92 THE FITNESS OF THE ENVIRONMENT 



B 



LATENT HEAT 



Very different from specific heat in their 

 relationship to the chemical constitution of 

 a substance, but not unlike it in biological 

 importance, are the so-called latent heats of 

 melting and of evaporation. 



The latent heat of melting is expressed as 

 the number of calories which are required to 

 convert one gram of solid at the freezing point 

 into one gram of liquid at the same tempera- 

 ture. For water its value is approximately 

 80, which indicates that the same quantity 

 of heat must be employed to melt ice as to 

 raise the temperature of the resulting ice- 

 water to 80° centigrade. 



The latent heat of evaporation is similarly 

 defined as the number of calories required to 

 change one gram of liquid into vapor. Its 

 magnitude depends upon the temperature at 

 which the process takes place. The latent 

 heat of evaporation of water is approximately 

 536. There is required, accordingly, as much 

 heat to boil away one gram of water as to 

 raise the temperature of 536 grams through 1° 

 centigrade. 



There are a number of important effects of 



