We have now seen what a wonderful change is brought about by heating water. At first, it expands gradually and slightly; but, when it reaches the boiling-point, it suddenly expands enormously, and is no longer a liquid, but a gas.
On the other hand, if warm water is allowed to cool, it gradually contracts till it reaches the ordinary temperature of the air in mild weather; but, if the weather is very cold, or if the water is cooled artificially, it goes on contracting only down to a certain temperature (39°), and then begins to expand again. In this peculiarity water is unlike all other bodies which are fluid at ordinary temperatures. Hence the temperature of 39° is that at which pure water has its greatest density or specific gravity, and water at this temperature is heavier, bulk for bulk, than the same water at any other temperature. Therefore if water at the top of a vessel is cooled down to this temperature, it falls to the bottom, and if the water at the bottom of a vessel is cooled below this temperature it rises to the top.
40. =Water cooled still further becomes the transparent brittle solid Ice.=
Our tumbler of water, if put out of doors on a cold winter’s night, would gradually cool until it assumed a temperature of 39° throughout. Cooling below this temperature, the water so cooled would gradually accumulate at the surface by reason of its less density, and its temperature would fall till the thermometer placed in it marked 32°. As soon as this upper water cooled ever so little below 32°, a film like glass would form on its surface by the conversion of the coldest fluid water into solid water or =ice=. And if all the water cooled down to the same degree it would all gradually change into the same kind of substance.
In this condition water is solid. It occupies space, offers resistance, has weight and transmits motion as the water did, but if you shake it out of the tumbler in a cold place it retains its form without the least change. If you press it, it proves to be exceedingly hard and unyielding; and, if the pressure is increased, it becomes crushed and breaks like glass. It may thus be crushed to powder, and the ice powder can be formed into heaps as if it were sand.