T4S 



WATER \-APOUR ON AJARS. 



pressure and the lower the boihng point of water. (27 mm. pres- 

 sure for 1° Cent.). 



Regnault's experiments have shown that in air at a density 

 •14 of our own. the position of equihbrium between the respective 

 pressures of the air and water would be reached at a temperature 

 of about 44° C. (111° Fahr.) ; according to Williams* at 59° C. 

 (138° Fahr.) ; according to Pickering, at 28° C. (82° Fahr.) This 

 then, the temperature of one of our summer days, would be the 

 boiling point of water on Mars. 



We all know that evaporation takes place long before the actual 

 bo'ling point is reached : in fact, on a winter's da^^ we can see 

 the process made manifest owing to the colder air condensing 

 the vapour. 



It follows then that owing to the lesser pressure, water in Mars 

 must be in a very unstable condition, and the air must of necessity 

 contain aqueous vapour, and in large quantities : in fact, must 

 nearly always be saturated with it. 



(2) From molecular physics and the kinetic theory of gases 



* The Fuel of the Sun, London, 1S70. 



