PHYSICAL AND LITERARY. 19 



tural to look upon this as the efFeft of the re- 

 action of light upon bodies, and that, at a 

 diftance from them ; for, there is no reafon. 

 to think that light produces heat by adtually 

 ftriking the folid parts of bodies, after we 

 are fatisfied that bodies produce the. reflexion, 

 andrefradion of light, without fuffering it 

 ,to come into contadt with them, fiJul -uuiii. 



10. From thefe principles it follows, ^ that 

 light, in pafling out of one medium into an- 

 other of different denfity, mufl: always , pro- 

 duce fome degree of heat ; becaufe it is part- 

 ly refradted and reflected at the common fur- 

 face : fecpndly, that, in paffing forwards 

 thro' the fame homogeneous or perfedly 

 tranfparent medium^ it can produce no heat ; 

 becaufe there is no reflexion - or refradiion, 

 no influence of the body upon the light, but 

 every ray purfues its own right-lined courfe, 

 as if it moved in a perfe<5t void *. 



11. Kence it appears, that, in water, 

 glafs, and other tranfparent mediums^ which 



are 



* Sir Ifaac Neiuton, in the third book of his Vrbictpia, 

 \yhere he difputes concerning the tails of comets, ' lays it 

 down as an obvious principle, ^oJ radii folis non agitant 

 media qua pertnanant, nifi in refltxione et refraSiione, 



