164 
time. Chalks are similar in one respect, that they 
are difficultly heated ; but being drier they retain 
their heat longer, less being consumed in causing 
the evaporation of their moisture. 
A black soil, containing much soft vegetable 
matter, is most heated by the sun and air; and 
the coloured soils, and the soils containing much 
carbonaceous matter, or ferruginous matter, ex- 
posed under equal circumstances to the sun, acquire 
a much higher temperature than pale- coloured 
soils. 
When soils are perfectly dry, those that most 
readily become heated by the solar rays likewise 
cool most rapidly, their power of losing heat by 
radiation being greatest ; but I have ascertained, 
by experiment, that the darkest coloured dry soil, 
(that which contains abundance of animal or ve- 
getable matter, substances which most facilitate 
the diminution of temperature,) when heated to 
the same degree, provided it be within the com- 
mon limits of the effect of solar heat, will cool 
more slowly than a wet pale soil, entirely com- 
posed of earthy matter. 
I found that a rich black mould, which con- 
tained nearly | of the vegetable matter, had its 
temperature increased in an hour from 65° to 88° 
by exposure to sunshine ; whilst a chalk soil was 
heated only to 69° under the same circumstances. 
But the mould removed into the shade, where the 
temperature was 62°, lost, in half an hour, 15° ; 
whereas the chalk, under the same circumstances, 
had lost only 4°, 
