Chapter 1 of J.C. Maxwell's treatise is entitled "Description of
Phenomena" and merely exposes the experimental phenomena to be modeled in
this work. Paragraph (28) reads in part:
"Experiment 2:
Let a hollow vessel of metal be hung up by white silk threads, and let a
similar thread be attached to the lid of the vessel so that the vessel may
be opened or closed without touching it. . . .
Let the vessel be originally unelectrified, then if an electrified piece
of glass is hung up within it by its thread without touching the vessel,
and the lid closed, the outside of the vessel will be found to be
vitreously electrified and it may be shewn that the electrification
outside of the vessel is exactly the same in whatever part of the interior
space the glass is suspended."
At this point one is referred to a footnote which reads: "This is an
illustration of Art. 100c"
Art. 100c is a derivation of a uniqueness theorem which Maxwell calls a
specific case of "Thomson's Theorem".
(Note that previously in "Experiment 1" Maxwell has associated vitreous
and resinous electrification with positive and negative electrification,
respectively.)