Page:Popular Science Monthly Volume 5.djvu/415

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THE PHYSICS OF ICE.

THE PHYSICS OF ICE.
By E. LEWIS Jr.

THREE-QUARTERS of a century ago a cargo of ice was obtained from a pond near the junction of Broadway and Canal Street, in New York, and sent to Charleston, South Carolina, in a vessel chartered by a gentleman of that city. But it was in 1805-'6 that Frederick Tudor, of Boston, inaugurated and laid the foundation of the now immense ice-trade of the United States by shipping, as a mercantile adventure, a cargo to St. Pierre, on the island of Martinique. This cargo, with several subsequent shipments to other West Indian ports, was largely unprofitable. The people to whom it was sent, unfamiliar with its use, knew little of its value.

In 1833, Mr. Tudor sent in the ship Tuscany the first cargo of ice from this country to Calcutta, and thus began the ice-trade of the United States with seaports of India. Several years after that event, the Hon. Edward Everett, then our minister to England, met in London a wealthy and eminent Hindoo, who cordially thanked the American people whom he represented for the great service they had done to his countrymen in shipping cargoes of ice to India. It is obvious that, in our zone of alternate heat and cold, ice is one of Winter's great benefactions. In the healthy preservation of food it is indispensable during summer's heat. In regions where little ice forms, the mountain-snows are economized. Some years since the supply on Mount Etna gave out, and a glacier buried beneath sand and lava was found, and worked as an ice-quarry to supply the necessities of the people. Ice of good quality is now produced by artificial methods, but it is our purpose to develop in this paper some of the physical properties and phenomena of ice, rather than its economic value.

Ice is simply water in a solid state. In ordinary conditions it begins to form at a temperature of 32° of Fahrenheit's thermometer, and this is the well-known freezing-point. Below it the molecules of water become fixed in the grasp of molecular force; above it they are separated by heat, and fall asunder, forming liquid water. But to this law are exceptions, in which water may be cooled many degrees below 32°, still remaining liquid. In glass vessels exposed in open air, water kept perfectly still has been reduced in temperature 15° below freezing, and in a vacuum much lower than this. In this condition there is a "tendency to freeze which is kept in check only by the difficulty of making a commencement," and the process begins by the slightest jar of the water. Fine particles of vapor, or mist, and water in fine capillary tubes, may remain unfrozen 20° or more below the freezing-point. Water thus cooled rises in temperature the instant crystallization begins, by liberation of its heat, and at 32°