. 
POLISHED AND OF STRIATED METALLIC PLATES. 421 
polished metallic surfaces an increase is obtained in their emis- 
sive and absorbing power: this increase cannot be attributed 
either to reflection from the points and asperities, or to any 
other direct and immediate action of these asperities; since we 
thave seen,—lst, that the striz have no sensible influence on 
non-metallic surfaces; 2ndly, that the effect is very various, ac- 
cording to the nature and condition of the plate used; 3rdly, 
‘that metals which are unalterable by the air, will, if prepa 
prepared, give an inverse effect, the presence of asperities then 
diminishing instead of increasing the emissive and absorbing 
powers. This last tact appears to me conclusive. Thus the in- 
crease long ago observed in the emissive and absorbing power 
of striated metallic plates is only a particular case. The absence 
f effects, and the diminution subsequently observed by us in 
narble and silver properly prepared, are also particular cases ; so 
that the variations in emissive and absorbing power which ac- 
mpany the conditions of polish and of roughness, in sub- 
ances susceptible of being made smooth and shining, have not 
i character of generality, but on the contrary alter with the na- 
ture of the bodies and the state of molecular equilibrium im- 
pressed on their superficial layers. These alterations however 
e only observed with metals, and we know that when sub- 
jected to the action of mechanical forces these bodies undergo 
permanent modifications in the specific gravity, hardness, and 
asticity of their superficial layers: now these modifications are 
the only effects known. In attributing therefore to them the 
shanges observed in the radiation and absorption, we are in 
reality not advancing any hypothesis; we are merely announ- 
g the phenomenon under a new form, expressing conditions 
were unknown before the experiments which we have been 
iling in this notice. 
