LIQUEFACTION. 317 



Recapitulation. In this section we have considered 

 the Nature of Heat; the meaning of Tem- 

 perature ; Thermometers and their graduation ; 

 the determination of the Freezing and Boiling 

 Points; thermometric Scales and Readings; 

 the Differential Thermometer ; Expansion 

 of Solids ; Expansion of Liquids, especially 

 the Expansion of Water ; the Expansion of 

 Gases and the Rate thereof; Absolute Zero of 

 temperature; the relation between Temperature, 

 Pressure and Volume. 



ECTfON il. 



LIQUEFACTION, VAPORIZATION, DISTILLATION. 



495. Liquefaction. In the last section we learned, 

 that heat is a form of energy. As energy, it is able to 

 perform work, such as overcoming or weakening the force 

 of cohesion. It is well known that when a solid is changed 

 to the liquid or aeriform condition, or when a liquid is 

 changed to a vapor, it is done by an increase of heat, and 

 that when the reverse operations are performed, it is by a 

 diminution of heat. Cohesion draws the particles together ; 

 heat pushes them asunder, and on the varying preponder- 

 ance of one or the other of these antagonistic powers, the 

 condition of the body seems to depend. When the firm 

 grip of cohesion has been so far weakened by heat that the 

 molecules easily change their relative positions ( 55), the 

 body passes from the solid into the liquid condition. This 

 change of condition is called liquefaction. 



