136 
During the experiment the force of the jet was altered, 
hut within moderate limits this did not affect the position 
of equilibrium. 
The following extract of a letter, dated March 21st, 1870, 
from Sir Wm. Thomson, D.C.L., F.R.S., Hon. Member of the 
Society, to the President, was read : — 
I have now at last got into good working order measure- 
ments of electrostatic capacity (which, perhaps, you may 
remember I was working on the first time you ever came 
to see me, and more or less almost ever since.) I have two 
students of last year, junior assistants in my laboratory, 
measuring electrostatic capacities of condensers, and varia- 
tions of specific inductive capacities of resistance, with 
sensibility of to per cent, and with constancy in spite of 
accidental variations, generally within \ or J per cent. 
My occupation on the Kinetic Theory of gases has led me 
at last to come to definite terms as to the size of molecules. 
Ever since about the first year of my professorship I have 
taught my students that Cauchy’s theory of Dispersion 
proves heterogeneousness, or molecular structure, to become 
sensible in contiguous portions of glass or water, of dimen- 
sions moderately small in comparison with the wave lengths 
of ordinary light. I have spoken to you also, I think, of 
the argument deducible from the contact electricity of 
metals. This, I now find, proves a limit to the dimensions 
of the molecules in metals quite corresponding to that 
established for transparent solids and liquids by the 
dynamics of dispersion. In experiments made about ten 
years ago, of which a slight sketch is published in the Pro- 
ceedings of the Literary and Philosophical Society of 
Manchester, I found that a plate of zinc and a plate of copper 
kept in metallic connection with one another (by a fine wire 
or otherwise) act electrically upon electrified bodies in their 
neighbourhood, and upon one another, as they would if they 
