ating agent by Pictst of Geneva (1876), and thereafter applied by Gamgee to the formation of his glaciarium or artificial skating rink, is in many respects far superior to any other known refrigerator. Thus it is more easily liquefied than ammonia and methylic ether, exerting a vapour pressure of only 4| atmospheres at 30 C. : it has no chemical action upon metals or fats ; it is incombustible ; it is obtainable at small expense ; and it has, besides, good lubricating properties : in short, it seems to possess all the essentials of an efficient and economical refrigerator. In Pictet s machine, the liquid sulphurous acid passes under pressure from the condenser to the refrigerator, where on the pressure being relieved it vaporizes, cooling to -7 C. a current of brine which then flows round the tanks containing the water to be frozen. The sulphurous acid gas in the refrigerator is drawn over by an aspirating force- pump and recondensed in the condenser, which is kept cool by an ample supply of cold water. By a special modification of the sulphurous acid machine, Pictet obtained as low a temperature as -73 C. ; under this low tempera ture he then compressed carbonic acid gas to a liquid, by the evaporation of which lie produced such intense cold as to enable him to liquefy the so-called permanent gases under a pressure of several hundreds of atmospheres (Bibliotheque Universelle, 1878). Gamgee uses as his congealing liquid a solution of 4 pirts of glycerin in 6 parts of water, which is conveyed in pipes beneath the water-surface to be frozen. Machines which are capable of freezing water may in certain circumstances be much more efficiently employed to produce cooling without freezing. For instance, in curing- houses, breweries, sugar refineries, provision stores in hot climates, and in ships engaged in the transport of meat, where it is of importance to have the temperature moderately cool, it is usually by no means necessary to obtain ice. In many such cases, indeed, the production of ice would be a mere waste of labour. In tropical and subtropical climates refrigeration is of high importance from a sanitary point of view ; and there seems little doubt that if a simple, economical, and thoroughly efficient means of cooling were discovered, houses would be cooled in warm weather with the same care and regularity with which they are when necessary heated. At present, however, the manufacture of ice and the artificial production of cold are arts still in their infancy, which have a powerful rival in the extensive and increasing ice-trade that has sprung up within the last half century.
The idea of trading in ice first occurred to a Boston merchant, named Tudor, who in 1805 shipped ice to Martinique. In 1833 American ice began to be imported into Calcutta, where it was sold for 3d. per pound exactly half the price of the Bengal manufactured ice. In America, which was for long the great ice-exporting country of the world, supplying especially the West Indies, India, and China, the cutting and storing of ice form an important industry during the winter months. When the ice is suffi ciently thick, 9 to 12 inches for home consumption, 20 inches for exportation, the surface is scraped free of all porous ice, and is marked out into squares of 5 feet each way. Along these lines the ice is grooved to a depth of 3 inches by means of a plough. An instrument like a harrow is drawn over the grooves so as to deepen them ; and, after the surface has been divided into smaller squares, the ice is cut up into blocks by means of handsaws. The blocks are then removed to large double-walled storehouses, many of which are capable of containing thousands of tons of ice, It is estimated that, in America, 2,000,000 tons of ice are cut and stored annually by companies supplying New York and the middle States. New York city alone consumes as much as 500,000 tons per annum. A considerable quantity of ice from Wenham Lake near Boston was at one time imported into Britain, but now the whole supply comes from Drobak near Christiania in Norway. The Norwegian ice is remark ably solid and pure, and is superior in its staying power to English ice or to manufactured ice. The total quantity imported into the United Kingdom may be estimated roughly at 150,000 tons per annum, of which the greater part is consumed in London, where it is retailed at from 2s. Gd. to 3s. Gd. per cwt. At present Norway is undoubtedly the great ice-store for the Old World ; and quite recently (1880) Norwegian ice has been sold in the United States more cheaply than native ice. The transport on board ship offers practically no difficulty, since, as long as the hold is kept dry and cool, there is very little loss, and in the lading no special care need be taken. For the storing in houses, see ICE-HOUSE. (c. G. K.)
ICEBERG, a floating mass of ice, which has broken off from such ice-sheets as cover Greenland, Spitzbergen, and other polar lands, constituting vast glacier systems ever creeping out and down from the central heights to the shores. As the glacier is pushed out to sea, the lower margin is exposed to the destructive action of the waves, and breaks up into fragments of endless variety of form. These severed blocks, many of which are hundreds of millions of tons in mass, drift to lower latitudes under the influence of polar currents, and gradually melt away in the warmer water. Such is the natural history of icebergs, which, in their freshwater origin, are to be distinguished from the ice-fields, ice-floes, pack ice, and ice-hummocks, so familiar to the polar voyager. The iceberg, as it drifts along, melts most rapidly under water ; and this unequal wasting must be accompanied by a change in the position of the centre of mass and a consequent shifting of the iceberg into its new position of equilibrium. Undermining and fracture also result ; so that at length the mass of ice, however square-shaped or symmetrical it may have been originally, assumes a form irregular and fantastic in the extreme. The densities of ice and sea-water are nearly - 92 and 1 03 respectively, so that only ~% or nearly ^ th of the iceberg is above water and visible. An iceberg observed by Sir John Ross and Lieutenant Parry was 2J miles long, 2i miles broad, and 153 feet high. Assuming the form to have been approximately a cone erected upon an elliptic base, the mass above water would be roughly 150 million tons giving a total mass of nearly 15 hundred million tons. This iceberg, however, was by no means of extraordinary dimensions. In the southern seas, great fleets of icebergs have been observed as far north as the latitude of Cape Horn ; and some of these ice masses have towered to a height of 700 or 800 feet. The limiting latitude to which icebergs drift is lower in the southern than in the northern hemisphere, probably because of the comparative scarcity of land in the south polar regions. Thus, icebergs have been observed off the Cape of Good Hope in 34 S. lat., while none has been noticed in the northern hemisphere lower than the 36th parallel. Generally speaking, the limiting latitudes may be fixed at 40 N. lat. and 35 S. lat. In the North Atlantic the distribution of icebergs is very remarkable, and indicates, in its peculiar way, the general set of ocean currents. Ice bergs, of course, can only drift along with some polar current, such, for example, as the Labrador current, which flows in a generally southerly direction round the coasts of Newfoundland and Nova Scotia. To the east of this track, in which icebergs abound during the early summer months, lies the region which is warmed by the waters of the Gulf Stream as it flows to the Scandinavian coast, and here floating ice is rarely seen. The same considerations regard ing prevailing currents determine the distribution and limit of icebergs in the southern oceans, the great antarctic current that cools the coasts of Chili and Peru bearing upon
