138 Mr Ritchie on Radiant Heat 



approaches nearer to glass or paper than to polished 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 its heat more co- 

 piously and energetically than a surface of metal in the same 

 condition.'*' If air never comes into actual contact with any 

 surface, by what power, we may ask, is caloric carried over the 

 space between the surface of the body and the atmospheric 

 boundary ? Surely not by the pulses excited in the ambient 

 air, since there is no air in which such pulses can be excited. 

 Is this power, whatever it may be, so feeble as to lose its in- 

 fluence just at the atmospheric boundary, when it has deliver- 

 ed its charge to the wings of the ambient air ? If a power, 

 different from the ambient air, exist, which is capable of tran- 

 sporting caloric over the 500th part of an inch, why should not 

 the same power carry it over the 100th part of an inch, or even 

 a whole inch ? The supposition which Mr Leslie has very in- 

 geniously made, even if it were well-founded, could not, on the 

 pulsatory system, account for the striking difference between 

 the radiating powers of glass and polished metal. The true 

 cause of this difference we have endeavoured to assign in one 

 of the numbers of the Edinburgh Philosophical Journal, 



The various contrivances to which the author had recourse 

 in supporting his favourite theory are numerous and truly in- 

 genious, though to us they appear to support with much greater 

 force the opposite theory. *' When a pellicle of gold-beater"'s 

 skin," he observes, " was applied, the metallic surface imme- 

 diately under it repelled partially the atmospheric boundary, 

 and reduced the darting efflux of heat from ten, which would 

 have been thrown by the skin alone, to about seven, or only six 

 more than the efficacy of the naked metal. The repelling in- 

 fluence of the metallic plate was sensible even under four coats, 

 or at the distance of the 750th part of an inch from the exter- 

 nal face."" The metals, as is well known, possess a powerful 

 attraction for the matter of heat. This attraction will there- 

 fore be exerted at some distance from the surface of the metal. 

 The calorific atoms ranged along the surface of the metal, and 

 also in the interior of the pellicle of gold-beater's skin, will there- 

 fore be attracted with considerable force towards the metallic 



