on the Temperature of Soils. 
127 
Leslie remarks that " the absorbent power of the earths 
depends as much on their mechanical condition as on the species 
of matter of which thev are composed. Whatever tends to 
harden them diminishes the measure of their effect ; and hence, 
apparently, the reason why the action of fire impairs their desic- 
calino: quality. '* 
Useful as is this contribution to the philosophy of soils, it must 
be deemed verv remarkable that the ingenious author altogether 
omitted to investigate the relation of the same substances to the 
absorption of heat as well as moisture. The importance of ascer- 
taining these double relations did not, however, escape the 
sagacitv of Daw, who preceded Leslie in this research, and 
whose remarks are so pertinent, and possess such intrinsic worth, 
that I trust the citation of them will not be thought tedious : — 
" 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 heated by the rays of the sun, all other 
circumstances being equal, than others ; and soils brought to the same 
degree of heat cool in difFereut times, i. e. some cool much faster than 
others. 
" This property has been little attended to in a philosophical point of 
view, yet it is of the highest importance in agriculture. In general, soils 
that consist principally of a stiff white clay are heated with difficulty ; 
aud, being usually very moist, they retain their heat only for a short 
time. Chalks are similar in one respect — that they are difficult to heat; 
but, being drier, they retain their heat longer, less being consumed m 
causing the evaporation of their moisture. 
" A black soil, containing much soft v^table matter, is most heated 
by the sun and air; and the coloured soils, and the s^iils containing 
much carbonaceous matter, or ferruginous matter, exposed under equ^ 
circumstances to the sun, acquire a much higher temperature than pale- 
coloured soils. 
" "When soils are perfectly dry, those that most rapidly become heated 
by the solar rays likewise cool most rapidly, their power of losing heat 
by radiation being the 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 dimi- 
nution of temperature), when heated to the same desree, provided it be 
withm the common limits of the effect of solar heat, will cool more 
slowly than a wet pale soil, entirely composed of eanhy matter. 
" I found that a rich black mould, which contained one-fourth 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'. 
• Leslie, on Heat and Moisture, p. 96, 1818. 
