s a characteristic distinction between
conducting and non-conducting substances, that the former contain
phlogiston intimately united with some base, and that the latter, if
they contain phlogiston at all, retain it more loosely. In what manner
this circumstance facilitates the passing of the electric matter through
one substance, and obstructs its passage through another, I do not
pretend to say. But it is no inconsiderable thing to have advanced but
_one step_ nearer to an explanation of so very capital a distinction of
natural bodies, as that into conductors and non-conductors of
electricity.

I beg leave to mention in this place, as favourable to this hypothesis,
a most curious discovery made very lately by Mr. Walsh, who being
assisted by Mr. De Luc to make a more perfect vacuum in the double or
arched barometer, by boiling the quicksilver in the tube, found that the
electric spark or shock would no more pass through it, than through a
stick of solid glass. He has also noted several circumstances that
affect this vacuum in a very extraordinary manner. But supposing that
vacuum to be perfect, I do not see how we can avoid inferring from the
fact, that some _substance_ is necessary to conduct electricity; and
that it is not capable, by its own expansive power, of extending itself
into spaces void of all matter, as has generally been supposed, on the
idea of there being nothing to obstruct its passage.

Indeed if this was the case, I do not see how the electric matter could
be retained within the body of the earth, or any of the planets, or
solid orbs of any kind. In nature we see it make the most splendid
appearance in the upper and thinner regions of the atmosphere, just as
it does in a glass tube nearly exhausted; but if it could expand itself
beyond that degree of rarity, it would necessarily be diffused into the
surrounding vacuum, and continue and be condensed there, at least in a
greater proportion than in or near any solid body, as Newton supposed
concerning his _ether_.

If that mode of vibration which constitutes heat be the means of
converting phlogiston from that state in which it makes a part of solid
bodies, and eminently contributes to the firmness of their texture into
that state in which it diminishes common air; may not that peculiar kind
of vibration by which Dr. Hartley supposes the brain to be affected, and
by which he endeavours to explain all the phenomena of sensation, ideas,
and muscular mo