90 GENERAL SCIENCE 



between 110 and 140 C., and kerosene, which boils be- 

 tween 150 and 300 C. At still higher temperatures the 

 heavy lubricating oils pass over, and lastly vaseline evap- 

 orates. Of the residue in the retort paraffine is made. 



The manufacture of alcohol is also a form of frac- 

 tional distillation. The first part of the distillate contains 

 a higher percentage of alcohol than that which comes off 

 later. 



64. Cooling by Evaporation. We have learned that 

 when water in an open vessel is boiling, its temperature 

 cannot be raised by applying more heat, because the 

 increased heat applied is carried off by an increase in the 

 rate of evaporation. That is, the water is kept cooled 

 to 100 C., by evaporation. To cause water or any 

 other liquid to evaporate requires heat. Water will 

 evaporate at any temperature, but most rapidly at boil- 

 ing point. If you cover your hand with water and then 

 swing the hand it soon feels cool because heat is taken 

 from the hand to make the water evaporate. In summer 

 your body is kept cool by the evaporation of the sweat 

 from the surface of the body. If it were not for this 

 natural cooling process man could not live comfortably 

 in hot climates. Most animals are kept cool in the same 

 way. The evaporation of moisture from the leaves of 

 plants keeps them cool and prevents the hot sun from 

 scorching them. A heavy rain cools the ground and 

 streets partially by evaporation. In cities the streets 

 are sometimes sprinkled to cool them. Water can be 

 made to freeze in summer by the rapid evaporation of 

 ammonia or ether. When liquid carbon dioxide is 

 allowed to escape from the containing cylinder, it evapo- 

 rates so rapidly that a temperature of 80 C. below zero 

 may be reached. Mercury can easily be frozen with the 



