and the Mode of its Communication. 81 



time, we may derive much useful information by a 

 careful examination of the phenomena which occasion- 

 ally fall under our observation. 



If it be true that the black colour of a negro, by 

 rendering him more sensible to the few frigorific rays 

 which are to be found in a very hot country, enables 

 him to support the great heats of tropical climates with- 

 out inconvenience, it might be asked how it happens 

 that he is able to support, naked, the direct rays of a 

 burning sun. 



Those who have seen negroes exposed naked to the 

 sun's rays, in hot countries, must have observed that 

 their skins, in that situation, are always very shining. 

 An oil exudes from their skin, which gives it that shin- 

 ing appearance ; and the polished surface of that oil 

 reflects the sun's calorific rays. 



If the heat be very intense, sweat makes its appear- 

 ance at the surface of the skin. This watery fluid not 

 only reflects very powerfully the calorific rays from the 

 sun which fall on its polished surface, but also, by its 

 evaporation, generates cold. 



When the sun is gone down, the sweat disappears ; 

 the oil at the surface of the skin retires inwards ; and 

 the skin is left in a state very favourable to the admis- 

 sion of those feeble frigorific rays which arrive from the 

 neighbouring objects. 



But I shall refrain from pursuing these speculations 

 any farther at present. 



I shall now proceed to give an account of several ex- 

 periments, of various kinds, which were made with a 

 view to a farther investigation of the radiations of cold 

 bodies. 



Having found, by several of the foregoing experi- 



