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. [Illustration: 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. Brass 0.000018 per degree C. Glass 0.000009 per degree C. Ice 0.000052 per degree C. Iron 0.000012 per degree C. Platinum 0.000009 per degree C. Zinc 0.000027 per degree C. _The coefficient of linear expansion is the fraction of its length that a body expands when heated one degree._ _The coefficient of cubical expansion is the fraction of its volume that a body expands when heated one degree._ The expansion of solids is used or allowed for in many cases: a. Joints between the rails on a railroad allow for the expansion of the rails in summer. b. One end of a steel truss bridge is usually supported on rollers so that it can expand and contract with changing temperatures. (See Fig. 128.) [Illustration: FIG. 128.--Truss bridge showing roller support at one end.] c. Suspension bridges have expansion joints where the ends of the iron girders can move in or out of an expansion joint thus making the bridge longer or shorter according to the temperature. d. Iron tires are heated, slipped on to wagon wheels and then cooled, the contraction on cooling setting them tightly in place.