Refrigeration

Refrigeration of a body is the loss of heat which it may undergo, its temperature being lowered in consequence. For a long time it was believed that rarefaction was alone sufficient to produce a lowering of temperature in a mass of gas, but it has been demonstrated by the experiments of Gay Lussac, Joule, and others that refrigeration will not be produced unless the gas expands against pressure - that is, unless it does work - and the loss of heat experienced is exactly equivalent to the amount of work done. Loss or gain of heat occurs when a body changes its state [Latent Heat], and this fact is practically employed in the artificial production of cold. Machines for the production of ice are known by the name of refrigerators, one of the commonest being that of M. Carre. Two strong metal cylinders A and B are connected by a metal tube, the whole being perfectly air-tight. Into A is placed a solution in water of ammonia gas. All the air is driven out of the apparatus, and the solution in A is heated, while B is cooled by running water. Ammonia gas is driven over from A to B, and soon condenses in B under its own pressure. After a suitable time, A is removed from the source of heat and is placed in cold water, while B is placed in connection with the water which it is required to freeze. This is suitably effected by having B in the form of a hollow jacket, capable of allowing a cylinder of water to be placed in its centre. As A cools, the ammonia is rapidly reabsorbed, the liquid ammonia in B volatilises, and thus seizes heat from surrounding bodies. The water in the central cylinder is the nearest body to supply this heat; it thus becomes itself intensely cold, and hence freezes. In "cold-air machines" air is compressed by a pump, cooled to the lowest convenient temperature (generally by cold-water circulation), and then allowed to expand against the atmospheric pressure. In so doing it takes up an amount of heat equivalent to the work done, and hence cools the surrounding bodies. This method is used for cold storage on board ship, etc. Artificial cold is also often produced by means of freezing mixtures (q.v.).