LATENT HEAT. 53 



familiar instance may be given. If a person slide down 

 from an elevated place by means of a rope held in the 

 hands and he descends rapidly he feels a burning sensation 

 in his hands and the skin is blistered precisely the same as 

 if he held a hot iron rod in his hands. This heat is the re- 

 sult of an intense vibration of the fibers of the muscles and 

 skin of the hands ; and is equal in degree exactly to the vi- 

 bration of the particles in an iron rod whose heat would 

 cause precisely the same sensation and result in the hands. 

 To study the relations of heat with intelligence it must not 

 be regarded as a thing, but as a condition of matter and an 

 effect of the change of a condition. 



The chief source of heat is the sun. All combustion is a 

 source of heat, and as we have seen, combustion is a chemi- 

 cal effect. Mechanical force is also a source of heat ; and 

 friction, pressure, or any other result of force is accompanied 

 by heat. Heat once produced is never lost or destroyed : it 

 may disappear but it always exists. The heat of the sun 

 communicated to the earth is absorbed in various ways, that 

 is we use this expression ; but in truth we should say the 

 force is communicated to every object brought under its 

 influence. It is absorbed by the waters of the ocean and 

 their particles move and separate more widely apart form- 

 ing vapor. 



The amount of force (which we call heat) thus commun- 

 icated has been accurately calculated. If we take an ounce 

 of ice at 32 degrees and one of water at 174 degrees and 

 put them together, the ice will be melted and there will be 

 two ounces of water; but the temperature will be only 32 

 degrees. Where has the excess of 142 degrees of heat which 

 has been apparently lost by the hot water, disappeared ? It 

 has not been lost but has become stored up in the water 

 and has become the latent or hidden heat of the liquid. 

 This latent heat can be found again when the water is froz- 

 en, for in the formation of ice precisely 142 degrees of heat 

 are given out by the water in the gradual change of the 

 liquid to a solid. 



In the same way when water is changed to steam a very 



