THEORY OF RADIATION. 193 



air, 50, the thermometer would sink much 

 more rapidly. It is equally obvious, that when 

 placed in water, mercury, and other dense liquids, 

 the effect is produced wholly by their attraction 

 for caloric, and without radiation. 



It was before stated, that the metallic oxides 

 have but little attraction for caloric, compared 

 with the pure metals ; and that their cohesion, 

 specific gravity, and conducting power, are ac- 

 cordingly small. The experiments of Leslie 

 prove that their radiating power is much greater. 

 He found that while the radiating power of clean 

 lead was only 19, it rose to 45 when tarnished 

 by oxidation ; that the radiating power of plum- 

 bago was 75, and deutoxide of lead (red lead) 80. 

 He also discovered, that while the radiating 

 power of gold, silver, copper, and polished tin, 

 was only 12, that of crown glass was 90, sealing- 

 wax 95, resin 96, writing-paper 98, and lamp- 

 black 100. 



In short, all light bodies, which are full of 

 latent aethereal fire, and therefore bad conductors, 

 are proportionally good radiators ; while the 

 denser metals, which contain less of it, hold on 

 to it for a longer time, under the same circum- 

 stances. But as their affinity for caloric di- 

 minishes in proportion as they acquire more of 

 it, they also radiate freely at very high tempe- 

 ratures. On the other hand, if the dense metals 

 could be reduced 1000 below zero, they would 



o 



