CHEMICAL FUNCTIONS OF RAIN. 287 



months in the year not a drop of rain falls, nor 

 is scarce a cloud to be seen on the molten sur- 

 face of the sky, yet many trees preserve all their 

 beauty and freshness of aspect. We are not, 

 however, therefore to suppose that the office of 

 rain to vegetation is either trifling or unim- 

 portant. In these peculiar circumstances, it 

 has been suggested that the appendages of the 

 stem, or the leaves themselves, are gifted with a 

 peculiar function of withdrawing watery vapour 

 from the air, and thus sustaining existence when 

 it would otherwise be impracticable. But it 

 must also not be forgotten that the soil itself 

 has the property, in a remarkable manner, of 

 absorbing watery vapour from the air, and no 

 soil possesses this property more remarkably 

 than that which is formed of decayed vegetable 

 tissues, and other matters called humus. Hence 

 in the tropics, where such a soil is rapidly pro- 

 duced, owing to the rapid decomposition caused 

 by the elevated temperature, plants do not 

 suffer as much as might be the case in time of 

 drought, were the soil not possessed of this 

 property. We may perceive, however, in the 

 picture of the effects of drought presented to 

 us by travellers in tropical regions, something 

 of the value and importance of rain to the 

 earth. The grass becomes burnt up, withered, 

 and dead. The leaves of the forest-trees hang 



