1173. When soils are perfectly dry, those which most readily become heated by the solar rays likewise cool most rapidly; but 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, provided 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. Sir H. Davy 'found that a rich black mould, which contained nearly one fourth of vegetable matter, had its temperature increased in an hour from 65ﾦ to 88ﾦ by exposure to sunshine; whilst a chalk soil was heated only to 69ﾦ under the same circumstances: but the mould removed into the shade, where the temperature was 62ﾦ, lost, in half an hour, 15ﾦ; whereas the chalk, under the same circumstances, had lost only 4ﾦ. We may also refer to the influence of black earth in melting snow, as practised empirically on the Alps, and tried philosophically by Franklin and Saussure. The latter placed on the top of the high Alpine mountain Cramont a box lined with black cloth, with the side next the sun closed by three panes of glass at a little distance apart the one from the other, and found the thermometer rise thirty degrees in two hours, from the concentration of the sun's rays. (La Chymie appliquee, &c., tom. i. 82.) A brown fertile soil and a cold barren clay were each artificially heated to 88ﾦ, having been previously dried, they were then exposed in a temperature of 57ﾦ; in half an hour the dark soil was found to have lost 9ﾦ of heat, the clay had lost only 6ﾦ. An equal portion of the clay containing moisture, after being heated to 88ﾦ, was exposed in a temperature of 55ﾦ; in less than a quarter of an hour it was found to have cooled to the temperature of the room. The soils in all these experiments were placed in small tin-plate trays, two inches square, and half an inch in depth; and the temperature was ascertained by a delicate thermometer. Thus the temperature of the surface, when bare and exposed to the rays of the sun, affords at least one indication of the degree of its fertility; and the thermometer may be sometimes an useful instrument to the purchaser or improver of lands.'
