Radiation of Rough and Polished Surfaces. 303 



tallic stihstances, being put out of the question, the inquiry still 

 remains, what is the alteration peculiar to metals which, in these 

 bodies accompanies the change, more or less considerable, of 

 the superficial layer ? 



It is no other, in my opinion, than a change of hardness or 

 of density. Jet, ivory, and marble are, in fact, substances 

 which are almost entirely destitute of compressibility, or at all 

 events they do not possess, in any sensible way, the property of 

 permanently retaining the modifications of density and hardness 

 impressed on them by the action of mechanical force. They 

 also readily arrange themselves in the form of plates without 

 being subjected to pressure. On the contrary, the metals are 

 compressible ; and the common metal plates of commerce are 

 obtained, as is known, by subjecting the metals to an extremely 

 severe pressure by means of the hammer and screw. Finally, 

 experiment also proves that these plates, as well as wires, have 

 a specific weight, and a hardness superior to those of east metal. 

 Who is the individual that will tell us, that this augmentation 

 of hardness and density is uniformly distributed throughout 

 every part of the mass ? Is it not, on the contrary, probable 

 that, during this operation, the surface undergoes a pressure 

 and condensation greater than any other part ; and that the 

 plate is truly covered by a kind of envelope, whose density and 

 hardness is superior to that of the internal layers ? 



This conceded, it is evident that by streaking the surface of 

 a plate the less dense and hard parts will be exposed. Upon 

 turning, then, to the tables which represent the emissive powers 

 of bodies, it will be at once perceived that these powers follow, 

 in general, the inverse ratio of their density. Let us admit that, 

 according to all analogy, the same law holds regarding the dif- 

 ferent degrees of condensation of the same substance, and we 

 may hence conclude, that, in producing these small furrows 

 at the surface of the plate, we must obtain an augmentation of 

 the radiating power. To this we have to add, that the parts 

 which compose the superficial layer being loosened by the sepa- 

 ration of their contiguous particles, must become softer, and thus 

 acquire, by the diminution of density, an emissive power which 

 will more nearly approach to the softer layers of the interior. 



This being the case, it will result, Isi, that a polished 



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