Page:Popular Science Monthly Volume 5.djvu/420

404 Had the heat-rays which destroyed the ice fallen into water, it would have been heated; but, falling into the cube of ice, and melting a portion of it, no change in temperature occurred. Ice melts at 32°, and the temperature of the water in which it floats is kept steadily at thai point until all of it has disappeared. The ices we consume, and the iced drinks for which we thirst in summer, are, at the freezing temperature, 68° colder than the internal organs with which they are brought into contact.

Ice, like other solids, may be cooled and warmed. That which Tyndall chilled 100° could be warmed steadily to the temperature of 32°, and a thermometer would indicate the change; but at that point the process is interrupted—the structure falls into pieces, and not until the mass is entirely liquid can the warming be resumed. From that point, however, it goes on until, at a temperature of 212°, it again ceases, and the molecules of water are separated into vapor.

But, in melting the ice by the dark or heat rays of the beam of light, a great quantity of heat was consumed, not in raising temperature, but in undoing what molecular force had done. To simply melt a pound of ice requires 142° of heat, that is, an amount which would raise the temperature of a pound of water 142°. Now, this is the equivalent of the molecular force exerted in solidifying the water, and the mechanical value of the two forces is the same. Expressed in figures, it is equal to lifting the same pound of ice 110,000 feet high. The mere melting of 20 pounds of ice, a quantity received daily by many families, is equivalent, in mechanical force, to lifting nearly 1,000 tons' weight a foot high, or to lifting two persons weighing 300 pounds 1,000 feet higher than the summit of Mount Washington. We may thus realize the enormous display of energy along the line where heat and molecular force contend for the mastery.

The transition of water to ice, and of ice to water, produces important changes in the temperature of surrounding objects. We are often made painfully sensible of the chilling influence of the atmosphere when its heat is rapidly abstracted in the melting of large masses of ice and snow. But the reverse of this takes place in freezing. The crystallization of water is attended with an elevation of temperature. The heat which vapor carries with it in its aërial journeys is liberated when those vapors are transformed into flakes of snow. The expression we often hear when a storm in winter is imminent, that "the cold is too great for snowing," is true enough. The air is made warmer when snow-flakes begin to form, and the temperature is higher than it would otherwise be while snowing continues. In this way the formation of ice and snow modifies and softens the temperature of arctic winters; and the blossoms which open with the spring-time are not more significant of milder airs than are those which are born of frost and vapor, and expand their petals to the winter's tempest. Snowflakes are stellate in form; the molecules of vapor in crystallizing