76 HEAT AND ENERGY 



hold. When air at any temperature holds all that it 

 can, it is said to be saturated. If now it be somewhat 

 cooled, this air can no longer hold all the vapor that is 

 in it, and some will change back to its liquid state. 

 This change is called condensation. The condensed water 

 vapor may then float about as tiny liquid drops; small 

 masses of these drops may pass to another place and 

 there evaporate again, like the cloud from a locomotive ; 

 while large masses would form a fog or cloud. If the 

 drops were large they would fall as rain. 



Experiment 68. Put ice or snow into a pitcher and take it 

 into a warm room. Watch the outside of the pitcher, and explain. 

 Breathe upon a cold piece of glass. Why does frost form on the 

 inside of a window pane ? Why do we " see our breath " in cold 

 weather ? 



87. Distillation. Important use is made of these 

 principles ( 84-86) in separating substances from each 

 other or from impurities. Since different sorts of mat- 

 ter vaporize at different temperatures, a mixture may be 

 heated to the low boiling point of one substance without 

 vaporizing the others ; the gas from this one may then 

 be cooled, giving us the desired liquid or solid, free 

 from the others. The process is called distillation. 



Experiment 69. A device for distillation may be arranged as 

 in Fig. 58. Instead of the condenser c, a long tube of glass or 

 metal may be run through a trough in which cold water is flowing. 

 Fig. 58 shows the condenser as generally used in distilling. 

 Muddy water may be boiled in a closed flask /; the steam runs 

 through a tube t which carries it to the coiled tube e in the con- 

 denser. Cold water running into c from a pipe p surrounds the 

 coiled tube e and runs out at o. The steam in e is cooled and 



