"Yes," said his father, "there is no difficulty about that. The vapor from a fire is carried up by the warm air, no doubt. Air swells when it is heated, and so becomes lighter, and rises; and the hot air from the top of the chimney carries the vapor up with it, no doubt. After it rises a little way, and becomes cool, it ceases to ascend, but floats away horizontally. Perhaps it begins to descend when it gets cool, though very slowly; and perhaps all clouds are really descending all the time, though too slowly for us to perceive the motion."
"Only," said Rollo, "after 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."
"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."