ticks and in all natural
motion; not by attenuating or by expelling the air, so that the body moves
down into the place of the air which has gone out[140]; for thus it would
have allured only and would not have retained; since it would at first also
have repelled approaching bodies just as it drives the air itself; but
indeed a particle, be it ever so small, does not avoid the first
application made very quickly after rubbing. An effluvium exhales from
amber and is emitted by rubbing: pearls, carnelian, agate, jasper,
chalcedony, coral, metals, {56} and other substances of that kind, when
they are rubbed, produce no effect. Is there not also something which is
exhaled from them by heat and attrition? Most truly; but from grosser
bodies more blended with the earthy nature, that which is exhaled is gross
and spent; for even towards very many electricks, if they are rubbed * too
hard, there is produced but a weak attraction of bodies, or none at all;
the attraction is best when the rubbing has been gentle and very quick; for
so the finest effluvia are evoked. The effluvia arise from the subtile
diffusion of humour, not from excessive and turbulent violence; especially
in the case of those substances which have been compacted from unctuous
matter, which when the atmosphere is very thin, when the North winds, and
amongst us (English) the East winds, are blowing, have a surer and firmer
effect, but during South winds and in damp weather, only a weak one; so
that those * substances which attract with difficulty in clear weather, in
thick weather produce no motion at all; both because in grosser air lighter
substances move with greater difficulty; and especially because the
effluvia are stifled, and the surface of the body that has been rubbed is
affected by the spent humour of the air, and the effluvia are stopped at
their very starting. On that account in the case of amber, jet, and
sulphur, because they do not so easily take up moist air on their surface
and are much more plenteously set free, that force is not so quickly
suppressed as in gems, crystal, glass, and substances of that kind which
collect on their surface the moister breath which has grown heavy. But it
may be asked why does amber allure water, when water placed on its surface
removes its action? Evidently because it is one thing to suppress it at its
very start, and quite another to extinguish it when it has been * emitted.
So also thin and very fine silk, in comm