oil is lighter than a quart of water, the oil will
float; since a pint of cream is lighter than a pint of milk, the cream
will rise. In the same way, anything that is lighter than the same
volume of air will be pushed up by the air. When a balloon with its
passengers weighs less than the amount of air that it takes the place
of at any one time, it will go up. Since a quart of warm air weighs
less than a quart of cold air, the warm air will rise.

You can see how a heavy substance like water pushes a lighter one,
like oil, up out of its way, in the following experiment:

EXPERIMENT 11. Fill one test tube to the brim with kerosene
slightly colored with a little iodine. Fill another test tube
to the brim with water, colored with a little blueing. Put a
small square of cardboard over the test tube of water, hold it
in place, and turn the test tube upside down. You can let go
of the cardboard now, as the air pressure will hold it up. Put
the mouth of the test tube of water exactly over the mouth of
the test tube of kerosene. Pull the cardboard out from between
the two tubes, or have some one else do this while you hold
the two tubes mouth to mouth. If you are careful, you will not
spill a drop. If nothing happens when the cardboard is pulled
away, gently rock the two tubes, holding their mouths tightly
together.

[Illustration: FIG. 12. The upper tube is filled with water and the
lower with oil. What will happen when she pulls the cardboard out?]

Oil is lighter than water, as you know, because you have seen a film
of oil floating on water. When you have the two test tubes in such a
position that the oil and water can change, the water is pulled down
under the kerosene because gravity is pulling harder on the water
than it is pulling on the kerosene. The water, therefore, goes to the
bottom and this forces the kerosene up.

_APPLICATION 6._ Three men were making a raft. For floats they
meant to use some air-tight galvanized iron cylinders. One
of them wanted to fill the cylinders with cork, "because," he
said, "cork is what you put in life preservers and it floats
better than anything I know of." "They'd be better with
nothing in them at all," said a second. "Pump all the air
out and leave vacuums. They're air-tight and they are strong
enough to resist the air pressure." But the third man said,
"Why, you've got to have some air in t