the top of a kettle of water to the bottom.
Then how does it so easily get from the bottom to the top?"

"Ah!" sighed Mr. Bagges. "In my young days we should have said, because
the heat rises, but that won't do now. What is all that about
the--eh--what--law of ex--what?--pansion --eh?"

"The law of expansion of fluids and gases by heat. This makes the
currents that I spoke of just now, mamma; and I should have spelt the
word to explain to you that I didn't mean plums. You know what a draught
is?"

"I am sorry to say I do," Mr. Bagges declared with much seriousness,
instinctively carrying his hand to the region of the human body from the
Latin for which is derived the term, Lumbago.

"Well," pursued Harry, "a draught is a current of air. Such currents are
now passing up the chimney, and simply owing to that trifling
circumstance, we are able to sit here now without being stifled and
poisoned."

"Goodness!" ejaculated Mrs. Wilkinson.

"To be sure. The fire, in burning, turns into gases, which are rank
poison--carbonic acid, for one; sulphurous acid, for another. Hold your
nose over a shovelful of hot cinders if you doubt the fact. The gases
produced by the fire expand; they increase in bulk without getting
heavier, so much so that they become lighter in proportion than the air,
and then they rise, and this rising of hot air is what is meant by heat
going upward. The currents of hot air that go up the chimney in this way
have currents of cold air rushing after them, to supply their place.
When you heat water, currents are formed just as when you heat gas or
air. The heated portion of water rises, and some colder water comes down
in its place; and these movements of the water keep going on till the
whole bulk of it is equally hot throughout."

"Well, now," interrupted Mr. Bagges, "I dare say this is all very true,
but how do you prove it?"

"Prove that water is heated by the rising and falling of hot currents?
Get a long, slender glass jar. Put a little water, colored with indigo,
or any thing you like, into the bottom of it. Pour clear water upon the
colored, gently, so as not to mix the two, and yet nearly to fill the
jar. Float a little spirit of wine on the top of the water, and set fire
to it. Let it blaze away as long as you like; the colored water will
remain steady at the bottom of the jar. But hold the flame of a
spirit-lamp under the jar, and the colored water will rise and mix with
the clear, in very li