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COLD (in O. Eng. cald and ceald, a word coming ultimately from a root cognate with the Lat. gelu, gelidus, and common in the Teutonic languages, which usually have two distinct forms for the substantive and the adjective, cf. Ger. Kolte, kalt, Dutch koude, koud),subjectively the sensation which is excited by contact with a substance whose temperature is lower than the normal; objectively a quality or condition of material bodies which gives rise to that sensation. Whether cold, in the objective sense, was to be regarded as a positive quality or merely as absence of heat was long a debated question. Thus Robert Boyle, who does not commit himself definitely to either view, says, in his New Experiments and Observations touching Cold, that " the dispute which is the primum frigidum is very well known among naturalists, some contending for the earth, others for water, others for the air, and some of the moderns for nitre, but all seeming to agree that there is some body or other that is of its own nature supremely cold and by participation of which all other bodies obtain that quality." But with the general acceptance of the dynamical theory of heat, cold naturally came to be regarded as a negative condition, depending on decrease in the amount of the molecular vibration that constitutes heat.

The question whether there is a limit to the degree of cold possible, and, if so, where the zero must be placed, was first attacked by the French physicist, G. Amontons, in 1702-1703, in connexion with his improvements in the air-thermometer. In his instrument temperatures were indicated by the height at which a column of mercury was sustained by a certain mass of air, the volume or " spring " of which of course varied with the heat to which it was exposed. Amontons therefore argued that the zero of his thermometer would be that temperature at which the spring of the air in it was reduced to nothing. On the scale he used the boiling-point of water was marked at 73 and the melting-point of ice at 511, so that the zero of his scale was equivalent to about - 240 on the centigrade scale. This remarkably close approximation to the modern value of - 273° for the zero of the air-thermometer was further improved on by J. H. Lambert (Pyrometrie, 1779), who gave the value - 270 and observed that this temperature might be regarded as absolute cold. Values of this order for the absolute zero were not, however, universally accepted about this period. Laplace and Lavoisier, for instance, in their treatise on heat (1780), arrived at values ranging from 150o to 3000 below the freezing-point of water, and thought that in any case it must be at least 60o below, while John Dalton in his Chemical Philosophy gave ten calculations of this value, and finally adopted - 3000° C. as the natural zero of temperature. After J. P. Joule had determined the mechanical equivalent of heat, Lord Kelvin approached the question from an entirely different point of view, and in 1848 devised a scale of absolute temperature which was inde pendent of the properties of any particular substance and was based solely on the fundamental laws of thermodynamics (see Heat and Thermodynamics). It followed from the principles on which this scale was constructed that its zero was placed at - 273°, at almost precisely the same point as the zero of the air-thermometer.

In nature the realms of space, on the probable assumption that the interstellar medium is perfectly transparent and diathermanous, must, as was pointed out by W. J. Macquorn Rankine, be incapable of acquiring any temperature, and must therefore be at the absolute zero. That, however, is not to say that if a suitable thermometer could be projected into space it would give a reading of - 273°. On the contrary, not being a transparent and diathermanous body, it would absorb radiation from the sun and other stars, and would thus become warmed. Professor J. H. Poynting (" Radiation in the Solar System," Phil. Trans., A, 1903, 202, p. 525) showed that as regards bodies in the solar system the effects of radiation from the stars are negligible, and calculated that by solar radiation alone a small absorbing sphere at the distance of Mercury from the sun would have its temperature raised to 483° Abs. (21 0° C.), at the distance of Venus to 358° Abs. (85° C.), of the earth to 300° Abs. (27° C.), of Mars to 243° Abs. (- 30° C.), and of Neptune to only 54° Abs. (- 219° C.). The French physicists of the early part of the 19th century held a different view, and rejected the hypothesis of the absolute cold of space. Fourier, for instance, postulated a fundamental temperature of space as necessary for the explanation of the heat-effects observed on the surface of the earth, and estimated that in the interplanetary regions it was little less than that of the terrestrial poles and below the freezing-point of mercury, though it was different in other parts of space (Ann. chim. phys., 1824, 27, pp. 141, r50). C. S. M. Pouillet, again, calculated the temperature of interplanetary space as - 142° C. (Comptes rendus, 1838, 7, p. 61), and Sir John Herschel as - 150° (Ency. Brit., 8th ed., art. " Meteorology," p. 643).

To attain the absolute zero in the laboratory, that is, to deprive a substance entirely of its heat, is a thermodynamical impossibility, and the most that the physicist can hope for is an indefinitely close approach to that point. The lowest steady temperature obtainable by the exhaustion of liquid hydrogen is about - 262° C. (I I° Abs.), and the liquefaction of helium by Professor Kamerlingh Onnes in 1908 yielded a liquid having a boiling-point of about 4.3° Abs., which on exhaustion must bring us to within about 22 degrees of the absolute zero. (See LIQUID GASES.) For a"cold," in the medical sense, see CATARRH and RESPIRATORY SYSTEM: Pathology.