Here is a glass flask with a long neck and an open mouth. If we pour
water in at the mouth until it rises to the lip we say that the flask is
full of water. If we now pour the water out we say that the flask is
empty. But is it empty? Press the flask mouth downwards into a glass jar
full of water. If the flask were empty there would be no reason why the
water should not enter the neck of the flask and stand at the same
height inside the neck as it does outside. If you take an “empty” glass
tube open at each end and press it down into the water, the water inside
and the water outside will stand at the same level. But if you put your
finger on the upper end of the tube so as to convert it into a closed
vessel, the water will enter the lower end only a little way. So with
the flask, the water enters the neck only a little way. Hence there is
something inside the “empty” tube and in the “empty” flask; something
which is material, because it occupies space and offers resistance. In
fact the flask is full of that form of matter which is termed =air=, a
thick coat of which surrounds the earth as the =atmosphere=. Air has
weight, as you will learn more fully by and by; and that air in motion
can transfer that motion to other bodies you are taught by the effects
of the winds, which are merely air in motion.
Air therefore has all the characters of a material substance. Moreover
it is a fluid, for it fits itself exactly to the shape of any vessel
which contains it; its parts are very easily moved, or we should feel
its resistance every time we move a limb; that it “flows” is seen in
every breeze and every time you use a pair of bellows, when the air is
driven in a stream out of the nozzle; and it presses on all sides
anything contained in it.
But though air is a fluid it is not a liquid. In the first place it is
very compressible. We saw that the water entered a little way into the
tube or the neck of the flask in the preceding experiment. The reason of
this is that the water compresses the air into a smaller volume. A bag
full of air, such as a common air-cushion, can be squeezed till the air
in its interior occupies a much smaller volume; and, if you treat a
syringe full of air in the same way as the syringe full of water was
treated, you will find, if the piston fits well, that it can be driven
down some distance and then springs back again. Air in fact is not only
a compressible, but it is an =elastic= fluid or =gas=. Heat expands air
just as it expands water, but the expansion of air for the same degree
of heat is much greater.