COLD, subjectively the sensation which is excited by con tact 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 Experi ments 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, de pending on decrease in the amount of the molecular vibration that constitutes heat.
The question whether there is a limit to the degree of cold pos sible, and, if so, where the zero must be placed, was first attacked by the French physicist, G. Amontons, in 1702-03, in connection 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 ther mometer 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 514, so that the zero of his scale was equivalent to about — 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 —27o° 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 (178o), arrived at values ranging from 1,500° to 3,000° below the freezing-point of water, and thought that in any case it must be at least 600° below, while John Dalton in his Chemical Phi losophy gave ten calculations of this value, and finally adopted —3,000°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 independent 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 diather manous, 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 suit able 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. (See J. H. Poynting, "Radiation in the Solar System," Phil. Trans. A, 19o3.) The French physicists of the early part of the 19th century held a dif ferent view, and rejected the hypothesis of the absolute cold of space. Fourier, for instance, postulated a fundamental tempera ture 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 dif ferent in other parts of space (Ann. chim. phys., 1824, 27, pp. 141, I 50) . C. S. M. Pouillet, again, calculated the temperature of inter planetary space as —142°C. (Comptes rendus, 1838, 7, P. 61), and Sir John Herschel as — 150°.
To attain the absolute zero in the laboratory, that is, to deprive a substance entirely of its heat, is a thermodynamical impossibil ity, 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 Prof. Kamerlingh Onnes in 1908 yielded a liquid having a boiling-point of about 4° Abs. which on exhaustion should bring us to within about I° of the absolute zero. Liquid helium has since been solidified by Prof. Keesom (19 26) at 1.13° Abs. under a pressure of 25 atmos. The freezing point rises to 4.2° Abs. under a pressure of 14o atmos. (See LIQUEFACTION OF GASES.)