162 LECTURES ON 



lowing order: Garden soil; yellow sandy clay; pipe clay; lime soils 

 having crystalline grains; and, lastly, a pulverulent chalk soil. 



To show what different degrees of warmth soils may acquire, under 

 the same circumstances, the following maximum temperatures may be 

 adduced: At noon of a July day, when the temperature of the air 

 was 90°, a thermometer placed at a depth of a little more than one 

 inch, gave these results : 



In quartz sand • • • 126° 



In crystalline lime soil 115° 



In garden soil 114° 



In yellow sandy clay 100° 



In pipe clay 94° 



In chalk soil 87° 



Here we observe a difference of nearly 40° in the temperature of 

 the coarse quartz and the chalk soil. The experimenters do not men- 

 tion the influence of water in affecting these results; they do not state 

 the degree of dryness of these soils. It will be seen, however, that 

 the warmest soils are those that retain least water, and doubtless 

 something of the slowness with which the fine soils increase in warmth 

 is connected with the fact that they retain much water, which, in 

 evaporating, appropriates and renders latent a large quantity of heat. 



The chalk soil is seen to be the coolest of all, its temperature in 

 these observations being three degrees lower than that of the atmos- 

 phere at noon day. In hot climates this coolness is sometimes of great 

 advantage as appears to happen in Spain, near Cadiz, where the 

 Sherry vineyards flourish. ' ' The Don said the Sherry wine district 

 was very small, not more than twelve' miles square. The sherry grape 

 grew only on certain low chalky hills where the earth being light- 

 colored, is not so much burnt ; did not chap and split so much by the 

 sun as darker and heavier soils do. A mile beyond these hills the 

 grape deteriorates." — (DicJcens' Household Words, November 13, 1858.) 



In explanation of these observations we must recall to mind the fact 

 that all bodies are capable of absorbing and radiating as well as 

 reflecting heat. These properties, although never disassociated from 

 color, are not necessarily dependent upon it. They chiefly depend 

 upon the character of the surface of bodies. Smooth polished sur- 

 faces absorb and radiate heat least readily ; they reflect it most per- 

 fectly. Radiation and absorption are opposed to each other, and the 

 power of an)'' body to radiate is precisely equal to its faculty of absorb- 

 ing heat. It must be understood, however, that bodies may differ in 

 their power of absorbing or radiating heat of different degrees of intensity. 

 Lampblack absorbs and radiates heat of all intensities in the same 

 degree. White lead absorbs heat of low intensity (such as radiates 

 from a vessel filled with boiling water) as fully as lampblack, but of 

 the intense heat of a lamp it absorbs only about one-half as much. 

 Snow seems to resemble white-lead in this respect. If a black cloth 

 or black paper be spread on the surface of snow, upon which the sun 

 is shining, it will melt much faster under the cloth than elsewhere, 

 and this too if the cloth be not in contact with, but suspended above 



