Page:Popular Science Monthly Volume 84.djvu/228

224 water is concerned is surface tension. It is this property which causes a liquid to rise in a capillary tube and also aids in the formation of drops (pressure of the atmosphere likewise tending to reduce the liquid to the smallest, most stable geometric shape possible). It is due to capillarity that the minute blood vessels of living animals are supplied with blood, that a blotter sucks up ink, that moisture tends to come to the surface of the earth and that a good many other essential things of a similar nature take place. In fact we could not do without this important force.

Solid Water.—Here we have snow, hail, frost and ordinary ice.

Snowflakes are assemblages of minute crystals of ice formed from the aqueous vapor in the atmosphere. They vary in size from one fourteenth of an inch to one inch in diameter. The smaller ones are formed when the temperature is very low, but the larger ones not until it is near 0° C. They always assume a hexagonal shape and from each corner of the hexagon protrudes a ray at an angle of 60° to the ray on either side of it. This fundamental form is the same, no matter how much the crystals otherwise vary in shape.

Snow is only white to the eye because of the great refractive power of the crystals, which, when examined under the microscope, are seen to be transparent. It forms whenever a cold enough wave passes over a moist atmosphere, the water condensing out as crystals. Hail, on the other hand, is formed when the rain passes through a region of the atmosphere sufficiently cold to freeze it.

Just as the dew condenses out of the atmosphere on a summer night, on a winter night, when the temperature is below 0° C. frost forms. The action of frost as a geological agent need hardly be mentioned, it is so well known. Suffice it to say that it has played and continues to play a very important role in the changing of the earth's topography.

Water, when it cools, contracts until it reaches a temperature of 4° C, and then it begins to expand, slowly at first, until it very nearly reaches 0° C. and is about to freeze, then it increases very markedly and suddenly in volume. The specific gravity of ice is only 0.920, whereas at 4° C, pure water has a specific gravity of 1.000, that is, at 4° C, one cubic centimeter of pure water, in a vacuum, weighs one gram. Water is therefore used as a unit for specific gravity measurements. If water contracted all the way down to its freezing point, as most liquids do, in one cold winter every river, lake, etc., would be frozen up and would stay so, because of the ice being so much heavier than water and sinking to the bottom.

In freezing, water gives off a very large amount of heat, 79.06 calories for every gram of ice formed. The amount of heat liberated in freezing a gram of water, stating it in other words, is sufficient to raise the temperature of 79.06 times its weight of water from 0° C, to