Climate 



157 



The difference in temperature between two substances, 

 one of which is white and the other black, when both are 

 equally exposed to the sun, is very remarkable. There 

 will be as much as thirteen or fourteen degrees difference, 

 for instance, in the temperatures of a piece of lamp-black 

 and of a piece of magnesia. 



But though color makes such an important difference 

 in the power to absorb heat, it has no influence, appar- 

 ently, upon the power to retain it. Indeed, though sand 

 may absorb heat less quickly than a darker soil does, when 

 once it is warm it remains warm longer than any other. 

 The coarser it is, too, the less quickly it cools; and as 

 coarse gravel, once thoroughly heated, retains some degree 

 of heat even through the night, it is found useful to put it 

 round grape-vines to keep them warm. 



Great, and especially sudden, changes of temperature 

 are particularly trying to most plants, though such as grow 

 in deserts have become so accustomed to being scorched 

 by day and frozen by night, as at least to manage to keep 

 ahve. But though an even temperature is in some ways 

 most desirable, it does not by any means follow that all 

 plants would thrive in the equatorial region. That region, 

 as has been said, is characterized less by its great heat 

 than by its remarkable uniformity of climate. It knows 

 no change of seasons : it is always summer, always spring, 

 and always autumn there, and many trees bear both flow- 

 ers and fruit at the same time. But this constant activity, 

 though it suits the trees and plants accustomed to it, is too 

 much for our European fruit-trees and other plants; for 

 these are in the habit of taking a winter sleep, and canno: 

 get on without it. Transported to the equatorial region, 

 they grow, indeed, freely enough, but too freely; for they 



