io6 The Realm of Nature CHAP. 



at every temperature when the vapour exerts a certain 

 definite pressure upon the liquid, evaporation is stopped, 

 and the vapour is said to be saturated at that tempera- 

 ture. 



157. Water-vapour and Temperature. At the freezing- 

 point (32) water-vapour is saturated, i.e. presses sufficiently 

 to stop evaporation, when its pressure is equal to that of 

 o- 1 8 inch of mercury ; at 50 it must exert twice this pressure, 

 or 0-36, before evaporation ceases ; at 70 it must exert a 

 pressure of 0-73, and at 90 a pressure of 1-45 inches, in 

 order to be saturated. These figures show that at 50 

 twice as much vapour is required to form a saturated 

 atmosphere as at 32, and at 70 twice as much as at 50, 

 and at 90 twice as much as at 70. If an atmosphere of 

 water- vapour saturated at 50 is warmed up to 70, evapora- 

 tion is at once allowed to commence and will continue 

 until the amount of vapour present above the water is 

 doubled. Then the vapour will exert pressure sufficient to 

 stop further change, and will be saturated. Again, if the 

 temperature of the saturated vapour is reduced from 70 to 

 50, half the vapour must return to the liquid state or 

 become condensed in order that the pressure may fall to 

 that which is just sufficient to prevent further evaporation. 

 Hence it is plain that every rise of temperature is ac- 

 companied necessarily by evaporation, every fall of temper- 

 ature is accompanied necessarily by condensation, until the 

 vapour exerts the pressure proper to its new temperature. 

 Precisely the same thing happens, as explained by Dalton's 

 law, when there are atmospheres of nitrogen, oxygen, and 

 carbonic acid surrounding the Earth. The pressure of 

 saturated water-vapour at 50 is still equal to 0-36 inches 

 of mercury, the only difference is that it takes a longer 

 time for the pressure to readjust itself to a change of 

 temperature, as a party of excursionists crossing a broad 

 railway platform reach their carriages, whether the platform 

 is left to themselves or is thronged by crowds moving in 

 different directions, only in the latter case the transference 

 takes a longer time. On account of the low temperature at 

 great elevations, water- vapour, although its density is only half 



