154 HEAT. 



Here, removing the pressure, the ether evaporated, and 

 deprived the water of its heat and it became frozen. Of 

 course evaporation always produces cold. 



It is owing to the evaporation of water, that showers in 

 summer cool and refresh the earth, by the evaporation of 

 the rain which is spread over its surface. 



Many practical uses are made of this property. The 

 natives of India wear moist cloths about their heads to 

 keep them cool, which is done by the evaporation of the 

 water. The same people preserve their apartments cool 

 during the night, by hanging them with wet cloths. 

 Wine-cockers produce their effect entirely by evaporation. 

 Caravans which cross the hot deserts of Asia and Africa, 

 carry earthen bottles filled with water, which is pre- 

 served cool by having the bottles wrapped around with 

 wet cloths. 



The effect of evaporation is of the greatest importance 

 to man, in preserving the human body at a liealthy tempe- 

 rature. The natural heat of the body is about 90. 

 Active exercise or exposure to great heat, raises the 

 temperature of the body, which is unhealthy ; but perspi- 

 ration, brings a watery fluid to the surface, which 

 by being evaporated, soon reduces the heat of the body 

 to its healthy state. 



Distillation. Some liquids are more easily converted 

 into vapor than others. If two liquids, which boil at dif- 

 ferent temperatures be mixed together, they may be again 

 separated by exposing them to such a heat as will cause 

 that which boils at the lowest temperature, to assume the 

 state of vapor, which by being collected and condensed 

 by cold, will form the original liquid, while the other is 

 left in the vessel. This is the process in the distillation 

 of all substances. 



If the pressure of the atmosphere is removed from the 

 substances to be distilled, the operation for some pur- 

 poses' is much better performed. Vinegar distilled in 

 this way is perfectly pellucid and of an agreeable flavor, 

 which it has not when distilled in the usual way. 



OF COLD. 



We have already explained, that when a body passes 

 from a solid to a liquid state, it must absorb a large 



