Radiation of Rough and Pollstied Surfaces, 301 



four sides. Each pair was composed of plates in every respect 

 alike, except as it regarded the external surface, one of which 

 was very smooth and brilliant, and the other was not polished, 

 and was streaked with emery. On precisely measuring with the 

 thermo-multiplier, the quantities of heat projected by the two 

 polished faces, when the vessel was filled with hot water, and in 

 comparing them with those which emanated from the corres- 

 ponding streaked faces, I could only perceive differences to the 

 extent of one or two hundredths (centiemes), and these some- 

 times on the one side and sometimes on the other. The mean 

 of twenty observations yielded only a variation which scarcely 

 amounted to a few thousandths fmilliemesj, and, ccfisequently^ 

 might be altogether disregarded. 



To this experiment it may perhaps be objected, that in 

 spite of the precautions taken for the establishment of a perfect 

 contact betwixt the plates and the vessel, we have nevertheless 

 no certainty that the two plates which composed each of the 

 pairs submitted to experiment, possessed the same tempera- 

 ture. To avoid this objection, I caused a square reservoir to 

 be hollowed out of a small block of marble, the sides of which 

 were reduced to a perfectly equal thickness, and the external 

 surfaces of which were differently prepared. The first was 

 smooth and brilliant ; the second was equatty smooth, but un- 

 polished and tarnished ; the third was streaked in one direction ; 

 and the fourth in two, crossed at right angles. The vessel was 

 then filled with hot water, and projected the same quantity of' 

 radiating caloric from each of the four sides. 



Hence, it appeared that the more or less irregular state of 

 the surface has no influence upon the emissive power, when the 

 radiating body is not of a metallic nature. 



I then covered with lamp-black one of the faces of my marble 

 vase, as well as one of each of the pairs of plates I had employ- 

 ed in the preceding experiment. As it has been agreed to re- 

 present by the number 100, the emissive power of lamp-black, 

 I could easily determine, by successive comparisons, the pro- 

 portional numbers which represented the emissive powers of 

 ivory, jet, and marble. The whole three were found to be com- 

 prised between the numbers 93 and 98. But here might it not 

 be said, that if in the substances which were thus employed, the 



