Radiation of Heat by Gases and Vapours. 283 



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 ab- 

 sorptive 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 Dalton'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 chemi- 

 cally united to form ammonia, produce an enormous effect. 

 Thus oxygen and hydrogen, which, when mixed in their elec- 

 trolytic proportions, show a scarcely sensible action, when chemi- 

 cally 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 ab- 

 sorption 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 ab- 

 sorption 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 present broad sides to the ether, while the 

 simple atoms with which we have operated do not, — that in con- 

 sequence of these differences the ether must swell into billows 

 when the former are moved, while it merely trembles into rip- 

 ples 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 consi- 

 deration remains. All the gases and vapours whose deport- 

 ment we have examined are transparent to light ; that is to say, 

 U2 



