Climate 185 



greater or less degree are the presence of forests, 

 rivers, lakes, mountains, which cannot now be dealt 

 with in detail. 



But plants are also affected in a minor degree by the 

 colour of the soils in which they grow. 



Dark substances absorb more heat than light ones 

 do ; and snow will melt more quickly if a piece of black 

 cloth be stretched above it, even though the two do 

 not touch one another. In the Arctic regions the ice 

 melts much faster wherever a small dark-brown plant, 

 of the same family as the Red Snow, grows, because it 

 attracts the heat. So, too, in some parts of Switzer- 

 land the peasants hasten the departure of the snow by 

 strewing it with black powdered slate. 



Dark soils are therefore usually warmer than light 

 ones ; and it is not an uncommon thing for gardeners 

 to sprinkle a light-coloured soil with peat, charcoal, or 

 vegetable mould to warm it, for these all act as sun- 

 traps. 



Melons are thus ripened, even in the coolest summers, 

 at Freiberg, in Saxony, by means of a layer of coal- 

 dust. 



Grapes, too, in the Rhine district, ripen best where 

 the ground is covered with fragments of black slate; 

 and the vines are purposely kept near the ground, that 

 they may have the full benefit of the heat which the 

 slate absorbs by day and gives up again by night. 



The difference in temperature between two sub- 

 stances, one of which is white and the other black, 

 when both are equally exposed to the sun, is very re- 

 markable. 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 pie e of magnesia. 



