Refrigeration is the process of making and keeping things cold. The way to make things cold is to remove heat from them.
In ancient times people used snow and ice mainly to refrigerate things. They also knew about the process of dissolving certain salts in water for cooling. Compounds like saltpeter and ammonium nitrate, remove heat from the water in which they dissolve. Thus they lower the temperature of water. Salt lowers the freezing point of water. When salt is put on ice, the ice changes to water. For this change to take place, heat is needed. This is supplied by the water and so the temperature falls.
So the earliest methods of refrigeration used natural ice or snow, or used salts dissolved in water. Another process that creates refrigeration is evaporation, the change of liquid to a vapor. When we keep a little alcohol on our hand, we feel coolness as the liquid takes heat from the hand evaporates.
Modern refrigerators are based on the principle of evaporation. In 1823, Michael Faraday learned how to change ammonia vapor to liquid by compression. The liquid is again converted to vapor by taking heat from the things to be refrigerated. By controlling the process of changing vapor to a liquid—and back from liquid to a vapor—taking up heat continuously we have our modern refrigerators.
The first refrigerating machine to use this principle was built by a Swiss man called Carl Lined in 1874 to cool beer. In 1877 Linde used ammonia as the liquid in refrigerators.
