322 RADIANT HEAT. 



sun's rays than another filled with the same kind of spirit uncolored; 

 but that the fluid rose equally in both when dipped together into the 

 same vessel of warm water. — (On Air and Fire, &c.) 



Dr. Franklin found that the hand, when applied alternately to a 

 black and to a white part of his dress in the sun, would feel a great 

 difference in their warmth. 



He observed that black paper was sooner fired by exposure to the 

 focus of a lens than white. 



His well known experiment of placing differently colored pieces of 

 cloth on the snow in the sun, and observing them sink deeper in pro- 

 portion to the darkness of color, was first suggested by Dr. Hooke. 



3.) Cavallo observed that a thermometer, with its bulb blackened, 

 stands higher than one which had its bulb clear when exposed to tho 

 light of the sun, or even of the clouds. — (Phil. Trans., 1780.) 



Pictet made a similar observation, observing that when the two 

 thermometers remained for some time in a dark place they acquired 

 precisely the same height. He also found that when they had both 

 been raised to a certain point, the clean one fell much faster than the 

 coated one. — (Stir h Feu. ch. iv. Thomson, i, 126.) This last state- 

 ment is so contrary to all other experiments that we must suppose 

 some mistake. 



De Saussure received the sun's rays into a box lined with charred 

 cork, containing a thermometer with a glass front; it rose in a few 

 minutes to 221°, when the temperature of the air was 75°. — (Voyages, 

 ii, 932.) 



Professor Robison, in a siinila%experiment, employed three vessels 

 of flint glass within each other at one-third of an inch distance, set on 

 a base of charred cork, and placed on down in a pasteboard cylinder; 

 the thermometer within, in clear sunshine, rose to 230°, and once to 

 237°.— (Black' s Led. i, 547. Thomson, i, 127.) 



Sir H. Davy took several small disks of copper of equal weight, size, 

 and figure, on one side painted respectively white, yellow, red, green, 

 blue, and black. A mixture of oil and wax, which became liquid at 

 a temperature of 7G° Fahr., was attached to the other surface of each 

 disk : and on exposing the colored surfaces together to the sun's rays, 

 the length of time elapsed before tho mixture on each began to be 

 affected was in tho order in which they are above enumerated.-— (Bed- 

 doe's dlcdical Contributions, p. 44.) 



4.) The experiments of Sir E. Home (PHI Trans., 1821, Parti,) 

 are particularly deserving of attention, as exhibiting what might at 

 first sight be considered an exception to the above remarks a greater 

 effect being produced in some instances on a white than on a black 

 surface. A more attentive examination, however, will show us that 

 these experiments prove thus much: The heat occasioned by the 

 rays of the sun when received directly, or when in some degree inter- 

 cepted, as by thin white cloth, on the skin, is greater than that com- 

 municated by conduction to the same skin through a black cloth in 

 contact with it, which is itself, in the first instance, heated by absorb- 

 ing the rays. 



He observes, also, that a white skin is scorched, and that of a negro 



