Cold

Cold, the absence of heat. No body is absolutely cold; it must contain a certain amount of heat. The degree of cold is usually measured by thermometry, that is, by noting its temperature. The sensation of cold that a body may give is only estimated relatively, by comparison with its surroundings and with the human body. If when left to itself it gives out less heat than it receives, it is said to be colder than its surroundings. The second law of thermodynamics states that heat cannot pass from a colder to a hotter body without employment of external energy to effect the transfer. It is not that the coldest body in a system does not radiate heat out from itself, but it receives more than it gives, and the exchange is in effect the same as if cold were existent and were radiated from the body. Varying degrees of cold may be obtained by the use of certain mixtures, generally termed freezing-mixtures (q.v.). Thus, if equal parts of ammonium-nitrate and water be mixed together a lowering of temperature to the extent of 26° C. may be effected. About the same effect is produced by mixing five parts of snow or pounded ice with two parts of common salt. The rapid evaporation of liquid carbonic acid or of sulphurous acid will also greatly lower the temperature of bodies in the immediate neighbourhood.