HEAT. 53 



This evaporation, which takes place at various temperatures and at 

 the surface only, is not to be confounded with boiling, which is the 

 rapid conversion of a liquid into a gas at a fixed temperature with 

 the phenomena of ebullition, due to the formation of gas in the mass 

 of liquid. Boiling-point may, therefore, be defined as the highest 

 point to which any liquid can be heated under the normal pressure of 

 one atmosphere. 



A liquid in a closed space evaporates until a definite pressure is 

 attained by the vapor at a fixed temperature, when the liquid and 

 vapor remain in equilibrium. When this limit is reached the vapor 

 is said to be " saturated." If the temperature is increased, more 

 liquid evaporates and the pressure increases. If the temperature of 

 a saturated vapor falls, some vapor is condensed to liquid, or if the 

 pressure is increased, some vapor will also condense to liquid. At a 

 given temperature a saturated vapor exerts a definite pressure, 

 which is different for different vapors. Thus, in terms of a column 

 of mercury, at 20 C. (68 F.), water vapor exerts a pressure of 17 

 mm., alcohol vapor, 60 mm., and ether vapor, 450 mm. 



As a rule, in experiments where gas volumes are measured, the gas is satu- 

 rated with aqueous vapor, which has to be taken into account in making calcu- 

 lations. If a volume of gas saturated with water-vapor is measured at atmo- 

 spheric pressure, the tension of the gas alone is the difference between the 

 barometric pressure and the tension of the saturated water-vapor at the given 

 temperature. 



Tension of Saturated Water-vapor (Regnault). 



Distillation is the conversion of a liquid into a gas, and the con- 



