516 



Anniversary Meeting, 



[Nov. 30, 



of inferior pressures. Among these substances may particularly be 

 mentioned, as being constituents of our atmosphere, carbonic acid and 

 aqueous vapour, both of which, but more especially the latter, were 

 found to exert a considerable absorptive action, so that Professor Tyn-. 

 dall concludes that it is to its aqueous vapour that the absorption of 

 radiant heat by our atmosphere is mainly due. He also shows that the 

 gases operated on when heated, radiate heat in an order corresponding 

 to that of their absorption. 



In his second memoir, besides continuing the same subject, he has 

 struck out a new and remarkable method of determining the absorption 

 and radiation of heat by gases and vapours, depending on what he calls 

 dynamic radiation and absorption. In this method the results are 

 arrived at without the use of any extraneous source of heat or cold 

 whatsoever, but simply by the heat or cold produced by the condensa- 

 tion or rarefaction of the gas itself, or of a mixture of the vapour with 

 air or a diathermanous gas. 



It may serve to show the difficulties which beset the inquiry, arising 

 from the interference of disturbing causes, to state that two such expe- 

 rienced physicists as Professor Tyndall and Professor Magnus of Berlin 

 should have arrived at, and long maintained, opposite conclusions re- 

 specting the absorption of radiant heat by air, and the influence of 

 aqueous vapour. This led Professor Tyndall in a third memoir to con- 

 sider more especially the case of aqueous vapour, which he had already 

 treated in his two former papers. The result is that his conclusions 

 have been so confirmed by a system of checks and counterchecks, and 

 by the complete harmony which they present with what we know to be 

 true in other cases, that it seems impossible to doubt their correctness. 



The conclusion that the chief absorbing action of the atmosphere on 

 non-luminous heat is due to the aqueous vapour which it contains, has 

 numerous and important bearings on meteorology, and has been applied 

 by Professor Tyndall to the explanation of some phenomena which 

 appear hitherto to have been imperfectly understood. 



In his fourth memoir, besides various other subjects of investigation, 

 he compares the absorption of heat by vapours with that produced by 

 the same substances in the liquid state, and concludes that for non- 

 luminous radiant heat (in accordance with what we know for light) the 

 general character of the absorption is the same, in whichever of the two 

 states the substance may be found. 



In a fifth memoir, just published, he examines among other things 

 the penetrative power of the heat radiated from various flames, and 

 shows that such heat is absorbed with especial facility by the gases 

 which result from the combustion. 



Professor Tyndall concludes from his researches that, as a general 

 rule, the opacity of a substance with respect to radiant heat from a 

 source of comparatively low temperature increases with the chemical 



