CHAPTER V. 



EVAPORATION. 



In whatever shape water may be thrown down upon the 

 earth — whether as rain or dew, as snow or hail — it must, at 

 one time, have existed in the state of invisible vapour 

 diffused through the atmosphere, and not to be distinguished 

 from the air itself. However diy the air may appear to be, 

 it always contains more or less of this moisture. Though 

 not recognized by the senses, its presence is readily revealed 

 by the behaviour of certain substances which greedily absorb 

 moisture, and are consequently said to be hygroscopic} Oil 

 of vitriol, or sulphuric acid, for example;, is one of these 

 hygroscopic substances. If a bottle of this corrosive liquid 

 be left without its stopper, it will be found that, after a few 

 hours' exposure, the bulk and weight of the liquid have 

 sensibly increased ; indeed a pound of oil of vitrol may in 

 this way become two pounds in the course of a few days. 

 This increase of weight is due to moisture absorbed from 

 the surrounding air, and, after exposure, the acid is con- 

 sequently found to be weaker. When the air is damp, the 

 increase of weight is rapid ; when dry, the increase is but 

 slow. Yet the liquid can never be exposed, even to the 

 driest air, without absorbing some amount of moisture, how- 

 1 Hygroscopic, from v-ypb-s, hvgros, moist. 



