104 THE FITNESS OF THE ENVIRONMENT 



There is still another beneficial result of 

 this property : the great variation in the vapor 

 tension of water which accompanies variation 

 in temperature. Vapor tension measures the 

 amount of vapor which is present in the 

 atmosphere when it is in contact with a liquid 

 and after it has become saturated with the 

 liquid's vapor. Now, according to a well- 

 known law, the rate of increase of vapor ten- 

 sion, or in other words the amount of vapor 

 which the air can hold, is greater the greater 

 the latent heat of vaporization. 1 Hence, 

 degree by degree there is more variation in 

 the vapor tension of water than there could 

 \be if the latent heat were lower. Such great 

 jvariability in the quantity of water which 

 'the air can hold is in meteorology the most 

 important characteristic of aqueous vapor. 2 

 The relationship between vapor tension and 

 temperature (centigrade) is shown in the 

 accompanying table. 



1 Near the freezing point an increase of 10° in temperature 

 doubles the amount of water which the air can hold. The 

 increase is proportional to the latent heat of vaporization 

 according to the formulae 



p c 1.99 \ I li / 

 where W stands for the latent heat of vaporization, p for vapor 

 tension, and T for temperature. 



2 Arrhenius, "Kosmische Physik," p. 612. 



