LATENT HEAT
Far more efficient are the machines which utilize latent heat. As explained in a previous chapter, whenever a solid is converted into a liquid or into a gas a certain amount of heat is absorbed and stored up in such a way as not to become apparent to the senses or to a thermometer. Such heat is known as “latent heat.” For instance, we can add a pound of water at 50 degrees temperature to a pound of water at 200 degrees, and the mixture will have a temperature of 125 degrees, or the mean of 200 + 50 degrees. But a pound of ice at 32 degrees mixed with a pound of water at 200 degrees will not give us 116 degrees ((200+32)/2), but only 44½ degrees. In other words, about 143 heat units will be rendered latent in converting solid water into liquid water, reducing the temperature of the water to 57 degrees and then the mean of 57 and 32 is 44.5 (200-143=57, (57+32)/2 = 44½). A more striking experiment is to mix a pound of water cooled to 32 degrees F. with a pound of water at 175 degrees F., and the result will be two pounds of water at 103.5 degrees, but if we mix a pound of chopped ice at 32 degrees F. with a pound of water at 175 degrees F., the result will be two pounds of water cooled to the freezing point.
In passing from a liquid into a gas water absorbs far more heat and renders it latent. For each pound of water converted into steam at atmospheric pressure 970 B. t. u. are absorbed. This storage of latent heat is utilized to good advantage in refrigerating machinery. The vacuum machine invented by Dr. Cullen in 1755 was a latent heat machine.