1840.] On ine Skin in the Dark Races of Mankind, 



179 



black surfaces rise to a higher temperature than 'white under the sun's 

 rays, yet they scorch the surface of the body less; the scorching eifect 

 depending on a union of the r-tys of heat with those of light, the latter 

 being supposed, by way of explanation, to be excluded by the black sur- 

 face. First, I shall state my repetition of the experiment, and then at- 

 tend to SirEverard's explanation of his supposed fact. 



I have attempted to ascertain the rise which the absorption of heat 

 by black and white cloths respectively gives to the thermometer ; to 

 compare this observation with the effects of the same cloths under the 

 sun's rays upon the body, and with the effect of the sun's rays on the 

 naked skin. When the thermometer stands at about 80" in the sun, the 

 solar rays concentrated on white cloth over the ball of a thermometer, to 

 a space of an inch and a half in diameter by a burning-glass, caused a 

 rise of the thermometer to 125" in a quarter of an hour. When black 

 cloth was substituted the rise during the same period was to 172°. In five 

 minutes, with the white, the rise was to 108°, with the black to 140°; 

 and in some experiments in a proportion nearer that given by the longer 

 period. 



When the black and white cloths were applied to the skin at the same 

 temperature, and with the same degree of concentration, as already men- 

 tioned, the black cloth generally caused intense pain in the course of a 

 few minutes, and on being allowed to remain for five or at most seven 

 minutes, produced blisters. During the same period "very little apparent 

 effect followed the application of the white cloth, though considerable 

 pain was sometimes produced. The experiment was at different times 

 performed on several individuals, all of whom found the black cloth give 

 the sensation of pain sooner than the white. On the whole, I found 

 nothing like the difference described by Sir Everard, though certainly 

 the vesicating effect of the black surface appeared to be much greater 

 than that of the white. From many experiments I conclude, that the 

 rays of the sun will scorch when they are applied to the surface so as to 

 cause a heat of about 130° and upwards. And from the experiment 

 related by Sir Everard, it appears that hot water is capable of producing 

 a similar effect at that temperature. From all this, I am inclined to 

 deny the existence of a scorching power in the sun's rays, independent 

 of the heat they contain, or at least of the effect they produce on the 

 thermometer. Moreover, if such a power do exist, black cloth should 

 yet scorch more than white, since it will absorb all the rays of light, 

 whereas the other surface will reflect them. 



In those experiments which I performed, care was taken to have the 

 white and black cloth nearly of the same density. Sir Everard does not 



