x 
ON POLISHED AND STRIATED METALLIC PLATES. 417 
We shall now see that it is the same with their absorbing 
power. But before entering on the experimental proofs, it is 
essential to guard against any mistake as to the exact meaning 
of what we have just advanced. Our proposition does not bear 
upon the fact itself, which we in no way dispute, but upon the 
explanation hitherto given of that fact. Thus, if the polish of 
a metallic body be destroyed by friction, with emery, or with a 
file, so as to render its surface rough and dull in place of its 
former smoothness and brilliancy, the proportion of heat ab- 
sorbed by this body in a given time, when exposed to calorific 
radiation, is unquestionably altered: this alteration may even 
be such that the metal becomes heated doubly or trebly what it 
_ was before; and yet we maintain that the roughness or polish 
‘ 
' 
t 
have no influence whatever in producing the phenomenon, and 
. 
_ that the change effected in the absorbing power of the metallic 
surface has an entirely different source. The following experi- 
ments prove this. 
If two small copper discs, the one striated or unpolished, the 
other polished and shining, be successively placed before a good 
thermoscope, each of the discs being blackened on the side 
turned towards it; and if we then cause exactly the same calo- 
rific radiation, concentrated by means of a lens of rock salt, to 
act on their anterior surfaces, we shall observe the effects which 
are above stated, viz. that the roughened disc becomes more 
‘intensely heated than the polished one. The result is the same 
‘if we act on polished and unpolished discs of steel, tin, silver, 
gold, or any other metal reduced into plates by means of the 
hammer or the flatting-mill. But if the experiments be repeated 
on two plates of tinned iron, one of which has been subjected 
to strong and quick blows of the hammer and the other left in 
its natural state, we shall find that the latter, which has an even 
and reflecting surface, becomes far more intensely heated than 
the former, whose surface is less shining and full of unevennesses. 
Again, if we take two plates of silver or of gold that have been 
melted and slowly cooled, one of the plates possessing the fine 
polish which can be imparted by means of oil and of fine char- 
coal, while the other, having been similarly polished, is after- 
wards unpolished by a series of lines traced with the diamond, 
we find, to our surprise, that the results are exactly contrary to 
those which occur under the ordinary circumstances ; that is to 
