304 Radiation of Rough and Polished Surfaces. 



plate of any given metal radiates a quantity of beat so much 

 greater in proportion as the density or the hardness of its su- 

 perficial layers is less ; and, S(i, that, in the case of inferior 

 density and hardness, the increase of the radiating power pro- 

 duced by the wnpolishing will be inferior to that obtained when 

 the plate is denser and more thoroughly compressed. 



It is almost unnecessary to add, that, for the verification of 

 these theoretic conclusions, we must avoid employing a metal 

 which is oxidated at a low temperature, for a plate of metal 

 of this kind is imbued with a tendency to, augment its emissive 

 power, which varies every moment with the condition of the 

 superficial layers, and so much the more as these layers are 

 soft and detached. 



Violent percussion, on the one hand, and a very slow transi- 

 tion from the state of fusion to solidity, on the other, are the 

 two means by which we succeed in endowing metallic substances 

 with greater or less degrees of density. Accordingly, I ordered 

 two pairs of plates to be made of very pure silver, and gave di- 

 rections that one of each pair should be hammered to a high 

 degree, and the other should be cooled down very slowly in its 

 sandy mould. Out of these I constructed, with the addition of 

 a metallic bottom, a small rectangular cistern, the sides of which 

 were united by means of soft solder, so that there might be no 

 risk of altering their densities or tempers, during the operation. 

 When so united, the four sides were perfectly polished, which 

 process had been accomplished by means of pumice-powder and 

 charcoal, without the help of either the hammer or the bur- 

 nisher. I then took coarse emery paper, and strongly rubbed, 

 in one direction, one of each pair of the plates ; the images of 

 objects which appeared very distinct and bright upon the surfaces 

 which were left in all their pristine polish were completely ef- 

 faced upon the rubbed surfaces, which were quite obscure and 

 covered with striae. The vase, thus prepared, was filled with 

 hot water ; and the four sides, successively presented to the 

 opening of my thermo-electrical apparatus, produced the fol- 

 lowing effects upon the galvanometer :— 



The forged and polished plate, . . 10*^ 

 The forged and roughened plate, . . 18« 

 The cast and polished plate, . . . 13°7 



