water, as we find it in the world, has a very useful and
accommodating disposition to find its own level. Pump all the air out of
water, however, and it loses this obliging character in a great measure.
Suppose I take a bent glass-tube, and fill one arm of it with airless
water. Then I turn the tube mouth upward, and if the water were common
water, it would instantly run from one arm into the other, and stand at
the same level in both. But if the water has been exhausted of its air,
it remains, most of it, in the one arm, and won't run till I give the
table a smart rap, and shake it. So, but for the air contained in water,
we could not make the water run up and down hill as we do. If water were
deprived of air, London would be almost deprived of water."

"And water," observed Mr. Bagges, "would be robbed of a very valuable
property."

"Good again, uncle. Now, if we could see through the kettle, we should
be able to observe the water boiling in it which is a curious sight when
looked into. To examine water boiling, we must boil the water in a glass
vessel--a long tube is the best--heated with a spirit lamp. Then first
you see the water in motion, and the air-bubbles being driven off by the
heat. As the water gets hotter, other bubbles appear, rising from the
bottom of the tube. They go up for a little way, and then they shrink,
and by the time they get to the top of the water, you can hardly
distinguish them. These are bubbles of steam, and they get smaller as
they rise, because at first the water is colder above than below, in
proportion to the distance from the flame, and the cold gradually
condenses the bubbles. But when the water gets thoroughly hot, the
bubbles grow larger and rise quicker, and go of the same size right up
to the top of the water, and there escape--if you choose to let them.
And steam was allowed to escape so for many, many ages, wasn't it,
uncle, before it was set to work to spin cotton for the world, and take
us to America within a fortnight, and whirl us over the ground as the
crow flies, and almost at a crow's pace?"

"For all which," remarked Mr. Bagges, "we have principally to thank
what's his name."

"Watt _was_ his name, I believe, uncle. Well; heat turns water into
steam, and I dare say I need not tell you that a quantity of water
becoming steam, fills an immense deal more space than it did as mere
water. Cold turns the steam back into water, and the water fills the
same space as it di