-1859] WRITINGS OF JOSEPH HENRY. 193 



a stove of a temperature below redness. Furthermore if the 

 same board were exposed to the clear sky and suffered to 

 cool by its own radiation no difference of temperature would 

 be observed in the different parts of its surface, except a 

 very slight one, which might be due to the difference of 

 the radiating power possessed by the substances of which 

 the black and white paints are composed. On this subject 

 Prof Bache, the Superintendent of the Coast Survey, has 

 made a series of very interesting experiments. He found 

 that canisters of tinned iron painted externally of different 

 colors and filled with heated water, required the same time 

 to cool through a given number of degrees. The facts in 

 regard to this point may be generalized by saying that color 

 has no influence whatever upon the emissive power of differ- 

 ent bodies, but that its influence is confined to the reception 

 of rays of high intensity, or those which approximate in 

 quality to the luminiferous emanations. Hence a black or 

 a white dress is equally cool in the night, though in the 

 sunshine the darker one would absorb the greater amount 

 of heat. 



Besides the color, the humidity of the soil has great influ- 

 ence upon the temperature it acquires, a portion of the heat 

 being expended in evaporating the water. We have seen 

 the statement somewhere that the average temperature of 

 whole districts in Great Britain has been elevated one de- 

 gree by the system of drainage adopted in that country. 



In addition to the preceding causes, there are two others 

 which affect the temperature of the soil, namely, conduction 

 and capacity for heat. In a porous, badly conducting sub- 

 stance the heat which may escape from the surface is not 

 readily supplied from the interior, and hence such bodies 

 are long in cooling. Again, different bodies contain very 

 different amounts of heat at the same temperature, and hence 

 one body may take a much longer time to cool down to the 

 same temperature through the same number of degrees than 

 another. That two different bodies of the same weight at 

 the same temperature possess different amounts of heat mav 

 be shown by first heating say a pound of each in boiling 



13-2 



