218 Is a Vacuum a Conductor of Electricity ? 
the whole system—conductor, magnet, battery and all—about 
a common centre of gravity. This result (as remarked) will 
appear improbable at first sight at least. If found to be con- 
firmed on making the experiment, it would not, as it seems 
to me (and I would put this as a query) necessarily invalidate 
Ampére’s theory, which accounts for equal forces of rotation 
in opposite directions. It would rather add an additional or 
new experimental fact to knowledge. 
P.S. The conditions stated in the above paper are, I think, 
capable of settling this question ; and in this I am confirmed 
by a letter recently received from Prof. Wilhelm Weber, of 
Gottingen, to whom the present manuscript was previously 
sent. I quote the following remark from Prof. Weber’s 
letter (dated February 15, 1885) :— 
“Ich habe sie (2. e. the present manuscript) mit groésstem 
Interesse gelesen und mich gefreut, dass die Frage iiber wni- 
polare Induction dadurch endlich zu definitiver Hrledigung 
gelangen werde.”’ 
I may say I had, some time ago, received a letter from Lord 
Rayleigh, in which he spoke favourably of my view in the last 
paper (Phil. Mag., Feb.), which was in opposition to that of 
Faraday. I pointed out in my last paper that the experiment 
on which Faraday founded his view was capable of a double 
meaning. The subject appears an extremely interesting one to 
follow out experimentally, as there are conditions in the 
inquiry which would seem, if investigated, to be capable of 
throwing a light on the physical nature of the electric “current,” 
aid even of magnetism. 
Paris, February 1885. 
XXIV. On Prof. Hdlund’s Theory that a Vacuum is a 
Conductor of Electricity. 
To the Editors of the Philosophical Magazine and Journal. 
GENTLEMEN, 
SHOULD like to call the attention of Prof. Hdlund to an 
objection to his view that a discharge-resisting vacuum 
is nevertheless a conductor of electricity. The objection is 
one that I urged in a letter to ‘ Nature’ in the spring of 1883, 
and, so far as I am aware, it has not yet been answered. i 
refer to the fact that a body surrounded by such a vacuum is 
acted upon inductively by another electrified body near it, 
and not screened from the action as it would be if surrounded 
by a conductor. 
