POLISHED AND OF STRIATED METALLIC PLATES. 419 
pose the softer parts of the interior, and allow the remaining com- 
pressed superficial layers, of which the particles were previously 
restrained by their mutual compression, to expand and dilate 
themselves in the solutions of continuity which have been made 
on the surface. 
The plate of gold or of silver which has been fused and 
slowly solidified, and has received a slight polish, loses on the 
contrary in absorbing power when it is striated; because the 
point of the diamond compresses a portion of the soft metallic 
surface, and thus imparts to it a greater degree of hardness. 
The influence which the degree of hardness or elasticity of 
metallic plates exerts on the calorific absorption, is especially 
evident from the following fact related to me by M. Saigey, and 
confirmed by M. Obelliane, Préparateur de Physique to the 
‘Ecole Polytechnique and to the Faculté des Sciences at Paris. 
Dulong had two large conjugate mirrors constructed of cast 
metal perfectly tempered, and on experimenting with this ap- 
paratus he was greatly astonished at finding it less effective 
_than another much smaller pair of mirrors which had been 
worked by the hammer, and had long been among the instru- 
ments of the latter establishment. There was at that time no 
means of accounting for this singular anomaly, which was merely 
suspected to result from a difference in the quality of the copper 
used in the manufacture of the two apparatus. Every one can 
now at once perceive it to be an immediate consequence of our 
principles. The cast mirrors had necessarily been less com- 
pressed, and were therefore less hard and elastic than the mir- 
rors that had been prepared with the hammer ; they would con- 
sequently absorb much more heat, and give a feebler reflection. 
Thus, to secure good calorific reflectors, it is not merely suffi- 
cient to polish their surfaces, but the metallic plate from which 
they are formed must be powerfully compressed, so as to com- 
municate at the same time to the metal a regular surface, the 
highest possible polish, and a great degree of hardness and 
elasticity *. 
* The great influence which the hardness or the elasticity of the superficial 
layers seems to exercise on the calorific reflection of metals, an influence far 
more decidedly marked than in the analogous case of light, is undoubtedly 
osely connected with the nature of heat itself. And it is much to be wished 
that this latter should become the subject of a profound investigation from those 
mathematicians, who now study, under all possible forms, the vibratory mo- 
tions of the fluid whence the phenomena of light, heat, and chemical action, 
manifested by the rays of incandescent bodies, are conjectured to be derived. 
