450 Relations of Air, Heat, and Cold. 



The rest of the heat, consisting of 1 part from the naked 

 metallic surface, and of 10 parts from the cased surface, is 

 propagated through the same medium, but with a certain 

 diffusive rapidity, which in a moment shoots its influence 

 to a distance, after a mode entirely peculiar to the gaseous 

 fluids. The very superior propellent energy of a surface 

 of glass or paper in comparison of that of a metallic one, 

 lies within the compass even of nrdinarv observation. If 

 a glass caraflTe or a pot of porcelain be filled with boiling 

 water, on bringing towards it the palm of the hand, an 

 agreeable warmth u ill be felt at the distance of an inch or 

 two from the healed surface; but if a silver pot be heated 

 in the same way, scarcely any heat is at all perceptible on 

 approaching the surface, till the fingers have almost touched 

 the metal itself. 



" It is curious to inquire how such a singular diversity 

 can arise. If the silver ball be covered with the thinnest 

 film of gold-beater's skin, and which exceeds not the 

 3000th part of an inch in thickness, the power of disper- 

 sion will be augmented from I to 7 ; if another pellicle be 

 added, there wil! be a further increase of this power, from 

 7 to 9; and so repeatedly growing, till after the application 

 of five coats, when the propellent energy will reach its ex- 

 treme limit, or the measure of 10. In this case, the me- 

 tallic surface is precluded from all contact with the air, and 

 it must, therefore, act in consequence of its mere approxi- 

 mation to the external boundary. We may thence infer, 

 that air never comes into actual contact with any surface, 

 but approaches much nearer to glass or paper than to po- 

 lished metal, from which it is separated by an interval of at 

 least the 500th part of an inch. A vitreous surface, from 

 its closer proximity to the recipient medium, must hence 

 impart it? heat more copiously and energetically, than a 

 surface of metal in the same condition ; and the metal, to 

 a certain extent, can act in reducing the power of the other. 

 When a pellicle was applied, the metallic surface imme- 

 diatelv under it repelled parti;-.lly the atmospheric boundary, 

 and reduced the darting efflux of heal from 10, which 

 would have been thrown by the skin alone, to about 7» or 

 only 6 more than the efiicacy of the naked metal. The 

 repelling influence of the metallic plate was sensible even 

 imder four coats, or at the distance of the 750th part of an 

 inch from the external surface. 



*' By what process the several portions of heat, thus de- 

 livered tn the atmosphere, shoot through the fluid mass, it 

 seems more difficult to conceive. They are not transported 



by 



