144 APPLIED SCIENCE 



of the liquid resting against the heated surface, and converts 

 a part of it into steam, which rises in the form of bubbles 

 that break on the surface of the liquid. The temperature at 

 which a liquid boils is called its boiling point. Each liquid 

 has its specific boiling point as well as its specific weight at 

 a specific atmospheric pressure. The boiling point remains 

 constant during ebullition. 



155. Evaporation. Evaporation is the process by which 

 a liquid is gradually changed into vapor which fumes into 

 the air. Evaporation may take place at any temperature, 

 but only on the surface of the liquid; thus it differs from 

 boiling which goes on inside the liquid. Since liquids 

 evaporate more or less at all temperatures, there is no specific 

 evaporating point, as there is a specific boiling point. 



156. Precipitation. Precipitation is the process of sepa- 

 rating solid particles from a solution by the action of either 

 heat, light, or chemical substances. The solid particles 

 separated are called the precipitate, and the liquid remaining 

 the supernatant liquid. A precipitate may either fall to the 

 bottom or rise to the top of the supernatant liquid. Pre- 

 cipitation caused by the action of heat is illustrated by the 

 coagulation and precipitation of albumin, when albuminous 

 fluids, such as the white of egg, are heated; precipitatidn of 

 silver salts by light as in photography illustrates precipita- 

 tion by light; and precipitation by chemical reaction occurs 

 in many instances when salts are mixed in solution. 



The objects of precipitation are: (1) to convert solid 

 substances into the form of powder; (2) to purify liquids; 

 (3) to test chemicals; and (4) to separate chemical substances. 



There is a distinct difference between a sediment and a 



