418 MELLONI ON THE ABSORBING POWERS OF 
say, the striated plate becomes less heated than the shining and — 
polished one*. 
But if, by destroying the polish, we can in one case increase 
the absorbing power, and in another diminish it, it is evident 
that the formation of points and asperities, through which a 
larger quantity of heat might be introduced, is not, as it has 
been generally supposed, the cause of the differences observed 
between polished and rough surfaces, but that this difference is 
rather to be attributed to changes in the hardness or in the 
elasticity of the superficial layers of the metal; for there is no 
doubt that the operations by means of which we render the plate 
rough or shining, necessarily produce at the same time displace- 
ments of the molecules; displacements which in some cases 
cause the integrant particles permanently to approach nearer, in 
others to recede from each other, and which increase the hard- 
ness and elasticity of the metal more or less, according to its 
previous consistence, and to the method adopted for imparting 
to the surface a more or less decided roughness or polish. 
As to the precise kind of effects respectively produced, it is © 
clear, from what we have just said, that the absorbing power | 
diminishes as the hardness or elasticity of the plate increases. 
Thus, hammered tinned iron, compressed by the percussion of 
the hammer, possesses a feebler absorbing power than when in 
its natural state. This inferiority is not the result of a more 
perfect polish, for we may easily give the hammered disc a polish 
very inferior to that of the unhammered one, without render- — 
ing by this the absorption superior to that of the unhammered 
disc. It is therefore really the greater hardness which dimi- 
nishes the absorbing power of the hammered plate. 
Polished copper prepared by the flatting-mill having by this 
means been in reality compressed, increases in absorbing power 
when it is subsequently striated ; because the furrowed lines ex- 
* It is necessary to use gold or silver, because in the case of copper or of any 
other oxidizable metal the striated surface would become coated with a film of 
oxide far more rapidly than the smooth surface. This would greatly increase 
the absorbing power, and it would be difficult to distinguish between this and 
the effects resulting from the respective influence of the polish and the striz. | 
For the same reason the surfaces of the gold or silver must on no account be 
roughened by means of emery or of a file, which, even after repeated washings, 
would always leave more or less abundant traces of heterogeneous matter in- 
crusted on the metal, and would produce on the unoxidizable surface the same — 
effect which oxidation would occasion on a plate of copper or any other metal 
alterable by the air. 
