192 



WRITINGS OF JOSEPH HENRY, 



[1855- 



stance has been strewed over the ground to increase its ab- 

 sorbtive power. 



The following table by M. Schubler is copied from Bec- 

 querel, and gives' the greatest elevation of temperature ob- 

 tained by different soils exposed to the direct rays of the sun, 

 while the surrounding air was at about 78°. 



Maximum of temperatures of various earths exposed to the sun, by Schubler. 



The differences of temperature exhibited by the two col- 

 umns are due to the heat expended in the evaporation of a 

 portion of the water in the moist earth, while the differences 

 between the substances are to be ascribed principally to the 

 colors, though the texture may have some effect. 



Absorptive power is connected with that of emission ; and 

 those bodies which possess the greatest absorptive power for 

 heat of a low intensity, also possess the greatest emissive 

 power for heat of the same kind. But the preceding re- 

 marks have reference to the rays from the sun and not to those 

 of dark heat, and here I must stop to recall the fact which 

 is frequently neglected, even by scientific men, namely, that 

 color has no effect upon the absorption or emission of rays 

 of low intensity. For example, if we pass our hands over 

 a sign-board on which dark letters upon a white ground are 

 exposed to the sun we can readily perceive with our eyes 

 shut the difference of temperature; but this would not be 

 the case were the board exposed in the dark to the heat of 



