RADIATION AND ABSORPTION. 65 
and a globe at 45° or 50° would be found to have only a mean 
temperature lower by some degrees than zero, its lower strata 
being warmer than the inclosure, and its upper strata much 
colder, following a certain law of decrease which may be caleu- 
lated when the proper data are obtained. 
Our present remarks (supposing the inclosure to be at the 
temperature of melting ice, or rather supposing that the heat 
which reaches the globe is uniformly distributed and equivalent 
in quantity to that which would proceed from such an inclosure 
possessing a maximum emissive power) apply, under the same 
conditions, to an inclosure of any temperature, provided that 
this temperature does not vary from the degrees of heat or cold 
to which the law of cooling may extend. 
Such, in general, are the effects produced by diathermanous 
envelopes from the inequality of the absorbing actions which 
they may exercise upon the different rays of heat which traverse 
them; with respect to the cause of these unequal absorptions, 
Delaroche has demonstrated, on one side, that it depends on the 
sources of heat themselves, and consequently on the peculiar 
nature of the calorific rays; and M. Melloni has shown, on the 
other hand, that it depends also, under certain relations, on the 
nature of the diathermanous substances. 
16. Hitherto it has been admitted that two athermanous sur- 
faces of the same temperature emit identical rays of heat, or at 
least rays of heat which experience always equal absorptions in 
' traversing the same media; but it is perhaps not impossible to 
arrive at the discovery, in this respect, of some differences which 
’ depend either upon the diversity of the emissive powers, or on 
the very nature of the bodies. 
This is an essential point, to which the researches of M. 
Melloni have, without doubt, not failed to call the attention of 
natural philosophers. If these rays, emanating from sources of 
equal temperature, resist all tests, if they preserve their identity 
in traversing the same diathermanous media, it will be impossible 
_ to obtain, in the experiments of the laboratory, any accumula- 
_ tion of heat by the interposition of diathermanous envelopes ; 
since in that case the absorbing powers of these envelopes would 
necessarily be the same upon the rays of the inclosure and on 
those of the globe or of the interior thermometer. 
This impossibility, however, could not affect the consequences 
which we shall proceed to deduce from the formulz with rela- 
VOL. IV. PART XIII. F 
