EADIATION OF HEAT BY GASES AND VAPOTJES. 
33 
becomes a temporary source of radiant heat, and diminishes the deflection produced in 
the first instance by its presence. The reverse occurs when the tube is exhausted ; the 
vapour’ is chilled, its great absorptive action on the heat radiated from the adjacent face 
of the pile comes more into play, and the original efiect is augmented. In both cases, 
however, the action is transient ; the vapour soon loses the heat communicated to it, 
and soon gains the heat which it has lost, and matters then take their normal course. 
§ 10. On the Physical Connexion of Radiation, Absorption, and Conduction. 
Notwithstanding the great accessions of late years to our knowledge of the nature of 
heat, we are as yet, I believe, quite ignorant of the atomic conditions on which radia- 
tion, absorption, and conduction depend. What are the specific qualities which cause 
one body to radiate copiously and another feebly"? Why, on theoretic grounds, must 
the equivalence of radiation and absorption exist? Why should a highly diatherma- 
nous body, as shown by Mr. Balfoue Stewaet, be a bad radiator, and an adiatherma- 
nous body a good radiator ? How is heat conducted ? and what is the strict physical 
meaning of good conduction and bad conduction? Why should good conductors be, 
in general, bad radiators, and bad conductors good radiators ? These, and other questions, 
referring to facts more or less estabhshed, have still to. receive their complete answers. 
It is less with a hope of furnishing such than of shadowing forth the possibility of 
uniting these various effects by a common bond, that I submit the following reflections 
to the notice of the Eoyal Society. 
In the experiments recorded in the foregoing pages, we have dealt with free atoms, 
both simple and compound, and it has been found that in all cases absorption takes 
place. The meaning of this, according to the dynamical theory of heat, is that no atom 
is capable of existing in vibrating ether without accepting a portion of its motion. We 
may, if we wish, imagine a certain roughness of the surface of the atoms which enables 
the ether to bite them and carry the atom along with it. But no matter what the 
quality may be which enables any atom to accept motion from the agitated ether, the 
same quality must enable it to impart motion to still ether when it is plunged in the 
latter and agitated. It is only necessary to imagine the case of a body immersed in 
water to see that this must be the case. There is a polarity here as rigid as that of 
magnetism. From the existence of absorption, we may on theoretic grounds infallibly 
infer a capacity for radiation ; from the existence of radiation, we may with equal cer- 
tainty infer a capacity for absorption ; and each of them must be regarded as the 
measure of the other*. 
This reasoning, founded simply on the mechanical relations of the ether and the 
atoms inmersed in it, is completely verified by experiment. Great differences have 
been shown to exist among gases as to their powers of absorption, and precisely similar 
differences as regards their powers of radiation. But what specific property is it which 
* This was written long before KiECHiiorr’s admirable papers on the relation of emission to absorption 
were known to me. 
MDCCCLXI. 
F 
