104 APPLIED SCIENCE 



105. Latent Heat.- Examine a pan of water over the fire. 

 Note that the heat passes first to the particles of the pan, then 

 to the water nearest to the source of heat. As these parti- 

 cles expand, they become lighter and pass to the surface 

 of the water. This process continues until the whole mass 

 of water reaches a uniform and fixed temperature called the 

 boiling point 212 F. under ordinary conditions. In the 

 generation of steam under pressure higher than the ordinary 

 air, the boiling point varies, increasing in proportion to the 

 pressure. With a pressure of 16 Ibs. to the square inch, water 

 boils at 212.1 F.; with a pressure of 20 Ibs. at 228.4, etc. 



After the boiling point has been reached the temperature 

 of the water remains constant, however long the heat is ap- 

 plied to the vessel. The steam bubbles will rise rapidly, 

 the whole mass will be in a state of agitation (ebullition), 

 and the steam vapor will be given off in large quantities. 

 The heat that is absorbed and given off without raising the 

 temperature of the water is called the latent heat of the steam. 

 This latent heat is either lost or dispelled in the air or is 

 given off when the steam is condensed. 



When a substance is heated as it passes from the solid 

 to the liquid state, and from the liquid to the gaseous state, 

 a certain amount of heat is expended in molecular work, 

 separating the molecules of the substance without raising 

 the temperature. The heat thus absorbed or lost is spoken 

 of as latent. For example, when a pound of ice is heated its 

 temperature remains the same until the melting point (32 F. 

 or C.) is reached; further application of heat, however 

 intense, will cause no further rise in temperature until the 

 ice has been entirely melted. Experiment shows that 144 

 B.T.U. are required to convert a pound of ice into water at 

 32 F. Further application of heat causes a rise in tempera- 



