1 86 1 .] of Gases on Radiant Heat. 297 



On a fair November day the aqueous vapour in tlie atmosphere 

 produced fifteen times the absorption of the true air itself. It is on 

 rays emanating from a source of comparatively low temperature that 

 this great absorptive energy is exerted ; hence the aqueous vapour of 

 the atmosphere must act powerfully in intercepting terrestrial radia- 

 tion ; its changes in quantity would produce corresponding changes of 

 climate ; subsequent researches must decide whether this vera causa 

 is competent to account for the climatal changes which geologic 

 researches reveal. 



Oxygen obtained from the electrolysis of water exerted four times 

 the absorptive energy of the same substance when caused to pass 

 through iodide of potassium ; the greater action being due to the 

 presence of ozone. 



The radiative power of gases was examined by causing them to pass 

 over a heated sphere of metal, and ascend in a column in front of the 

 thermo-electric pile ; various precautions were taken to secure 

 accuracy in the results. It was found that the order of radiation was 

 exactly that of absorption ; that any atom or molecule which is capable 

 of accepting motion from agitated ether, is capable in precisely the 

 same degree of imparting motion to still ether. Films of gas on sur- 

 faces of polished metal were found to act like coats of varnish. 



The speaker also investigated the physical connection of radiation, 

 absorption, and conduction. In the foregoing experiments free atoms 

 and molecules were dealt with, and upon them individually was fixed 

 the responsibility of the effects ojjserved. These effects are thus 

 detached from considerations of cohesion and aggregation, which sug- 

 gest themselves in the case of liquids and solids. 



The reciprocity of absorption and radiation is a simple mechanical 

 consequence of the theory of an ether. 



But why is one molecule competent to stop or generate a calorific 

 flux so much more powerfully than another ? The experiments prompt 

 the following reply : — The elementary gases which have been examined 

 all exhibit extremely feeble powers both* of absorption and radiation, 

 in comparison with the compound ones. In the former case we have 

 oscillating atoms, in the latter oscillating systems of atoms. Uniting 

 the atomic theory with the conception of an ether, it follows that the 

 compound molecule which furnishes points d'appui to the ether must 

 be capable of accepting and generating motion in a far greater degree 

 than the single atom, which we may figure to our minds as an oscillat- 

 ing sphere. Thus oxygen and hydrogen, which, taken separately, or 

 united mechanically, produce a scarcely sensible effect, when united 

 chemically to form oscillating systems as in aqueous vapour, produce a 

 powerful effect. Thus also, nitrogen and hydrogen, which when 

 separate or mixed,. produce but little action, when combined to form 

 ammonia, produce a great action. So also nitrogen and oxygen, which 

 when mixed as in air, are feeble absorbers and radiators, when united 

 to oscillating systems, as in nitrous oxide, are very powerful in both 

 capacities. Comparing small volumes and equal tensions, the actioa 



