escape of the air, which is driven off by the
heat. The air sticks and hangs in the water,
till the heat expands it and makes it rise.
Put a glass of water under the receiver of an
air-pump, and exhaust the receiver. As you
pump, the water begins to bubble, as if it
were boiling; but the bubbles are the air con-
tained in the water, being pumped out. The
air-bubbles act like wedges between the little
invisible drops that make up the whole water.
If it were not for them, the water would be a
mass which would hold together so hard that
it would not go into steam, or boil, till it was
heated to two hundred and seventy degrees,
as may be proved by boiling some water quite
deprived of air. And not only that, but
when it did boil, it would boil all at once, and
blow up with a tremendous explosion; which
would be a still greater inconvenience in
boiling a kettle."

"A pretty kettle of fish, indeed! " Mr.
Bagges observed.

"So," said Harry, " strictly pure water
would not be quite so great a blessing to
us as you might think. Of course, you
know, uncle, I don't mean to say that
there is any advantage in the impurity of
such water as the Thames, except when used
for the purpose of fertilising the earth. I am
speaking of water so pure as to contain no air.
Water of such severe purity would be very
unmanageable stuff. No fishes could live in
it, for one thing. I have already given you
one good reason why it would be unsuitable
to our kettle; and another is, that it would
not be good to drink. Then 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 upwards, 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 vessela 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, was'nt 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 some space as it did before.
Water, in swelling into steam and shrinking
back into water again, moves, of course, twice,
and mighty motions these are, and mighty
uses are made of them, I should rather
think."

"I believe you, my boy," said Mr. Bagges.

"And now," asked Harry, "have you any
idea of what a deal of heat there is in
steam?"

"It is hot enough to scald you," answered
his mamma, " I know that."

"Yes; and hot enough, too, to cook
potatoes. But there is much more heat in it
than that comes to. Take a kettle of cold
water. See at what degree the thermometer
stands in the water. Put the kettle on the
fire and observe how long it takes to boil. It
will boil at two hundred and twelve degrees;
and therefore, during the time it has taken to
boil, there has gone into it the difference of
heat between two hundred and twelve degrees
and the degree it stood at when first put on
the fire. Keep up the same strength of fire,
so that the heat may continue to go into the
water at the same rate. Let the water boil
quite away, and note how long it is in doing
so. You can then calculate how much heat
has gone into the water while the water has
been boiling away. You will find that
quantity of heat great enough to have made
the water red-hot, if all the water, and all the
heat, had remained in the kettle. But the
water in your kettle will have continued at
two hundred and twelve degrees to the last
drop, and all the steam that it has turned into
will not have been hotter according to the
thermometer than two hundred and twelve
degrees; whereas a red heat is one thousand
degrees. The difference between two hundred