, if the above theory be correct, to be much smaller than the 
value calculated by the usual assumption of a uniform dielectric, for 
experiment has already shown that the spark potential is very much 
greater. I have devised an arrangement for putting this conclusion 
to an experimental proof, and hope in the course of next summer to 
be able to lay the result before the Society. 
There is one other point on which I should like to remark. 
However good the methods of Sir William Thomson may be, I 
think that little is to be gained by following them to the exclusion 
of others. It is clearly desirable to experiment with other forms of 
terminals for which the surface density can be calculated, and to 
examine whether the results can be explained in all cases by a 
common hypothesis. I think it might be possible by careful atten- 
tion to insulation, to measure with some accuracy the potential at 
which disruptive discharge begins at the sharpest extremity of an 
