Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/628

612 612 ICE According to Pliicker, the coefficient of cubical dilatation at moderately low temperatures is 0001585. From a series of elaborate experiments, Person deduced 505 as the specific heat of ice, or about half that of water ; in other words, the heat required to raise 1 5) of water 1 C. will raise 2 fib of ice through the same range of temperature or 1 ft) of ice through 2 C. Though no rise of temperature accompanies the melting of ice, there is yet a definite quantity of heat absorbed, and a corresponding amount of work done mainly in altering the physical condition of the substance. The heat which disappears is transformed into other and less evident forms of energy, as, for example, the energy of translatory motion, which is the chief characteristic, according to the recognized molecular theory of matter, of the molecule in the liquid as compared with the molecule in the solid. The heat which is thus absorbed during the melting of unit miss of ice is called the latent heat of water, and its value in ordinary heat-units is 79 25, according to the determina tion of Person. Hence as much heat is required to trans form 1 fi&amp;gt; of ice at C. into water at the same tempera ture as would raise in temperature 1 fib of water through a range of 79 0- 25 C., or 79 25 fib of water through a range of 1 C. The same amount of heat which is absorbed when ice becomes water is evolved when water becomes ice, so that the melting of ice is accompanied by the abstraction of heat from surrounding objects, that is, by a cooling effect ; and the freezing of water by a heating effect. These thermal effects are generally masked by the pro cesses whereby the change of state is effected ; but the cooling which accompanies the melting of ice may be observed when pressure is used as the agent for accom plishing the change. That ice can be so melted by increase of pressure was first pointed out by Professor James Thomson (now of Glasgow) in a paper published in the Transactions of the Royal Society of Edinburgh for 1849 ; previous to that time the temperature of melting ice was believed to be absolutely constant under all conditions. Thomson showed that, since water expands on freezing, the laws of thermodynamics require that its freezing-point must be lowered by increase of pressure ; and, by an ap plication of Carnot s principle, he calculated that for every additional atmosphere of pressure the freezing-point of water was lowered by 0075 of a degree centigrade. This remarkable result was soon after verified, even to its numerical details, by his brother, Sir William Thomson (Proceedings of the Royal Society of Edinburgh, 1850). The Thomsons and Helmholtz have since then successfully ap plied this behaviour of ice under pressure to the explanation of many curious properties of the substance. When two blocks of ice at C. are pressed together or even simply laid in contact, they gradually unite along their touching surfaces till they form one block. This regelation, as it is called, is due to the increased pressure at the various points of contact causing the ice there to melt and cool. The water so formed tends to escape, thus relieving the pres sure for an instant, refreezing, and returning to the original temperature. This succession of melting and freezing, with their accompanying thermal effects, goes on until the two blocks are cemented into one. Thus it is that a snowball is formed ; and in virtue of the same succession of pheno mena does the glacier mould itself to its rocky bed and flow down the valley, behaving in many respects like a viscous fluid. Ice forms over fresh water if the temperature of the air has been for a sufficient time at or below the freezing-point ; but not until the whole mass of water has been cooled down to its point of maximum density, so that the subsequent cooling of the surface can give rise to no convection currents, is the freezing possible. Sea-water, in the most favourable circumstances, does not freeze till its temperature is reduced to about - 2 C. ; and the ice, when formed, is found to have rejected four-fifths of the salt which was originally present. In the upper provinces of India, water is made to freeze during cold clear nights by leaving it overnight in porous vessels, or in bottles which are enwrapped in moistened cloth. The water then freezes in virtue of the cold produced by its own evaporation or by the drying of the moistened wrapper. In Bengal the natives resort to a still more elaborate forcing of the conditions. Shallow pits are dug about 2 feet deep and filled three-quarters full with dry straw, on which are set flat porous pans containing the water to be frozen. Exposed overnight to a cool dry gentle wind from the north-west, the water evaporates at the expense of its own heat, and the consequent cooling takes place with sufficient rapidity to overbalance the slow influx of heat from above through the cooled dense air or from below through the badly conducting straw. The growing demand for ice for domestic, medicinal, and i c , other purposes has led, not only to the development of anig regularly organized ice trade, but also to the invention of machines for the manufacture of ice in countries which do not possess a sufficient home supply. The various types of machines which have been or are in use call for a brief description. Freezing-mixtures, such as the familiar snow and salt or the mixture of sulphate or phosphate of sodium and dilute nitric acid, may be dismissed with a word, since they are restricted in use to the production o-f intense cold for a brief period of time, and are incapable of economic application to the formation of large quantities of ice. All ice-machines which have proved of practical utility may be grouped under two great classes : those which utilize the lowering of temperature that accompanies the rapid expansion of a compressed gas, and those which make use of the like thermal effect that results from the vola tilization of some liquid. In machines of the first type, the gas usually employed is atmospheric air, which is first compressed to three or four atmospheres, and kept cool by circulating water or by other suitable means. It is then allowed to expand, and the heat necessarily absorbed daring the expansion is drawn either from the water to be frozen or from a solution of brine which does not freeze at the ordinary freezing temperature, and thus becomes, so to speak, a vehicle for the cold. In 1849 Gorrie constructed such a machine, which, however, was unsatisfactory in its action, probably because the compressed air was not sufficiently cooled and dried. More efficient in their action were Kirk s machine (patented in 1863), and Windhausen s (1870), one of which at the Vienna exhibition produced 30 cwts. of ice per hour, at the cost of Is. per cwt. The mode of action of Windhausen s is as follows. A piston works to and fro in a cylinder, compressing the air in the one end and allowing it to expand in the other. The compressed and therefore heated air forces its way through a valve to the cooling chambers, from which it is led towards the other end of the cylinder. Here the inlet valve is so arranged that it closes at a certain position of the receding piston, thus permitting what air has entered to expand and cool. At the return stroke this cooled air is forced out through easily opening valves, part going to cool the chambers into which the heated compressed air enters from the cylinder, and part passing to the refrigerator, from which after serving its purpose it is pushed on by the fresh supply of cooled air to the compressing end of the piston chamber. Such machines, to work economically, require large cylinders, tight-fitting pistons working with little friction, and perfect regulation in the motions of the various parts conditions so difficult to fulfil that refrigeration by means of compressed air may be regarded as a practical failure. The machines constructed by the Bell-Coleman Mechanical Refrigeration