169 
174 Soils, their Temperature. “Many soils are popularly 
distinguished as cold ; and the distinction, though at first view 
it may appear to be founded on prejudice, is really just. Some 
soils are much more heiated by the rays of the sun, all other 
circumstances being equal, than others ; and soils brought to the 
same degree of heat, cool in different times, i. e. some cool much 
faster than others. 
This property has been very little attended to in a philosophi- 
cal point of view; yet it is of the highest importance in agriculture. 
In general, soils that consist principally of a still’ white clay are 
difficultly heated ; and iieing usually very moist, they retain 
their heat only for a short time. Chalks are similar, in one re- 
spect, 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 carbonaceous matter, or ferruginous mat- 
ter exposed under equal circumstances to the sun, accpiire a much 
higher temperature than pale-coloured soils. When soils are per- 
fectly dry, those that most readily become heated by the solar rays, 
likewise cool most ra]>idly, their power of losing heat by radi- 
ation being greatest ; but I have ascertained, by experiment, that 
the darkest coloured dry soil, (that which contains abundance of 
animal or vegetable matter, substances which most facilitate the 
diminution of temperature,) when heated to the same degree, 
])rovided it be within the common limits of the effect of solar 
heat, will cool more slowly than a wet pale soil, entirely composed 
of earthy matter. 1 found that a rich black mould, which 
contained nearly i of the vegetable matter, had its temper- 
ature increased in an hour from 6o° to 88“ by exposure to 
sunshine ; whilst a chalk soil was heated only to 69“ under the 
same circumstances. But the mould removed into fhe shade, 
where the temperature was 62”, lost, in half an hour, 15“; 
whereas the chalk, under the .same circumstances, had lost 
only 4“. A brown fertile soil, and a cold barren clay, were each 
artificially heated to 88“, having been previously dried : they 
were then cxjiosed in a temperature of 57“ ; in half an hour the 
dark soil was found to have lost 9“ of heat ; the clay had lost 
only 6". An equal portion of the clay containing moisture, after 
being heated to 88“, was exposed in a temperature of 55°; in less 
185 AHCTARIUM. Davy’i Agricul. Chemitt. 
