Book III. USES OF THE SOIL TO VEGETABLES. 321 



much more lieatcd by the rays of the sun, all other circumstances being equal, than otliers; 

 and soils brought to the saine degree af heat cool in different times, i. e, some cool much 

 faster than others. This property has been very little attended to in a philosophical point 

 of view ; yet it is of the highest importance in culture. In general, soils wliich consist 

 principally of a stiff white clay are with difficulty heated ; and, being usually very moist, 

 they retain their heat but for a short time. Chalks are similar in one respect, the difficulty 

 with which they are 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 or ferruginous matter, exposed under equal circum- 

 stances to the sun, acquire a much higher temperature than pale soils. 



21 58. When soils are perfectly dry, those which most readily become heated by the solar rays 

 likewise cool most rapidly ; but the dai'kest-coloured dry soil (that which contains abund- 

 ance 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 65 to 88 

 by exposure to sunshine ; whilst a chalk soil was heated only to 69 under the same cir- 

 cumstances ; 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. We may also refer to the influence of black earth in melting snow, as prac- 

 tised 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 b'ned 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. [Agriculture appliquee, ^c torn. i. 82. ) A 

 brown fertile soil and a cold barren clay were each artificially heated to 88, having been 

 previously dried, they were then exposed 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 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 ther- 

 mometer may be sometimes a useful instrument to the purchaser or improver of lands." 



2159. The moisture in the soil and sub-soil materially affects their temperature, and pre- 

 vents, as in the case of constantly saturated aquatic soils, their ever attaining to any great 

 degree either of heat or cold. The same observation will apply to moist peaty soils, or 

 peat-bogs. 



2160. Chemical agency of soils. Besides these uses of soils, which may be considered 

 mechanical, there is. Sir H. Davy observes, another agency between soils and organisable 

 matters, which may be regarded as chemical in its nature. The earths, and even the 

 earthy carbonates, have a certain degree of chemical attraction for many of the princi- 

 ples of vegetable and animal substances. This is easily exemplified in the instance of 

 alumina and oil ; if an acid solution of alumina be mixed vrith a solution of soap, which 

 consists of oily matter and potassa, the oil and the alumina will unite and form a white 

 powder, which will sink to the bottom of the fluid. The extract from decomposing 

 vegetable matter, when boiled with pipe-clay or chalk, forms a combination by which the 

 vegetable matter is rendered more difficult of decomposition and of solution. Pure 

 silica and silicious sands have little action of this kind ; and the soils which contain the 

 most alumina and carbonate of lime are those which act with the greatest chemical energy 

 in preserving manures. Such soils merit the appellation, which is commonly given to 

 them, of rich soils ; for the vegetable nourishment is long preserved in them, unless 

 taken up by the organs of plants. Silicious sands, on the contrary, deser\'e the term 

 hungry, which is commonly applied to them ; for the vegetable and animal matters they 

 contain, not being attracted by the earthy constituent parts of the soil, are more liable to 

 be decomposed by the action of the atmosphere, or carried off from them by water. In 

 most of the black and brown rich vegetable moulds, the earths seem to be in combination 

 with a peculiar extractive matter, afforded during the decomposition of vegetables ; this is 

 slowly taken up or attracted from the earths by water, and appears to constitute a prime 

 cause of the fertility of the soil. 



2161. Th7is all soils are useful to plants, as affording them a fixed abode and a range 

 for their roots to spread in search of food ; but some are much more so than others, as 

 better adapted by their constituent parts, climate, inclination of surface, and sub-soil, 

 for attracting and supplying food. 



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