XXXVII. It is certainly and indispensably necessary, to make experimental
observations with the most exquisite attention, in order to avoid our
being deceived by them, as our forefathers have been, and as many people
are at this day; who, confiding in a superficial and careless experiment,
have precipitated their conclusions, and without giving themselves time
to reflect, have deduced consequences, from the first information of
their senses. From the ascent of the water in the pump, by considering
the thing inattentively, was inferred the repugnance of nature to a
vacuum. To what labours did Torrizeli, Pascal, and other sublime geniuses
submit, what a variety of experiments did they make, and with what
ingenuity did they combine and compare them, in order to undeceive the
world, and convince mankind, that the true cause of this phenomenon was
the weight of the air!
XXXVIII. From the flame flying upwards, was inferred the chimerical
region of fire, immediately adjoining to the heaven of the moon. In
our essay on Physical Paradoxes, we related the ingenious experiment
with which my Lord Bacon proved, that the flame did not ascend from any
natural propensity it had to fly upwards, but because it was compelled to
it, by the lateral pressure of the air.
XXXIX. I was once in conversation with some school philosophers, and
our discourse happening to turn upon physical matters, I took occasion
to propose a question to them, which was, whether cold water (and the
same may be said of every other liquid) was more subtil and penetrating
than hot, which it seemed evidently to be, by the following experiment,
that when we poured iced water to drink in summer time into a glass,
we perceived the glass wet and moist on the outside, which had the
appearance of the water having transuded through the pores of the
glass; and as this did not happen when the water was luke warm, or in
a temperate state, it was inferred that such water, was not so subtil
and thin as cold. As the bystanders seemed converts to the force of the
before-named experiment, it gave me a good deal of trouble, to undeceive
and convince them of their mistake, although at length I accomplished
doing it, by making it evident to them by various most clear experiments,
that the moisture which adhered to the outside of the glass, did not
transude through its pores, but that it consisted of the coagulated
vapours, of the circumambient air, which being in a warm state, condensed
and were converted into water, upon their coming in contact with any
very cold body, and that they became more condensed the less porous this
body was. For this reason, the vapours that are raised by fire, condense
as soon as they come to the head of the alembick; and for the same, if
we breathe upon a bright iron bar, or any other metallic body, that has
a smooth or polished surface, and is very cold, the vapour we breathe
from our mouths, when it comes in contact with that body, will condense,
and adhere to it; and it is also for the same reason, that in frosty
nights, we see the inside of the glass of windows moist, when they are
dry without, which appearance, I have known surprise many people, who
thought that that humidity was a portion of the external air, which had
penetrated through the glass; and it is likewise for the same reason,
that our breath, and the breath of other animals, is visible in cold
weather, it being then sufficiently condensed by the circumambient air,
to make it become apparent. But the most convincing argument I made use
of in the dispute, and which I advanced as a conclusive one, was, that if
you covered the exterior superficies of the glass with paper, there would
not the least moisture adhere to it, in the whole course of a frosty
night, and it is clear that the paper is not impenetrable to moisture,
but can only prevent the external air, from coming immediately in contact
with the glass.
XL. As it relates to the subject of the impenetrability of glass by
liquids, I cannot forbear in this place taking notice of another very
common error, which has originated from drawing conclusions hastily from
experiments, without making proper reflexions on them. It is generally
believed, that the zest of the rind of a lemon penetrates through glass,
which opinion is founded on the persuasion, that if you press out the
zest on the outside of a glass, the liquor that is within will taste of
it. I concluding this penetration to be impossible, after meditating
upon the matter, easily discovered the cause of this error; which is as
follows, upon pressing out the zest, some of the particles are apt to
fly and rest on the edges of the glass or very near it, and adhere to
that part, which in drinking the lip is applied to; and thus the palate
perceives the taste of the zest, which is communicated to it, from the
edge of the glass. To determine whether this was not the case, I squeezed
some of the zest in the ordinary way against the outside of the glass,
and then turning it half round, applied my lips to the opposite side,
and did not perceive the least taste or flavour of the zest of the lemon.
And it will happen the same to any one, who will give himself the trouble
to make the same experiment.