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. Badges.

"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
vapor. 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 vapor in the flask presses on the surface of the hot
water. The cold condenses the vapor--turns it back to water. That takes
off the pressure for the time; and then the hot water directly flies
into vapor, 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 shouldn't 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