The investigation of the properties of Atmospheric Air having been
deemed worthy of close attention by the ancient philosophers and
mechanists, the former deducing them theoretically, the latter
from the action of sensible bodies, we also have thought proper to
arrange in order what has been handed down by former writers, and to
add thereto our own discoveries: a task from which much advantage
will result to those who shall hereafter devote themselves to the
study of mathematics. We are further led to write this work from the
consideration that it is fitting that the treatment of this subject
should correspond with the method given by us in our treatise, in four
books, on water-clocks. For, by the union of air, earth, fire and
water, and the concurrence of three, or four, elementary principles,
various combinations are effected, some of which supply the most
pressing wants of human life, while others produce amazement and alarm.
But, before proceeding to our proper subject, we must treat of the
vacuum. Some assert that there is absolutely no vacuum; others that,
while no continuous vacuum is exhibited in nature, it is to be found
distributed in minute portions through air, water, fire and all
other substances: and this latter opinion, which we will presently
demonstrate to be true from sensible phenomena, we adopt. Vessels which
seem to most men empty are not empty, as they suppose, but full of
air. Now the air, as those who have treated of physics are agreed,
is composed of particles minute and light, and for the most part
invisible. If, then, we pour water into an apparently empty vessel,
air will leave the vessel proportioned in quantity to the water which
enters it. This may be seen from the following experiment. Let the
vessel which seems to be empty be inverted, and, being carefully kept
upright, pressed down into water; the water will not enter it even
though it be entirely immersed: so that it is manifest that the air,
being matter, and having itself filled all the space in the vessel,
does not allow the water to enter. Now, if we bore the bottom of the
vessel, the water will enter through the mouth, but the air will escape
through the hole. Again, if, before perforating the bottom, we raise
the vessel vertically, and turn it up, we shall find the inner surface
of the vessel entirely free from moisture, exactly as it was before
immersion. Hence it must be assumed that the air is matter. The air
when set in motion becomes wind, (for wind is nothing else but air
in motion), and if, when the bottom of the vessel has been pierced
and the water is entering, we place the hand over the hole, we shall
feel the wind escaping from the vessel; and this is nothing else but
the air which is being driven out by the water. It is not then to be
supposed that there exists in nature a distinct and continuous vacuum,
but that it is distributed in small measures through air and liquid
and all other bodies. Adamant alone might be thought not to partake of
this quality, as it does not admit of fusion or fracture, and, when
beaten against anvils or hammers, buries itself in them entire. This
peculiarity however is due to its excessive density: for the particles
of fire, being coarser than the void spaces in the stone, do not pass
through them, but only touch the outer surface; consequently, as they
do not penetrate into this, as into other substances, no heat results.
The particles of the air are in contact with each other, yet they do
not fit closely in every part, but void spaces are left between them,
as in the sand on the sea shore: the grains of sand must be imagined
to correspond to the particles of air, and the air between the grains
of sand to the void spaces between the particles of air. Hence, when
any force is applied to it, the air is compressed, and, contrary to
its nature, falls into the vacant spaces from the pressure exerted on
its particles: but when the force is withdrawn, the air returns again
to its former position from the elasticity of its particles, as is the
case with horn shavings and sponge, which, when compressed and set
free again, return to the same position and exhibit the same bulk.
Similarly, if from the application of force the particles of air be
divided and a vacuum be produced larger than is natural, the particles
unite again afterwards; for bodies will have a rapid motion through a
vacuum, where there is nothing to obstruct or repel them, until they
are in contact. Thus, if a light vessel with a narrow mouth be taken
and applied to the lips, and the air be sucked out and discharged, the
vessel will be suspended from the lips, the vacuum drawing the flesh
towards it that the exhausted space may be filled. It is manifest from
this that there was a continuous vacuum in the vessel. The same may be
shown by means of the egg-shaped cups used by physicians, which are of
glass,[5] and have narrow mouths. When they wish to fill these with
liquid, after sucking out the contained air, they place the finger on
the vessel’s mouth and invert it into the liquid; then, the finger
being withdrawn, the water is drawn up into the exhausted space, though
the upward motion is against its nature. Very similar is the operation
of cupping-glasses, which, when applied to the body, not only do not
fall though of considerable weight, but even draw the contiguous matter
toward them through the apertures of the body. The explanation is that
the fire placed in them consumes and rarefies the air they contain,
just as other substances, water, air or earth are consumed and pass
over into more subtle substances.
That something is consumed by the action of fire is manifest from
coal-cinders, which, preserving the same bulk as they had before
combustion, or nearly so, differ very much in weight. The consumed
parts pass away with the smoke into a substance of fire or air or
earth: the subtlest parts pass into the highest region where fire is;
the parts somewhat coarser than these into air, and those coarser
still, having been borne with the others a certain space by the
current, descend again into the lower regions and mingle with earthy
substances. Water also, when consumed by the action of fire, is
transformed into air; for the vapour arising from cauldrons placed
upon flames is nothing but the evaporation from the liquid passing
into air. That fire, then, dissolves and transforms all bodies grosser
than itself is evident from the above facts. Again, in the exhalations
that rise from the earth the grosser kinds of matter are changed into
subtler substances; for dew is sent up from the evaporation of the
water contained in the earth by exhalation; and this exhalation is
produced by some igneous substance, when the sun is under the earth
and warms the ground below, especially if the soil be sulphureous or
bituminous, and the ground thus warmed increases the exhalation. The
warm springs found in the earth are due to the same cause. The lighter
portions of the dew, then, pass into air; the grosser, after being
borne upwards for a certain space from the force of the exhalation,
when this has cooled at the return of the sun, descend again to the
surface.
Winds are produced from excessive exhalation, whereby the air is
disturbed and rarefied, and sets in motion the air in immediate
contact with it. This movement of the air, however, is not everywhere
of uniform velocity: it is more violent in the neighbourhood of the
exhalation, where the motion began; fainter at a greater distance from
it: just as heavy bodies, when rising, move more rapidly in the lower
region where the propelling force is, and more slowly in the higher;
and when the force which originally propelled them no longer acts upon
them, they return to their natural position, that is, to the surface
of the earth. If the propelling force continued to urge them onward
with equal velocity, they would never have stopped; but now the force
gradually ceases, being as it were expended, and the speed of the
motion ceases with it.
Water, again, is transformed into an earthy substance: if we pour
water into an earthy and hollow place, after a short time the water
disappears, being absorbed by the earthy substance, so that it mingles
with, and is actually transformed into, earth. And if any one says
that it is not transformed or absorbed by the earth, but is drawn
out by heat, either of the sun or some other body, he shall be shewn
to be mistaken: for if the same water be put into a vessel of glass,
or bronze, or any other solid material, and placed in the sun, for a
considerable time it is not diminished except in a very small degree.
Water, therefore, is transformed into an earthy substance: indeed,
slime and mud are transformations of water into earth.
Moreover, the more subtle substance is transformed into the grosser;
as in the case of the flame of a lamp dying out for want of oil,—we
see it for a time borne upwards and, as it were, striving to reach its
proper region, that is, the highest of all above the atmosphere, till,
overpowered by the mass of intervening air, it no longer tends to its
kindred place, but, as though mixed and interwoven with the particles
of air, becomes air itself. The same may be observed with air. For, if
a small vessel containing air and carefully closed be placed in water
with the mouth uppermost, and then, the vessel being uncovered, the
water be allowed to rush in, the air escapes from the vessel; but,
being overpowered by the mass of water, it mingles with it again and is
transformed so as to become water.
When, therefore, the air in the cupping-glasses, being in like manner
consumed and rarefied by fire, issues through the pores in the sides
of the glass, the space within is exhausted and draws towards it the
matter adjacent, of whatever kind it may be. But, if the cupping-glass
be slightly raised, the air will enter the exhausted space and no more
matter will be drawn up.
They, then, who assert that there is absolutely no vacuum may invent
many arguments on this subject, and perhaps seem to discourse most
plausibly though they offer no tangible proof. If, however, it be
shewn by an appeal to sensible phenomena that there is such a thing
as a continuous vacuum, but artificially produced; that a vacuum
exists also naturally, but scattered in minute portions; and that by
compression bodies fill up these scattered vacua, those who bring
forward such plausible arguments in this matter will no longer be able
to make good their ground.
Provide a spherical vessel, of the thickness of metal plate so as not
to be easily crushed, containing about 8 cotylæ (2 quarts). When this
has been tightly closed on every side, pierce a hole in it, and insert
a siphon, or slender tube, of bronze, so as not to touch the part
diametrically opposite to the point of perforation, that a passage
may be left for water. The other end of the siphon must project about
3 fingers’ breadth (2 in.) above the globe, and the circumference
of the aperture through which the siphon is inserted must be closed
with tin applied both to the siphon and to the outer surface of the
globe, so that when it is desired to breathe through the siphon no
air may possibly escape from the vessel. Let us watch the result. The
globe, like other vessels commonly said to be empty, contains air,
and as this air fills all the space within it and presses uniformly
against the inner surface of the vessel, if there is no vacuum, as
some suppose, we can neither introduce water nor more air, unless the
air contained before make way for it; and if by the application of
force we make the attempt, the vessel, being full, will burst sooner
than admit it. For the particles of air cannot be condensed, as there
must in that case be interstices between them, by compression into
which their bulk may become less; but this is not credible if there
is no vacuum: nor again, as the particles press against one another
throughout their whole surface and likewise against the sides of the
vessel, can they be pushed away so as to make room if there is no
vacuum. Thus in no way can anything from without be introduced into
the globe unless some portion of the previously contained air escape;
if, that is to say, the whole space is closely and uniformly filled,
as the objectors suppose. And yet, if any one, inserting the siphon
in his mouth, shall blow into the globe, he will introduce much wind
without any of the previously contained air giving way. And, this being
the uniform result, it is clearly shewn that a condensation takes place
of the particles contained in the globe into the interspersed vacua.
The condensation however is effected artificially by the forcible
introduction of air. Now if, after blowing into the vessel, we bring
the hand close to the mouth, and quickly cover the siphon with the
finger, the air remains the whole time pent up in the globe; and on
the removal of the finger the introduced air will rush out again with
a loud noise, being thrust out, as we stated, by the expansion of
the original air which takes place from its elasticity. Again, if we
draw out the air in the globe by suction through the siphon, it will
follow abundantly, though no other substance take its place in the
vessel, as has been said in the case of the egg. By this experiment
it is completely proved that an accumulation of vacuum goes on in the
globe; for the particles of air left behind cannot grow larger in the
interval so as to occupy the space left by the particles driven out.
For if they increase in magnitude when no foreign substance can be
added, it must be supposed that this increase arises from expansion,
which is equivalent to a re-arrangement of the particles through the
production of a vacuum. But it is maintained that there is no vacuum;
the particles therefore will not become larger, for it is not possible
to imagine for them any other mode of increase. It is clear, then, from
what has been said that certain void spaces are interspersed between
the particles of the air, into which, when force is applied, they fall
contrary to their natural action.
The air contained in the vessel inverted in water does not undergo
much compression, for the compressing force is not considerable,
seeing that water, in its own nature, possesses neither weight nor
power of excessive pressure. Whence it is that, though divers to the
bottom of the sea support an immense weight of water on their backs,
respiration is not compelled by the water, though the air contained in
their nostrils is extremely little. It is worth while here to examine
what reason is given why those who dive deep, supporting on their
backs an immense weight of water, are not crushed. Some say that it
is because water is of uniform weight: but these give no reason why
divers are not crushed by the water above. The true reason may be shewn
as follows. Let us imagine the column of liquid which is directly over
the surface of the object under pressure, (in immediate contact with
which the water is,) to be a body of the same weight and form as the
superincumbent liquid, and that this is so placed in the water that its
under surface coincides with the surface of the body pressed, resting
upon it in the same manner as the previously superincumbent liquid,
with which it exactly corresponds. It is clear, then, that this body
does not project above the liquid in which it is immersed, and will
not sink beneath its surface. For Archimedes has shewn, in his work
on ‘Floating Bodies,’ that bodies of equal weight with any liquid,
when immersed in it, will neither project above nor sink beneath its
surface: therefore they will not exert pressure on objects beneath.
Again, such a body, if all objects which exert pressure from above be
removed, remains in the same place; how then can a body which has no
tendency downward exert pressure? Similarly, the liquid displaced by
the body will not exert pressure on objects beneath; for, as regards
rest and motion, the body in question does [not] differ from the liquid
which occupies the same space.
Again, that void spaces exist may be seen from the following
considerations: for, if there were not such spaces, neither light,
nor heat, nor any other material force could penetrate through water,
or air, or any body whatever. How could the rays of the sun, for
example, penetrate through water to the bottom of the vessel? If there
were no pores in the fluid, and the rays thrust the water aside by
force, the consequence would be that full vessels would overflow,
which however does not take place. Again, if the rays thrust the water
aside by force, it would not be found that some were reflected while
others penetrated below; but now all those rays that impinge upon the
particles of the water are driven back, as it were, and reflected,
while those that come in contact with the void spaces, meeting with
but few particles, penetrate to the bottom of the vessel. It is clear,
too, that void spaces exist in water from this, that, when wine is
poured into water, it is seen to spread itself through every part
of the water, which it would not do if there were no vacua in the
water. Again, one light traverses another; for, when several lamps are
lighted, all objects are brilliantly illuminated, the rays passing
in every direction through each other. And indeed it is possible to
penetrate through bronze, iron, and all other bodies, as is seen in the
instance of the marine torpedo.
That a continuous vacuum can be artificially produced has been shewn
by the application of a light vessel to the mouth, and by the egg of
physicians. With regard, then, to the nature of the vacuum, though
other proofs exist, we deem those that have been given, and which are
founded on sensible phenomena, to be sufficient. It may, therefore,
be affirmed in this matter that every body is composed of minute
particles, between which are empty spaces less than the particles of
the body, (so that we erroneously say that there is no vacuum except
by the application of force, and that every place is full either of
air, or water, or some other substance), and, in proportion as any one
of these particles recedes, some other follows it and fills the vacant
space: that there is no continuous vacuum except by the application of
some force: and again, that the absolute vacuum is never found, but is
produced artificially.
These things having been clearly explained, let us treat of the
theorems resulting from the combination of these principles; for, by
means of them, many curious and astonishing kinds of motion may be
discovered. After these preliminary considerations we will begin by
treating of the bent siphon, which is most useful in many ways in
Pneumatics.