Other views of the question are numerous, for instance, that of Pcrsoz, adopted by Forbes, in which ice is considered as essentially colder than water, and as passing through a sort of viscous state before liquefying, as metals do during the process of melting. This idea, however, has not of late found much support; and it is probable that the true solution of the question is, as J. Thomson has lately pointed out, to be found in the analogy of the crystallization of salts from their aqueous solutions.
However that may be, there is no doubt about the following property of ice, theoreti cally predicted by J. Thomson from the experimental 'fact of its expanding in the act of freezing, and demonstrated by means of the piczometer by sir W. Thomson—viz., that the freezing-point of water, or the melting-point of ice, is lowered by pressure; and the brothers have, with singular ingenuity, applied this to the explanation of the motion of glaciers. That a mass of glacier-ice moves in its channel like a viscous fluid, was first completely established by Forbes. Thomson's explanation of this motion is of the following nature: In the immense mass of the glacier (even if it. were homogeneous, much more so when full of cracks and fissures, as it always is), there are portions sub jected to 'a much greater strain than others. The pressure to which they are subjected is such as corresponds to a melting-point considerably below the temperature of the mass —and therefore, at such points, the ice melts, the strain is relieved, and the whole mass is free for an instant to move nearly as a fluid would move in its place. But, the strains being thus for au instant removed, the temperature and pressure of the water are again consistent with thih layer of water quickly solidifies, and then matters proceed as before. Thus, at every instant, the strains at different parts of the mass melt it at those places where they are greatest, and so produce the extraordinary phenome non of a mass which may in CO1111E011 language be termed solid, and even rigid, slowly creeping down its rocky bed like a stream of tar or treacle.
lee-Trade and .:11ilnufacture. —The trade in ice is now one of great and increasing importance. Ice has always been esteemed as a luxury in warm weather; and this early led to the storing of it in winter and preserving it for slimmer use. The Greeks. and after wards the Romans, at first preserved snow, closely packed in deep underground cellars. Nero, at a later period, established ice-houses in Rome, similar to those in use in most European countries imp to the present time. But these means were not enough to supply the luxurious Romans with ice for cooling beverages, and they actually established a trade in snow, which was brought to Rome from the summits of distant mountains.
The trade in ice in this country has, until lately, been very limited, having been chiefly confined to the supply required by a few of the first-class fishulongers and con fectioners—the private residences of the more opulent families being furnished with ice houses in which it sufficiency is kept for fouivate use. But the North Americans have started a tmde in this article in their own cities, which has extended to Europe and Asia, and has, in an incredibly short space of time, attained a surprising magnitude. The export of ice from America was commenced about 1820 by a,merehant named Tudor, who sent ice from Boston to the West Indies. After persevering .against many losses he succeeded in establishing a trade with Calcutta, 'Madras, and Bombay; and nom- not only is it sent in vast quantities to those places, but also to Hong-Kong, 'N'ilitampoa, and Batavia. About the year 1840 the Wenham lake ice company commenced sending ice to this country from Boston, which is the great American port for shipment of this material; and since then not only has there been a continually increasing demand, hilt the success of the company has been so great as to tempt others into the market. The supply for Great Britain, however, now Conies almost wholly from Norway; 184,021 tons having been imported thence in 1876, of the value of S2167,092, whilst only 10 tons were received from other countries. Fifteen or twenty years previous America had sent to Great
Britain on an average 20,000 tons annually, costing as many thousand pbunds.
In America the ice is chiefly collected in the neighborhood of Boston, Philadelphia, Baltimore, Washington, and NeW York, and the supply it form no small p of the property of those whose lands border thereon; these have all been carefully marked out, and the right secured, so that, when the winter collies, and the ice is formed, the harvest begins with great regularity. The ice is cleared from snow by means of an implement called the plane. An ice-plow, drawn by horses, and driven by a man riding upon it, is then made to cut deep parallel grooves in the ice, and these. are again crossed by other grooves at right angles, so that the whole of the surface is deeply marked out into small squares, measuring a little more than 3 feet. A few of these square blocks being detached by hand-saws, the remainder are easily broken off with crowbars, and floated away to the ice-storehouses, are usually built of wood, on the borders of the lake. Some of these are of vast dimensions, and contain vaults of great depth; the walls are double, and sometimes treble, being altogether as much as 4 ft. in thickness, and having hollow spaces between to render them less heat-conducting. The blocks of ice are covered up with sawdust, a layer being placed between each tier of blocks. Many of these ice-houses arc made large enough to hold from 40,000 to 50.0U0 tons of ice. When fully stored a large quantity of dried marsh grass is trodden in upon the top, to the thickness of several feet, and the doors are then securely closed. The total value of the ice which is stored iu America has been computed at 4'900,000, and if to this we add the ice-trade of Norway, Sweden, Russia, which, from time slight data we possess, is estimated at .E600,000. we have the astounding fact that a value of one million and a half sterling is added to a comparatively small body of water by the mere act of freezing.
Notwithstanding the facilities for importing ice, there is, and always will be, a neces sity for procuring it locally if possible. Hence there has been much talent brought to bear in devising means far artificial freezing. See Fa EEZ/NG MIXTURES. But until recently the modes of producing ice artificially were too costly to be practically useful. The desideratum, however, may now be considered as supplied; for machines are con structed by Mr. Siebe of Lambeth capable of producing ten tons per day. Such a machine requires steam-power to work it, and its action consists in evaporating ether or any similarly volatile liquid us verve, and again condensing the vapor to liqua so as to be used afresh. By this machine, 20°• F. below zero (52° of cold) has been easily obtained, and such a machine is now regularly producing large quantities of ice nearly under the equator in Peru, where previously ice had never been seen; and others are used in India and at the Cape in the service of the government, for the use of the troops and hospitals. Another machine was first shown in the London exhibition of 18U2 by the French firm of Carro & Co. which, from its extreme simplicity, and the fact that it can be adapted to the humblest kitchen, has led to its extensive application. The price is from 4'4 upwards; and as the volatile liquid is only the aqueous solution of ammonia, the cost of working it is very slight. The importance of low temperatures in many manufactures has lately increased the demand for freezing-machines, and they are now of great size and power. The machine-made ice has the same valuable quality that distinguishes the imported lake ice, viz., its perfect purity, so that pieces can be put into tile drink that is to be cooled.