68 PHYSIOGRAPHY. [chap. 



the leaves of plants and from the lungs and skin of animals. 

 Decay, and other chemical phenomena likewise contribute 

 their quota to the moisture of the atmosphere. Evaporation, 

 however, remains the principal source of watery vapour in 

 the air. 



It need hardly be said that the rapidity of evaporation 

 may be materially affected in a variety of ways. If you 

 vi^ish to dry a damp object quickly, you at once place it 

 before the fire. Temperature, then, clearly affects the rate 

 of evaporation ; the higher the temperature the quicker the 

 process, other conditions remaining the same. Again, the 

 rate of evaporation is greatly affected by the hygrometric 

 state of the air; in other words by the proportion of 

 moisture already present in the atmosphere. If the air 

 were perfectly dry, evaporation would be extremely rapid, 

 and the vapour greedily licked up ; if, on the other hand, the 

 air were thoroughly saturated with moisture, evaporation 

 would be utterly impossible. As a matter of fact we rarely, 

 if ever, experience either one or the other of these extreme 

 conditions ; but, between these extremes, there are any 

 number of intermediate states. Every laundress knows that 

 there are " good drying days " and bad ones. When there 

 is but little moisture in the air the clothes dry quickly ; 

 when there is much moisture, they dry but slowly. Let it 

 not be supposed, however, that the proportion of moisture 

 in the air is easily estimated by our sensations. True, we 

 say that one day is dry, and another damp ; but, after all, it 

 is not so much the absolute quantity of moisture in the air 

 as its relative humidity that determines these sensations ; 

 that is to say, it is the ratio of the vapour actually present 

 to the amount which would saturate the air at the given 

 temperature. The hotter the air the greater its capacity for 

 moisture, and consequently the air may seem dry, though 



