296 POWER OF SOILS TO RETAIN HEAT, 



extracting watery vapour from the air during the absence of the sun 

 Hence the influence of the dews and of gentle showers on the progress 

 of vegetation, is not limited to the mere supply of water to the thirsty 

 ground, and of those vapours which they bring with them as they descend 

 to the earth, but is partly due also to the power which they impart to the 

 moistened soil, of extracting for itself new supplies of gaseous matter 

 from the surrounding atmosphere. 



IV. RELATIONS OF THE SOIL TO HEAT. 



There are some of the relations of soils to heat, which have considera- 

 ble influence upon their power of promoting vegetation. These are the 

 rapidity with which they absorJa heat from the air, the temperature they 

 are capable of attaining under the direct action of the sun's rays, and the 

 length of time during which they are able to retain this heat. 



1°. Power of absorbing heat. — It is an important fact, in reference to 

 thegrowthof plants, that during sunshine, when the sun's rays beat upon 

 it, the earth acquires a much higher temperature than the surrounding 

 air. This temperature very often amounts to 110°, and sometimes to 

 nearly 150°, while the air in the shade is between 70° and 80° only. 

 Thus the roots of plants are supplied with that amount of warmth which 

 is most favourable to their rapid growth. 



Dark-coloured — such as black and brownish red — soils absorb the 

 heat of the sun most rapidly, and therefore become warm the soonest. 

 They also attain a higher temperature — by a few degrees only, how-^ 

 ever (3° to 8°), — ^than soils of other colours, and thus, under the action 

 of the same sun, will more rapidly promote vegetation. In climates, 

 such as ours, where the presence of the sun is often wished for in vain 

 in time of harvest, this property of the soil possesses a considerable eco- 

 nomical value. In other parts of the world, where sunshine abounds, 

 it becomes of less importance. 



Every one will understand that the above differences are observed 

 among such soils only as are exposed to the same sun under the same 

 circumstances. "Where the exposure or aspect of the soil is such as to 

 give it the prolonged benefit of the sun's rays, or to shelter it from cold 

 winds, it will prove more propitious to vegetation than many others less 

 favourably situated, though darker in colour and more free from super- 

 fluous moisture. 



2°. Power of retaining heat. — But soils differ more in their power of 

 retaining the heat they have thus absorbed. You know that all hot bodies, 

 when exposed to the air, gradually become cool. So do all soils ; but a 

 sandy soil will cool more slowly than a clay, and the latter than a soil 

 which is rich in vegetable matter. The difference, according to Schiib- 

 ler, is so great, that a peaty soil cools as much in one hour as the same 

 bulk of clay in two, or of sand in three hours. This may no doubt have 

 considerable influence upon growing crops, inasmuch as, after the sun 

 goes down, the sandy soil will be three hours in cooling, while the clavs 

 will cool to the same temperature in two, and rich vegetable mould in 

 one hour. But on those soils which cool the soonest, dew will first l)(i:i i 

 to be deposited, and it is doubtful, where the soils are equally drninod, 

 whether, in summer weather, the greater proportion of dew deposited oi 

 the clays and vegetable moulds mnv not more than compensate to t:.tj 



