When the frosts come the ice hardens. A winter road can be laid on the river. But under the thick layer of ice water continues to flow as before. The river never freezes to the bottom. Ice, the solid state of water, is a very peculiar substance. 

   There are several kinds of ice. The one found in nature is that which melts at zero (on the Celsius scale). By using high pressures scientists have obtained six more varieties of ice in the laboratory. The most fantastic of them (ice VII), occurring at pressures over 21,700 atmospheres, might be called red-hot ice. It melts at 192°C above zero when the pressure is 32,000 atmospheres. It would seem that there could hardly be anything more familiar than the picture of ice melting. But what surprising things it involves!

   After melting any solid begins to expand. But the water that forms when ice melts behaves quite differently: it contracts and only afterwards, if the temperature continues to rise, does it begin to expand. This is again due to the ability of water molecules to attract one another. At four degrees above zero this ability becomes especially pronounced, and therefore at this temperature the density of water is at its highest; that is why our rivers, ponds and lakes do not freeze to the bottom even in the coldest weather. The coming of spring makes everyone glad; we have all taken pleasure in the golden days of autumn. The joyous spring thaw and the crimson attire of the woods . . .

   Again the result of an anomalous property of water!

   A great deal of heat is required to melt ice, much more than to melt the same quantity of any other substance. When water freezes this heat is evolved, and in returning it, ice and snow warm the Earth and the air. They soften the sharp transition to severe winter and enable autumn to reign for several weeks. In the spring, on the contrary, the melting of the ice holds back the sultry weather for a time.

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