Reflections on Heat. 183 



winter, to the air of a large quiet room, they all 

 passed, in cooling, through the given interval of 10 de- 

 grees in from 45 to 46 minutes. 



This equality in the degree of readiness with which 

 all the metals become cool or acquire heat is certainly 

 very remarkable ; and it seems to me very difficult of 

 explanation except by adopting the hypothesis that heat 

 is communicated by means of radiations. 



As it might be supposed that a film of air, attached 

 by a certain force of attraction to the surfaces of the 

 metallic vessels, could have caused this apparent equal- 

 ity in their rate of cooling, I made the following ex- 

 periments to elucidate this point. 



One of the two brass vessels was covered, first with 

 one, next with two, then with four, and finally with 

 eight coatings of spirit varnish, and the experiment with 

 the two vessels was repeated with each of these coatings. 

 While the vessel, the surface of which was bare, cooled 

 invariably through the given interval of 10 degrees in 

 45 minutes, the other vessel, which was varnished, 

 cooled more or less rapidly according to the thickness 

 of the coating of varnish with which its surface was cov- 

 ered, but always in a sensible degree more rapidly than 

 the one whose surface was naked : 



Minutes. 



With one coating of varnish it cooled in . . . 34^ 



With two coatings, in ...... 29 



With four coatings, in ...... 24^ 



And with eight coatings, in . . . . . 27 



As the film of air which is supposed to have been 

 attached to the surface of the vessel when this metal- 

 lic surface was not covered with varnish ought to have 

 been as completely driven off by one coating of varnish 



