386 Polmaise Method of Heati?ig Gree7iIiouses, S^c. 



evidently the effects of radiation and conduction. The air, 

 however, as a body, has no conductive power, at least it 

 may be ranked as the worst conductor with which we are 

 acquainted. The conductive power it possesses, as an atomic 

 mass, is principally owing to the extreme mobility of its par- 

 ticles, for when confined in such a manner as to prevent its 

 freedom of motion, it will not conduct heat, and is then a 

 most useful medium of non-conduction. 



The cooling of a body by the conduction of the air, i. e. 

 by the mobility of its atoms, is somewhat different from the 

 effect of radiation, that while the ratio of loss by conduction 

 continues the same, for the same excess of temperature, 

 whatever be the absolute temperatures of the air and heated 

 bod}^, radiation increases in velocity for like excess of tem- 

 perature, when the absolute temperatures of the air and 

 heated body increase. 



Hence it is evident that, while the conductive power of 

 Polmaise drains, or channels of conduction, does not increase 

 in proportion to the increase of generated heat, the radiating 

 power of hotwater pipes, or other media of radiation, in- 

 creases in proportion to the increase of conducted heat, and 

 that the air of a hothouse increases in proportion to the 

 absolute temperature of the medium of radiation. 



The following table shows the law of cooling by radia- 

 tion for the same body at different temperatures, which will 

 render this argument clearer to those who have not atten- 

 tively studied the subject : 



The fact that the ratio of cooling of those bodies that 

 radiate heat, is more rapid at low temperatures and less 



