ter a little time, they would get down to the
ground."

"Perhaps not," said his father; "for, when they get down nearer the
earth, where it is warm, they may be gradually dissolved, and disappear,
and thus never reach the earth. I should think they would descend, being
composed of globules of water, which, however small, must, I should
think, be heavier than air."

"A soap bubble will float in the air," said Rollo.

"I never saw one that would," said his father, "unless it got into a
current, which carried it up. A soap bubble--make it ever so thin--shows
a tendency to descend, unless you put it out in the open air, where
there are currents to carry it up. It descends very slowly, but still it
descends. It is heavier than the air. I am not absolutely certain, but I
believe there is no visible substance that is lighter than the air; and
it is very well for us there is not."

"Why, father?" said Rollo.

"Because, if there were any, they would immediately rise from the earth,
and float upwards, till they got up where the air was so light and thin,
that they could not go up any higher."

"And so," said Rollo, "we should lose them."

"That would not be all," said his father. "They would float about, above
us, and, if there were enough of them, they would form a perpetual cloud
over our heads, to keep out the sun, and to make the world dark and
gloomy. There seems to have been no way to keep all the solid and
visible substances of the earth down upon its surface, but to make them
all heavier than the air.

"And thus," continued his father, "all solid substances being heavier
than the air, they sink in it, like stones or iron in water. Only those
that are very much expanded in surface, sink very slowly, and sometimes
almost seem to float."

"What do you mean by _expanded_, father?" said Nathan.

"Spread out," said his father. "An umbrella, for example, when it is
spread out, is said to be expanded; other things are expanded in a
little different way. A feather is expanded, that is, it is spread out
in fine filaments, which extend, in every direction, into the air, all
around the stem of it. Things that are expanded take a great deal of air
with them when they descend, and so can only descend slowly."

"And water is expanded in a soap bubble," said Rollo.

"Yes," replied his father, "and there is a great deal of air included in
it, which all has to be brought down when the bubble itself descends.
And thus, you se