Because the heat of the fire passes into the water, and drives its atoms apart, making those of them that rise quickly to the surface lighter than the air, upon which they consequently rise.
751. Why does water become solid when it freezes?
Because the latent heat of the water passes away from between its atoms into the air; the atoms, therefore, draw closer together.
752. Why, if the atoms of water draw closer together when freezing, does ice expand, and occupy greater space than water?
Because, when the atoms of water are congealing, they do not form a compact mass, but arrange themselves in groups of crystal points, which occupy greater space. Water contracts when freezing until it sinks to 40 deg., and then it expands as ice is formed.
32 deg. is said to be the freezing point, but it should be called the frozen point.
Because heat, entering into the lower portions of the water, expands it; the heated portions are then specifically lighter than those that are cooler; the hot water therefore rises upward, and forces the cooler water down.
754. What proportion of the earth's surface is covered with water?
There are about one hundred and forty seven millions of square miles of water, to forty-nine and a half millions of square miles of land.