A refrigeration by which ordinary ice is artificially produced, perishable food of all kinds preserved for long times, and transported for great distances, which has proved an immense advantage to mankind everywhere and is still daily practised to the gratification and comfort of millions of men, must receive at least a passing notice. The Messrs. E. and F. Carré of France invented successful machines about 1870 for making ice by the rapid absorption and evaporation of heat by the ammonia process. The discoveries and inventions of others in the artificial production of cold by means of volatile liquids, whether for the making of ice or other purposes, constituted a great step in the art of refrigeration. Vaporisation, absorption, compression or reduction of atmospheric pressure are the principal methods of producing cold. By vaporisation, water, ether, sulphuric acid, ammonia, etc., in assuming the vaporous form change sensible heat to latent heat and produce a degree of cold which freezes an adjacent body of water. The principle of making ice by evaporation and absorption may be illustrated by two examples of the Carré methods:--It is well known what a great attraction sulphuric acid has for water. Water to be frozen is placed in a vessel connected by a pipe to a reservoir containing sulphuric acid. A vacuum is produced in this reservoir by the use of an air pump, while the acid is being constantly stirred. Lessening of the atmospheric pressure upon water causes its evaporation, and as the vapour is quietly absorbed by the sulphuric acid the water is quickly congealed. It is known that ammonia can be condensed into liquid form by pressure or cold, and is absorbed by and soluble in water to an extraordinary degree. A generator containing a strong solution of ammonia is connected by a pipe to an empty receiver immersed in cold water. The ammonia generator is then heated, its vapour driven off and conducted to a jacket around the centre of the receiver and is there condensed by pressure of an air pump. The central cylindrical space in the receiver is now filled with water, and the operation is reversed. The generator is immersed in cold water and pressure on the liquid ammonia removed. The liquid ammonia now passes into the gaseous state, and is conducted to and reabsorbed by the water in the generator. But in this evaporation great cold is produced and the water in the receiver is soon frozen.