98 Experiments of Mr. Boy k and Dr. Franklin. [Book II. 



iatisfactorily accounted for, on the principle that fire 

 is a diftind fubftance, than on that which fuppofes it a 

 mere quality, depending on the tremor or inteftine 

 motion of bodies. 



Heat and light are the only means by which we 

 are enabled to difcover the prefence of fire, I con- 

 clude, therefore, that they are both effects of the fame 

 caufe. The rays of the fun, when concentrated to a 

 certain degree, produce intenfe heatj and heat, when 

 violently excited by attrition, &c. if the body in which 

 it is excited is in favourable circumftances, will gene- 

 rally terminate in flame, and confequendy in the emif- 

 jRon of light. This hypothefis receives a ftrong con- 

 firmation from an experiment of Mr. Boyle. He co- 

 loured the furface of a large tile, one half white, and 

 the other black : after fuiFering it to lie for fome 

 time, expofed to the fummer fun, he found that while 

 the whited part of the furface, or that part which re- 

 flecled back the rays of light, remained quite cool, 

 the black part, which imbibed them, was grown ex- 

 tremely hot. He occafionally left a part of the tile of 

 its native red ; and, after expofing the whole to the 

 fun, found that this part grew hotter than the whice, 

 but not quite fo hot as the black part. He obferves, 

 that rooms hung with black are not only the darkeft 

 but the warmeft alfo ; and a virtuofo of unfufpected 

 credit aflured him, that in hot climates he had feen 

 eggs well roafted in a fhort time, by only blacking the 

 fhells, and expoiing them to the fun. This fad: was 

 afterwards completely eftablifhed by Dr. Franklin, 

 who expofed feveral pieces of cloth of different co- 

 lours upon the furface of fnow ; he found that the 

 black funlc confiderably beneath the furface, confe- 

 quently that it imbibed a large quantity of heat, 

 whereas the white, which reflected the greater part of 



the 



