CHAPTER II. 



LATENT HEAT 



This excess energy heat which is used in 

 turning the water into steam and which the thermom- 

 eter does not register is termed "Latent Heat." 

 Latent comes from the Latin word meaning "con- 

 cealed" or "hidden" and hence is very appropriate. 



If we place water in a cylinder, the lower end of 

 which is closed, and bring a sliding piston down on 

 it, we may heat the water to the 212 degree point and 

 not see that any appreciable change has taken place. 

 If, however, we continue to impart heat to the water 

 the thermometer does not show an increase, provided 

 that the piston be not too heavy. Roughly speaking, 

 as long as the piston is free to move the water does 

 not rise in its temperature; that is, the specific heat 

 does not increase, but the latent or non-apparent heat 

 does. As a result the energy of vibration of the 

 molecules increases to such an extent that the water 

 or steam increases in volume 1,700 times; and as a 

 consequence the piston must move or force must be 

 applied to the other side to hold it in place against 

 the steam. 



In other words, the heat which has been given to 

 the water, or the energy, or work-doing power, has 

 so set its molecules vibrating that their motion has 



