XXVII. The endeavour of the flame to ascend, is the proof which is
vulgarly exhibited, of there being an element or sphere of fire above
us, by those who maintain that position; but we shall shew, that this is
a very feeble proof, even admitting their own state of the case, because
every thing that is lighter than the fluid that surrounds it, will, if it
is not by violence restrained from doing so, rise above it, without being
invited to ascend by a sphere of its own species that is aloft; and it is
from hence, that flame, smoke, vapour, an infinity of elemental effluvia,
and divers other species of things ascend, without the assistance or
intervention of any other cause, except that of their being lighter than
the inferior gross air.
XXVIII. To this we shall add, that there is not in the flame, the
propensity to ascend which they suppose, and which seems to be indicated
by its rising upwards in a conic form; for this ascent is violent,
and not natural to the flame. We shall illustrate this paradox, by
an experiment which my Lord Bacon mentions in the first Book of his
Centuries. A small wax candle, fitted into an iron tube, and placed
perpendicular in a vessel filled with spirit of wine, in which it must
be immersed to such a depth, that when they are both set fire to, the
flame of the candle shall not rise higher than that of the spirit; and
you will see, the flame of the candle and that of the spirit being
distinguishable by their different colour, that the flame of the candle
will appear in the middle of that of the spirit, extending itself in
breadth, and not in a pyramidical, but a round form, that occupies four
or five times as much space, as that which it used to occupy when burning
in the open air. This experiment proves, that the pyramidical form which
the flame regularly assumes, is caused by the pressure of the air that
surrounds it; and this is the sentiment of the modern philosophers, who
without adverting to the experiment we have mentioned, would make no
scruple of pronouncing, that the form it assumes is a violence done the
flame.
XXIX. By this example it may be seen, that experiments, provided they
are made with judgment, and attended to with subtil reflection, are
the only means by which we can attain any certain knowledge of natural
things; but if they are made in a lumping way, and without due attention,
and exactness, they are the occasion of innumerable errors. Many have
arisen, from attributing to a native inclination, or intrinsic virtue of
some body, effects, which are only caused by the impulse of some other
neighbouring body. Before the gravity and elasticity of the air were
discovered, it was looked upon as a thing demonstrated by experience,
that the water’s rising in the pump, was from its inclination to obstruct
or oppose a vacuum, and now it is a thing evinced by experience, that the
air is what impels it to that ascent.