A test-tube filled with cold water is closed by a stopper containing a small glass tube, the water extending up into the small tube. (See Fig. 127.) The test-tube is placed in a freezing mixture of salt and ice contained in a tumbler. As the water cools, the level of the water in the small tube at first sinks. But before the water freezes it rises again, showing that after the water cools to a certain temperature that expansion of the water occurs with further cooling.

Careful tests show that the water on cooling contracts until it reaches 4°C. On cooling below this temperature it expands. For this reason, when the water of a lake or river freezes, the coldest water is at the surface. On account of this the ice forms at the top instead of at the bottom. If water contracted as it cooled to the freezing temperature the coldest water would be at the bottom. Freezing would begin at the bottom instead of at the surface. Lakes and rivers would freeze solid. In the summer only in shallow waters would all the ice melt. The result would be that fish and other aquatic life would be killed. Climate would be so changed that the earth might become uninhabitable. Since water is densest at 4°C. all the water in a lake or river, when it is covered with ice, is at 4°C. except that near the surface.

Fig. 127.—Apparatus used in testing the expansion of water.

149. The Expansion Of Solids.—Most solids when heated expand less than liquids and gases. Careful experiments show that expansion is:

(a) Proportional to the change in temperature.

(b) Different in different solids.

Here are a few coefficients of linear (length) expansion.

Brass0.000018 per degree C.
Glass0.000009 per degree C.
Ice0.000052 per degree C.
Iron0.000012 per degree C.
Platinum0.000009 per degree C.
Zinc0.000027 per degree C.