ON THE NATURE OF HEAT. 197 



heat, would radiate quite as much as ice; and a bullet of 

 freezing mercury would radiate fcarcely more than another of 

 boiling water: both which hot temperalures are very trifling 

 in comparifon to the heat in which we are habituated to notice 

 and obferve this phenomenon. 



Exp. 24<. After the proof that cold bodies of the fame kind Exp. 24. The 

 affect the thermofcope equally, when equally diftant frOm the ^j^j^,^ produce 

 common temperature, it remained to be determined whether moft heat by ra- 

 the different modifications of furface have the fame effe^s in ^^^^'^^^ ^^^^"^ 

 the ready propagation of lower temperatures, as they had cold. 

 (hewn before in higher. To fliew this, it was only required 

 to oppofe them to each other, as to their a61ion upon the fame 

 ball, as in the laff experiment. With this view both difcs 

 were blackened, and, the temperature of the room, being 72^ 

 as before, one of them was charged with ice and water, and 

 the other with water at 112^. Thefe, at equal diftances from 

 the ball as before, did not affect the bubble, and therefore their 

 a6tions were precifely equal. 



In the confideration of two kinds of rays, calorific and Do not the fame 

 frigorific, it did not efcape the attention of our author, ^^'^^^^yHI^^I^- ^^l^ as 

 hot and cold are terms denoting mere relations; fo that the the temperature 

 fame body will be either hot or cold accordingly as the com- bodt'sThrhJ? 

 mon temperature, or temperature of the bodies of com pari- or lower than the 

 fon, is lower or higher. Queftioning, therefore, as to the »^eceiving body, 

 difference between calorific and frigorific rays, he demands 

 whether the fame rays may not be either calorific or frigorific, 

 accordingly as the body at whofe furface they arrive, is hotter 

 or colder than that from which they proceed ? 



Exp. 25. The whole of the external furface of one of the E*P* ^5* Metal 

 large cylindrical palfage thermometers, Fig. 1, Plate I. ^^^ goU-ht7ter'& 

 covered with gold-beater^s fkin, and, along with the ftandard fkin cools fafter« 

 inflrument of the fame kind, was filled with hot water. The 

 covered inftrument cooled through the fiandard interval of 

 ten degrees, namely, from 101 ^ to 9 If, in twenty-feven and 

 three quarter minutes ; but the naked inflrument employed 45 

 minutes in paffing through the fame interval. 



Exp. 26. Both inflruments were fuffered to remain in the ^fp- ^6. And 

 cold all night, when the temperature in the naked inflrument ^jga^ ^o^g*^^^ 

 was 50|-^, and that of the covered inftrument 49|° ; the air quickly. 

 Qf the room being 48^. Both inflruments were then removed 

 into a warm room, of which the temperature continued ber 



tween 



