Another very interesting experiment is to secure a long-stemmed glass
bulb (see Fig. 32). Arrange this apparatus as illustrated, with the stem
of the bulb immersed in the water. The glass bulb condenses the air.
When you first put it into the water nothing happens, but as soon as you
apply heat the air bubbles come out of the end of the tube. This means
that the air in the tube has expanded and part of it has come out
through the stem of the tube and the remainder is lighter. It is well to
remember, when air is heated it expands and becomes lighter. This fact
is extremely important to remember, because it has a great deal to do
with the important instrument, the barometer, which is used to measure
the pressure of the atmosphere and is an important element in the
question of humidity, as you will learn later. By this time you no doubt
have learned that: