Boiling is the escaping of steam. That causes
the bubbling; so the bubbling of water over
the fire is only the sign that the water boils.
But what occasions the escape of the steam?"

"The heat, of course—the—what is the
right word?—the caloric," answered Mr.
Bagges.

"True; but what heat? Why, the excess
of heat over two hundred and twelve degrees
—taking that as the average boiling point of
water. You can heat water up to that point,
and it remains water; but every degree of
heat you cause to pass into it above that,
turns a quantity of the water into steam; and
flies off in the steam, unless the steam is
hindered from escaping by extraordinary
pressure. Blow the fire under that kettle
as much as you will, and you will make
the water boil faster, but you won't make it
a bit hotter than two hundred and twelve
degrees."

"Well, to be sure!" Mrs. Wilkinson
exclaimed.

"If water," continued Harry, " could keep
on getting hotter and hotter above the boiling
point, why, we might have our potatoes
charred in the pot, or our mutton boiled to a
cinder. When water is confined in a strong
vessel—and strong it must be to prevent a
tremendous blow-up—confined, I say, so that
no steam can escape, it may be heated almost
red-hot; and there is a vessel made for heat-
ing water under pressure, called Papin's
Digester, which will digest almost anything."

"What an enviable apparatus! " exclaimed
Mr. Bagges.

"Well," resumed Harry; " so the boiling
point of water depends on the degree of
force which the air or what not is pressing
on its surface with. The higher the spot on
which you boil your water, the lower the
point it boils at. Therefore, water boiling at
the top of a mountain is not so hot as water
boiling at the mountain's base. The boiling
point of water on the summit of Mont Blanc,
is as low as one hundred and eighty-four de-
grees. So, if water must be at two hundred
and twelve degrees, to make good tea, don't
choose too high a hill to build a temperance
hall on. The heavier, also, the air is, from
the quantity of moisture in it, the hotter
water becomes before it boils. If the
atmosphere were carbonic acid gas, water would
get much hotter without boiling than it can
under"—

"Present arrangements," interposed Mr.
Bagges.

"Consisting of a mixture of nitrogen and
oxygen," continued Harry. " Water requires
only a very low heat to make it boil in an
exhausted receiver, out of which the air has
been pumped, so as to leave none to press
upon its surface. Owing to boiling depending
upon pressure, you can actually make
water boil by means of cold."

"What next??" sighed Mrs. Wilkinson.

"You can, indeed. Put a little boiling
water in a salad-oil flask; so that the flask
may be a quarter full, say. Cork the flask
tightly. The boiling stops; and the upper
three-fourths of the flask are full of vapour.
Squirt a jet of ice-cold water upon the
flask, above where the water is, and the
water below will instantly begin to boil.
The reason why, is this. The vapour in the
flask presses on the surface of the hot water.
The cold condenses the vapour—turns it back
to water. That takes off the pressure for the
time; and then the hot water directly flies
into vapour, and boils, and so on, till it cools
down too low to boil any longer. What
reduces the boiling point of water on a hill
or a mountain is, that the pressure of the
atmosphere decreases as you ascend. A rise
of five hundred and thirty feet in height
above the level of the sea, makes a difference
of one degree; so, give me a kettle of water
and a thermometer, and I'll tell you exactly
how near the moon we are."

"I should'nt think one could make good
hot mixed punch up in a balloon, now,
observed Mr. Bagges, reflectively.

"Then," Harry proceeded, " it requires
more heat to make water boil in a glass vessel
than it does in a metal one. A metal vessel's
inner surface is made up of very small points
and dents. Scratching the inside of the
glass so as to give it a roughness
something like what the metal has, makes the
boiling point lower; and a few iron filings
thrown into water boiling in glass at two
hundred and fourteen degrees, will bring it
down to two hundred and twelve. The
filings, and the roughness of the glass, are so
many more points for the heat to pass into
the water from, and form steam, and the
water does not cling to them so hard as it
clings to a smooth surface. Throw a lot of
hay into a pan of hot water, and it makes a
quantity of steam rise directly; and I have
heard a 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