Radiation of Rough and Polished Surfaces, 305 



The cast and roughened plate, . . 11^3 



In comparing these four radiations, we perceive, 1^^, as to 

 the polished plates, that the cast metal affords nearly one-third 

 more than the forged metal ; which most clearly demonstrates 

 the influence we have assigned to the inferior density ; and, 2c?, 

 That the effects of the streaking upon the two sorts of plates 

 differs not only in intensity, as we had foreseen, but also in 

 Tcind, for if the radiating power of the forged plate received an 

 augmentation of four-fifths by the roughening action of the 

 emery, on the other hand, that of the cast silver experienced a 

 dijwinw^io7i of nearly one-fifth. 



This unexpected result, which irrefragably proves the truth 

 of our fundamental proposition, is most satisfactorily explained 

 by the theory we have just been developing. For the pressure 

 of a hard body, such as the emery, upon the soft surface of the 

 cast metal, somewhat compresses and condenses the rubbed 

 parts, and makes the bottom of the striae, produced upon the 

 superficial layer, harder than the whole surface of the corres- 

 ponding plate. 



I have to regret that I had not an opportunity of making 

 the same expeiiment upon vessels of gold and platina, where 

 the same kind of manifestations, in all probability, would ex- 

 hibit themselves upon a larger scale, on account of the very 

 great difference of density we can impart to these metals by 

 means of fusion and percussion. 



And now, turning to the first observations of the late Profes- 

 sor Leslie, we observe that the different metallic plates which 

 he subjected to experiment, gave constantly a greater emissive 

 power when they were rough and irregular, than when they 

 were smooth and polished. This being the case, nothing ap- 

 peared more natural than to conclude, regarding the phenome- 

 na of the emission of caloric, that, apart from the influence de- 

 pendent upon the quality of the superficial layers, there was 

 some peculiar influence dependent upon their degree of polish, 

 so far at least as the metals were concerned. This was the 



t conclusion drawn from the facts observed by Mr Leslie ; and, 

 notwithstanding, this conclusion — so simple and direct in ap- 

 pearance, was unwarrantable. 

 Here, then, is a beacon where needed, against the unfortu* 



