I 10 ON RADIANT HEAT. 



already granted, that with this apparatus the thermometer is 

 immediately affected by the screen, which however affords 

 no support to Mr. Leslie's general position. 



Htsrxpert- j± s Mr. Leslie considers his 10th as an experimentum cm- 



mcntum cm- . . ' 



eis examined. c,s » which establishes Ins theory beyond the power of con- 

 tradiction ; I would occupy a moment of your time in ob- 

 serving, that although we admit, that, when a single reflec- 

 tor is used, the thermometer derives its temperature imme- 

 diately from the skreen, and that the compound skreen is 

 heated much sooner when the glass surface is opposed to the 

 hot body than when the metallic surface is; yet we by no 

 means find ourselves constrained to adopt Mr. Leslie's hy- 

 pothesis for explaining the radiation of caloric ; as it rests 

 not on these facts, but on the general conclusion deduced 

 from them — That caloric passes through no medium inde- 

 pendently of raising its temperature. — A conclusion, which 

 does not necessarily follow from any of the facts or reason- 

 ings to be found in the experimental inquiry, and which, 

 from my own observations, 1 feel myself authorized to re- 

 ject. 

 Made with I am inclined to think, that, had Mr. Leslie's experimen- 



it might have tu,n crqcis heen made with two reflectors, and had he noted 

 turned out the time occupied by it, he would have obtained results si- 

 differently. m ^ w tQ thoge wll j cn \ ie has re | ate( J p< 35 Q f n j s b 00 k ; and 



they would have admitted of easy and satisfactory ex- 

 planation on the established principles, which Mr. Leslie 

 considers as wholly unfounded, but to which, on account of 

 their simplicity and apparent truth, I confess myself at- 

 tached. 

 Heat reflected I pointed out in experiment 10th and 11th, that the ther- 

 surfaceon mometer is differently affected as the blackened or the plain 

 •which itim- surface of a sheet of glass is opposed to the hot body. Yet 

 pmges. in both experiments the matter of the skreen is the same ; 



and the radiating and absorbing powers are nearly equal. I 

 think the phenomenon may be explained in the following 

 manner. Mr. Leslie's 4th experiment shows, that the calo- 

 rific rays are reflected only from the surface on which they 

 impinge; the degree or reflection from a glass mirror being 

 always the same, whether the back of the mirror was sil- 

 vered, preserved clean, or ground with sand or emery. It 



seems 



