34 
PEOPESSOE TYNDALL ON THE ABSOEPTION AND 
makes one free molecule a strong absorber, while another offers scarcely any impedi- 
ment to the passage of radiant heat 1 I think the experiments throw some light upon 
this question. If we inspect the results above recorded, we shall find that the 
elementary gases hydrogen, oxygen, nitrogen, and the mixture atmospheric air, possess 
absorptive and radiative powers beyond comparison less than those of the compound 
gases. Uniting the atomic theory with the conception of an ether, this result appears 
to be exactly what ought to be expected. Taking Daltox’s idea of an elementary body 
as a single sphere, and supposing such a sphere to be set in motion in still ether, or 
placed without motion in moving ether, the communication of motion by the atom in 
the first instance, and the acceptance of it in the second, must be less than when a 
number of such atoms are grouped together and move as a system. Thus we see that 
hydrogen and nitrogen, which, when mixed together, produce a small effect, when 
chemically united to form ammonia, produce an enormous effect. Thus oxygen and 
hydrogen, which, when mixed in their electrolytic proportions, show a scarcely sensible 
action, when chemically combined to form aqueous vapour, exert a powerful action. So 
also with oxygen and nitrogen, which, when mixed, as in our atmosphere, both absorb 
and radiate feebly, when united to form oscillating systems, as in nitrous oxide, have 
their powers vastly augmented. Pure atmospheric air, of 5 inches tension, does not 
effect an absorption equivalent to more than the one-fifth of a degree, while nitrous 
oxide of the same tension effects an absorption equivalent to fifty-one such degrees. 
Hence the absorption by nitrous oxide at this tension is about 250 times that of air. 
No fact in chemistry carries the same conviction to my mind that air is a mixture and 
not a compound, as that just cited. In like manner, the absorption by carbonic oxide of 
this tension is nearly 100 times that of oxygen alone; the absorption by carbonic acid 
is about 150 times that of oxygen; while the absorption by olefiant gas of this tension 
is 1000 times that of its constituent hydrogen. Even the enormous action last men- 
tioned is surpassed by the vapours of many of the volatile liquids, in which the atomic 
groups are known to attain their highest degree of complexity. 
I have hitherto limited myself to the consideration that the compound molecules pre- 
sent broad sides to the ether, while the simple atoms with which we have operated do 
not ; that in consequence of these differences the ether must swell into billows when the 
former are moved, while it merely trembles into ripples when the latter are agitated ; that 
in the interception of motion also the former, other things being equal, must be far more 
influential than the latter. But another important consideration remains. All the gases 
and vapours, whose deportment we have examined, are transparent to light ; that is to 
say, the waves of the visible spectrum pass among them without sensible absorption. 
Hence it is plain that their absorptive power depends on the periodicity of the undula- 
tions which strike them. At this point the present inquiry connects itself with the 
experiments of Niepce, the observation of Foucault, the surmises of Angsteom, Stokes, 
and Thomson, and those splendid researches of Kikchhofe and Bunsen, which so 
immeasurably extend our experimental range. By Kiechhoff it has been conclusively 
