383 
ArticLe XVII. 
Note on the Reflection of Radiant Heat ; by M. MELton1. 
(Read before the Academy of Sciences of Paris, November 2nd, 1835.) 
From the dnnales de Chimie et de Physique, vol. ux. p. 402. 
A.ruoucu the researches of Leslie and Rumford established the 
fact that the quantities of calorific rays reflected by different bodies 
depend on the nature of those bodies and the polish of their surfaces, 
the proportion of the reflected to the incident heat in each particular 
case remained yet undetermined. The results presented by my experi- 
ments on the immediate transmission of radiant heat enable us to solve 
this question with considerable exactness. 
When the calorific rays fall perpendicularly on the anterior surface 
of a diathermanous plate with parallel faces, they undergo a certain re- 
flection, then penetrate into the interior of the plate, reach its further 
surface, and, after there undergoing a second reflection, issue forth into 
the air, pursuing their original direction. In certain cases there is no 
internal absorption, and consequently the difference between the incident 
heat and that transmitted, will in such cases be the value of the reflec- 
tions produced at both surfaces. The substance in which this fact is 
most distinctly perceived is rock salt. We know that plates of this sub- 
stance, when very pure and well polished, transmit 0°923 of the incident 
heat, whatever may be the thickness of the plates, the nature of the 
rays, or the modifications which these may have previously suffered in 
their passage through other plates. 
Let us suppose, for example, two plates of rock salt, the one measur- 
ing one millimetre*, and the other ten millimetres+ in thickness. Ac- 
cording to what we have just stated, the transmissions of these two plates 
will be equal; and if we imagine the thick plate divided into ten layers, 
each one millimetre thick, the absorbent power of the nine layers that 
succeed the first will be of no appreciable value. Hence we may infer 
that, if the rays suffer any absorption, it must be in the first layer. Let 
us suppose for a moment that this takes place. On this hypothesis, the 
molecules which constitute the first layer of one millimetre in thickness 
will retain all the heat that is not completely transmissible by rock salt, 
and the quantity of heat lost in the passage through either of the two 
plates, that is, 1 — 0-923, or 0-077, will be but the sum of the rays ab- 
sorbed or retained, and of those reflected at the two surfaces. Let the 
* 0°03987 in. + 0:394 in. 
