M. Melloni oH the Theory 0/ Dert. I8t 



ened apparatus vibrates to the sky, and not at all to the con- 

 tact of the external air, which takes place equally on it and 

 on the polished metal apparatus of the other exposed ther- 

 mometer. 



My memoir contains the detail of all the precautions which 

 it is necessary to observe, in order to obtain the comparative 

 degrees of cold produced by the radiation of silver and lamp- 

 black. The definitive results have confirmed, in a striking 

 manner, the fact lately announced to the Academy by MM, 

 de la Provostaye and Desains, namely, that the emissive power 

 of metals is much less than has hitherto been believed, ac* 

 cording to the experiments of Leslie, Dulong, and Petit.* 



* The radiation of lamp-black being equal to ICO, laminated silver would 

 radiate, according to my experiments, about 3026. MM. de la Provostaye and 

 Desains found 5'37 for silver precipitated chemically on copper, and 2*10 when 

 this precipitate is polished with the burnisher. According to them, the emis- 

 sive power of silver just from the rollers is 2-94 j and 2*38 that of laminated 

 and burnished silver. 



Observations made in 1838 led me to this result, that the difference of radi- 

 ating force manifested in Leslie's famous experiment on the two faces of a cube, 

 the one smooth and polished, the other rendered more or less rough with grooves, 

 does not arise, as was generally believed at the time, from a variation in the 

 mechanical state of the two surfaces, but from a change in the degree of density 

 resulting fi'om the operation, by means of which the smooth surface was trans- 

 formed into a rough one. This proposition appears to me to be perfectly esta- 

 blished by the three following facts : 1. The variation of emissive power produced 

 by grooves does not appear much exce^it in the metals ; marble, jet, and ivory, 

 grooved or polished, always radiate with the same energy. 2. Silver melted, 

 and cooled slowly in moulds of sand, polished with oil and charcoal, and then 

 grooved with a diamond in such a manner as to compress and condense the bot- 

 tom of the rays, diminishes instead of augmenting its radiating force in passing 

 from the polished to the rough state. 3. This same kind of melted and polished 

 silver becomes much less radiating when beaten on an anvil, or passed under 

 the laminoir. 



It is easy to see that the experiments of the skilful natural philosophers above 

 mentioned, furnished results altogether analogous, and, consequently, shew the 

 same thing ; for silver precipitated chemically on copper, being much less 

 dense than laminated silver, and the latter being inferior in density to bur- 

 nished silver, this latter property, according to the preceding numbers, is in 

 the inverse sense of the corresponding emissive powers. 



The only difference between tlie two demonstrations of the principle is, that 

 my measures related to the most intense emissive i)Ower of silver, while those 

 of M\f. Provostaye and Desains referred to the emissive power of silver and 

 other metals relative to the lamp-black. 



It is, perhaps, to this unnoticed difference, or some other inaccurate data, 



