Refrigeration is the process of making and keeping things cold. The way to make things cold is to remove heat from them. So refrigeration is any process that removes heat to make cold.

In ancient times, of course, snow and ice were the natural way to refrigerate things. Wines were cooled in this way. But even in ancient times another way to produce cold was known.

This was the process of dissolving certain salts in water. Materials 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, energy or heat is needed. This is supplied by the water and so its temperature falls.

So the earliest methods of refrigeration used natural ice or snow, or used salts dissolved in water. But there is another process that creates refrigeration. This is called evaporation, the change of a liquid to a vapor. When you put a little water or alcohol on your hand, you feel the coolness as the liquid takes heat from the hand and evaporates.

It was this principle of evaporation that led to the creation of our modern refrigerators. In 1823 Michael Faraday learned how to change ammonia vapor to a liquid by compressing or squeezing it and then removing heat from it. When the pressure is removed and this liquid is allowed to evaporate again, it takes up heat and produces cold.

Why did this make our modern refrigeration possible? Because we now had a way to change something first from a vapor to a liquid giving up its heat. Then we could change it back from a liquid to a vapor-taking up heat. By controlling this process and having it take place continuously, we have our modern refrigerators

The first refrigerating machine to use this principle was built by a Swiss inventor named Carl Linde in 1874 to cool beer. In 1877 Linde used ammonia as the liquid in such a machine and modern refrigeration was on its way.