FORCE OF EVAPORATION. 281 



temperatures. Ice evaporates in the open air, 

 even when the temperature is below the freezing 

 point. This process goes on until the vapour 

 has attained what has been before called its 

 maximum density; evaporation then ceases, and 

 if its temperature becomes suddenly lowered, 

 the vapour condenses into the form of a liquid. 

 It is remarkable, that while the presence of air 

 checks the rapidity with which evaporation takes 

 place, it does not interfere with its amount. 

 Thus, if we could empty a sitting-room of air, 

 and put a basin of water into it, the vapour of 

 the water would rise almost instantly, and fill 

 the apartment, until it became as dense as it can 

 be in this form ; yet, if we put the same basin 

 of water into the room full of dry air, precisely 

 the same quantity of vapour would rise from it, 

 and fill the room just as in the former case, as 

 if there were no air present, only it would 

 occupy a longer time in so doing. Hence we 

 may perceive, that liquids evaporate with a cer~ 

 tain degree of force. In short, when a drop of 

 water spilled on a slab disappears, it does so 

 because it is able to pass into the invisible con- 

 dition of a vapour, in spite of the pressure of 

 the air on its surface. It has as much the power 

 to resolve itself into fine particles which elude 

 our notice, as if no air were present. 



It has already been mentioned, that watery 



