REJ.ATION.S OF THE SOIL Tu HEAT. 193 



in Explanation of tliose ob.sci-vutions we must recall to 

 TAmd the fact that all bodies are capable of absorbing and 

 radiating as well as reflecting heat. These properties, al- 

 though never dissoci;ited fi'oni color, are not necessarily 

 dependent upon it. They chiefly depend upon the char- 

 acter of the surface of bo lies. Smooth, polished surfaces 

 absorb and radiate lieat least readily; they reflect it most 

 perfectly. Ivadiation and absorption aie opposed to each 

 other, and the power of any body to radiate, is precisely 

 equal to its faculty of absorbing heat. 



It must be understood, however, that bodies may differ 

 in their power of absorbing or radiating heat of different 

 degrees of intensity. Lamp-blaclc absorbs and ra<liatcs 

 heat of all intensities in the same degree. White-lead 

 absorbs heat of low intensity (such as radiates from a ves- 

 sel filled with boiling Avater) as fully as lamp-black, but 

 of the intense heat of a lamp it absorbs only about one- 

 half as much. Snow seem-; to resemble white-lead in this 

 respect. If a bluck cloth or black paper be spread on the 

 surflice of snow, iipon whicli the sun is shining, it will 

 inelt much faster iinder the cloth than elsewhere, and this, 

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

 above, the snow. In our latitude every one has had op- 

 jiortunity to observe that snow thaws most rapidly when 

 covered by or lying oa black earth. The people of Cham- 

 ouni, in the Swiss Alps, strew the surface of their fields 

 Avith black-slate poAvder to hasten the melting of the suoav. 

 The reason is that snow absorbs heat of low intensity 

 Avith greatest facility. Tiie heat of the sun is con\'erted 

 from a higli to a Ioav intensity by being absoi'bed and then 

 radiated by the black naaterial. But it is not color that 

 determines this difference of absorjjtive power, for indigo 

 and Prussian blue, though of nearly the same color, have 

 very different absorptive powers. So fir, however, ns oi;r 

 observations extend, it appears that, usually, dark-coloi"ed 

 soils absorb heat most rapidly, and that the sun's rays 

 9 



