“Impossible,” you say?
Many things may seem to you impossible, which are nevertheless facts!
Things which are every-day experiences to us were “impossible” for
the last generation. We must be careful before pronouncing that word
“impossible.” For, strange as it may seem, this is a simple little
experiment which you yourself can perform in a few minutes, with a
little apparatus, procured from Mr. Woolworth or any drug store.
If you take an ordinary dish-pan, fill it with water, and drop a piece
of ice into it, the ice will of course melt long before the water comes
to a boil. But there is a way of doing this so that the ice does not
melt, and the water is boiling nevertheless!
Procure, if possible, a “Bunsen burner,” and a few feet of rubber
tubing. A Bunsen burner is merely a gas jet with a small hole in the
pipe, allowing a certain amount of air to mix with the gas as it burns.
In this way, you get a small, pointed flame of gas, hot and blue, like
the flame seen on a gas stove. Connect the tube to your gas supply, and
light it.
You must have already provided yourself with a glass “test tube,” about
eight inches long and a half an inch in width. (This can be obtained
for a few cents.) Also a piece of ice about an inch long, and just
large enough to fit into the tube; a couple of leaden shot or small
weights, and a handle for holding the test tube.
Place the ice in the tube, and drop the shot or weights upon it. Then
fill it nearly full of ice-cold water. The weights will keep the piece
of ice at the bottom of the tube, and prevent it from floating to the
top of the water.
Then hold the test tube, by means of the handle, over the gas flame,
at an angle of about forty-five degrees, in such a way that the flame
strikes the side of the tube about an inch below the surface of the
water. Do not heat the _bottom_ of the tube, containing the ice! (It is
best to approach the tube to the flame several times slowly, to prevent
the glass from cracking.)
If you have carried out these instructions, you will find that the
upper layer of water will be heated, and will boil, giving off steam,
whereas the lower portion of the test tube is still cold, and you can
invert the tube--when the solid block of ice will tumble out onto the
table.
The reason for this is that, whereas solids are heated by what is known
as “conduction” (one particle passing on its heat to the next), liquids
are heated by “convection” (circulation of the fluids). In this case,
no circulation takes place below the spot where the heat was applied,
and hence the lower portion of the tube remained cold.
You can see “convection” going on in any ordinary pan or kettle, by
dropping a few bits of bran or saw-dust into the water.