420 MELLONI ON THE ABSORBING POWERS OF 
This result, which might have been deduced by analogy from 
our first experiments on the emissive power of polished and 
striated surfaces, had not escaped the penetration of M. Saigey, 
who has since then applied the principle very successfully in 
the construction of conjugate mirrors, and of other apparatus 
intended for the reflection of heat. 
The new theory, which takes from the points and asperities 
their supposed influence on calorific absorption, and attributes 
it to variations in hardness or in elasticity, is strikingly con- — 
firmed by the constancy of absorbing power remarked in all 
substances which are incapable of retaining any state of com- 
pression impressed on their superficial layers by any mechanical 
means whatsoever. Thus we find that a disc of marble, jet or 
ivory absorbs the same quantity of heat in their natural state, as 
when they have eitherreceived the highest possible polish, or been 
- roughened by means of coarse sand or emery ; because in these 
classes of substances the methods which either develope or de- 
stroy asperities and inequalities of surface, do not in a perma- 
nent manner affect the hardness or the elasticity of the superficial 
layers. Finally, I may remark, that in the course of my various — 
experiments I have never been able to detect any difference in 
the amount of heat acquired by bodies exposed to calorific ra- 
diation, when their surfaces have been successively painted over 
with colouring matter of various degrees of fineness. In this 
case, as in that of the discs of marble, jet or ivory, there is a 
greater or less variation in the regular distribution of the super- 
ficial points and asperities, but without any appreciable alteration 
in the state of hardness or of elasticity. 
When I showed the insufficiency of the received theory as to 
the action of superficial asperities on the radiation of bodies, it 
was objected that irregularities of surface would necessarily 
cause the quantity of heat received through any one given point 
to vary, merely from the circumstance of reflection. As the 
same objection might be raised respecting absorption, we will 
remark, first, that in speaking of asperities in a radiating or ab- 
sorbing surface, we mean only those minute irregularities pro- 
duced in destroying polish; for it is obvious that all very sen- 
sible protuberances or concavities might act as real reflectors, 
and accumulate larger quantities of heat in certain directions. 
Secondly, we must observe that it is not here the question of a 
general law, but of a particular fact. By the striation of certain 
