VAPORIZATION. 



319 



ization. Vaporization may be of two kinds evaporation 

 and ebullition. 



499. Evaporation. Evaporation signifies the 

 quiet formation of vapor at the surface of a 

 liquid. 



(a.) With reference to the rapidity with which evaporation takes 

 place, it may be remarked that 



(1.) It varies with the temperature. 

 (2.) It varies with the extent of surface. 



(3.) It varies with pressure upon the liquid, being exceedingly 

 rapid in a vacuum. 



500. Evaporation in Vacuo. The rapid forma- 

 tion of vapors in a vacuum is prettily illustrated by the 

 following experiment : 



Torricellian vacua are 

 formed at the top of four 

 barometer tubes, A, B, 

 G and/), Fig. 245. Into 

 the mouth of B pass a 

 few drops of water. They 

 will rise through the mer- 

 cury to the vacuum at 

 the top. Upon reaching 

 this open space they are 

 instantly vaporized. The 

 tension of the aqueous 

 vapor shows itself by 

 lowering the mercury 

 column. This depression 

 is due to the tension 

 rather than to the weight 

 of the vapor, because the 

 water weighs scarcely anything compared with the mer- 



