doctor say
that some poor people are in the habit of giving themselves cheap
steam-baths by this means."

"A very good thing for rheumatic pains, I should think; certainly a much
more rational remedy than patent medicines or Government poison," Mr.
Bagges remarked.

"There are some salts," continued Harry, "which, if dissolved in water,
will prevent it from boiling till it is heated to two hundred and
sixty-four degrees, as if they held the water back from flying into
steam. So, then, the boiling of water may be hindered, more or less, by
pressure from without, and attraction from within. The boiling point of
water depends on another important fact which the kettle always mentions
before it boils, although we don't all of us understand the kettle's
language. The singing of the kettle tells us--"

"That the water is going to boil," interrupted mamma.

"Yes, and that water contains air. The singing of the kettle is the
noise made by the 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 contained 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 fertilizing 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