THEORY OF RADIATION. 191 



The connexion of this subject with the philo- 

 sophy of radiation is exceedingly important, and 

 seems never to have been rightly understood. It 

 was ascertained by Leslie as a very general fact, 

 that the radiating power of bodies was inversely 

 as their conducting power. This result was sup- 

 ported by a great variety of experiments per- 

 formed by Dr. Wells, with a view of explaining 

 the production of dew; and still farther con- 

 firmed by the recent researches of Melloni. It 

 was found by Dr. Wells, that eider down, wool, 

 cotton, grass, the leaves of trees, and all vege- 

 table substances, parted with their caloric more 

 rapidly by radiation, than rocks and metals 

 that while dew collected in large quantities on 

 the former, during clear nights, there was gene- 

 rally little or none on the latter. But no one 

 seems to have suspected, that the radiating 

 power of bodies was determined by their latent 



oily nature, or to the air which they contain. Dr. Thomson 

 thinks that it is owing to the air within their pores ; while the 

 great mass of writers on Chemistry and Physics, resolve the whole 

 mystery by telling us, that they are bad conductors of caloric, 

 which is the very thing to be explained. The hypothesis of Dr. 

 Thomson is so far from affording a solution of the difficulty, that 

 he does not explain why air is a bad conductor. The simple 

 matter of fact is, that all those articles of clothing which retain 

 the caloric of the body most effectually, contain a large amount of 

 the igneous principle, and are therefore bad conductors. For the 

 same reason, they are highly combustible ; and, when submitted 

 to friction, afford abundance of the electric aether on which ac- 

 count^ they have been called electrics. 



