[The Lecturer here tried the experiment, and succeeded in blowing the egg
from one egg-cup to the other.]

You see that the air which I blow goes downwards between the egg and the
cup, and makes a blast under the egg, and is thus able to lift a heavy
thing--for a full egg is a very heavy thing for air to lift. If you want
to make the experiment, you had better boil the egg quite hard first, and
then you may very safely try to blow it from one cup to the other, with a
little care.

I have now kept you long enough upon this property of the weight of the
air, but there is another thing I should like to mention. You saw the way
in which, in this popgun, I was able to drive the second piece of potato
half or two-thirds of an inch before the first piece started, by virtue of
the elasticity of the air--just as I pressed into the copper bottle the
particles of air by means of the pump. Now, this depends upon a wonderful
property in the air, namely, its elasticity; and I should like to give you
a good illustration of this. If I take anything that confines the air
properly, as this membrane, which also is able to contract and expand so
as to give us a measure of the elasticity of the air, and confine in this
bladder a certain portion of air; and then, if we take the atmosphere off
from the outside of it, just as in these cases we put the pressure on--if
we take the pressure off, you will see how it will then go on expanding
and expanding, larger and larger, until it will fill the whole of this
bell-jar, shewing you that wonderful property of the air, its elasticity,
its compressibility, and expansibility, to an exceedingly large extent,
and which is very essential for the purposes and services it performs in
the economy of creation.

We will now turn to another very important part of our subject,