108 



ON RADIANT HEAT. 



In some in- 

 stances Lett 

 passe; through 

 opaqtM 



dily as through 

 transparent 

 media. 



Attempt to 

 explain this. 



ter the painted tin has the decided advantage: yet in Exp, 

 2nd the thermometer indicated a higher temperature than 

 in Exp. 3rd. I consider this unexpected result as depend- 

 ing on the great difficulty of so completely painting the 

 surfaces of the glass and of the tin as entirely to destroy 

 the transparency of the one, and the polish of the other. 



We learn from Exp. 12th, that, when a single mirror is 

 employed, and boiling water is the source of heat, caloric 

 does not pass more readily through glass than through an 

 opaque body; and Mr. Leslie's experiments show, that, as 

 the skreen is removed from the hot body, the effect on the 

 thermometer diminishes, and at last entirely disappears. 

 But even these results admit of explanation on principles 

 very d liferent from those assumed by Mr. Leslie. 



We well know, that diaphanous media always intercept 

 a number of the calorific rays; and may therefore fairly 

 conclude, that such media offer a resistance to the passage 

 of caloric. It seems to me reasonable to suppose, that it is 

 by their momentum, that the rays of paloric overcome this 

 resistance; whence I infer, that, a perpendicular direction is 

 that most favourable for a ray of caloric to impinge on the 

 surface of glass that it may pass through its substance; and 

 conclude, that the greater is the angle of incidence of a ray, 

 the less will it be able to overcome the resistance offered to 

 its passage. When a single mirror is employed comparatively 

 few caloiiac rays impinge on the surface of the glass in such 

 a direction as enables them to overcome the resistance; too 

 few sensibly to affect the thermometer. Of the rays not 

 transmitted some will be reflected, some will be absorbed 

 by the skreen; its temperature will be raised, and it v M 

 consequently radiate caloric to the thermometer. When 

 therefore we operate with one mirror, the thermometer seems 

 to derive its temperature immediately from the interposed 

 skreen, whether it be plain or painted glass. But when we 

 employ two mirrors, a number of rays are made to fall per- 

 pendicularly on the surface of the skreen; they are trans- 

 mitted by the plain glass; they are intercepted by the 

 painted glass. Hence, in Experiment 9th, with plain glass 

 the thermometer rose as 7, with painted glass only as 5. 



Yet 



