94 Transactions of the South African Philosophical Society. 



Some very elaborate theories have been constructed upon a basis 

 of the absorption of heat by aqueous vapour. Tyndall and others have 

 thought that the energy of terrestrial radiation is determined almost 

 entirely by the quantity of transparent aqueous vapour in the air. 

 " The presence of the vapour checks the loss, while its removal 

 favours radiation and promotes the nocturnal chill." Tyndall also 

 was of opinion that the same radiation was largely responsible for 

 the heavy rainfall of tropical regions, and he added : " The aqueous 

 vapour which absorbs heat thus greedily, radiates it copiously ; and 

 this fact must come powerfully into play in the tropics. We know 

 that the sun raises from the equatorial ocean enormous quantities of 

 vapour, and that immediately under him, in the region of calms, 

 the rain, due to the condensation of the vapour, descends in deluges. 

 Hitherto this has been ascribed to the chilling that accompanies the 

 expansion of the ascending air ; and no doubt this, as a true cause, 

 must produce its proportionate effect. But the radiation from the 

 vapour itself must also be influential. When a column of saturated 

 air ascends from the equatorial ocean, the radiation from it is for 

 some time intercepted, and in great part returned to it, by the 

 surrounding vapour. But the quantity of vapour in the atmosphere 

 diminishes rapidly as we ascend; the decrement of vapour tension, 

 as proved by Hooker, Strachey, and Welsh, is much more speedy 

 than that of the air itself ; and, finally, our vaporous column finds 

 itself elevated beyond the protecting screen which, during the first 

 portion of its ascent, was spread above it. It is now in the 

 presence of pure space, and into space it pours its heat without 

 stoppage or requital. To the loss of heat thus endured, the con- 

 densation of the vapour, and its torrential descent, must certainly 

 be in part ascribed." * 



An explanation running upon similar lines was the carbonic acid 

 theory of S. Arrhenius, invoked primarily to explain the Great Ice 

 Age. On account of its historical importance, I venture to quote 

 a passage from a most enthusiastic account of it by Dr. Nils 

 Ekholm : — 



" Among all the numerous hypotheses imagined in order to explain 

 the great climatic changes of the geological ages, that worked out by 

 S. Arrhenius on the ground gradually laid by Fourier, Pouillet, 



o 



Tyndall, Langley, Knut Angstrom, Paschen, and others, is the only 

 one which has stood the test of a scientific examination. It is 

 founded on the fact that carbonic acid, though as transparent as 

 pure air to the solar rays, is partly opaque to the heat radiating from 



* J. Tyndall, " Heat a Mode of Motion," 1880, p. 383. Also by the same author, 

 " On the Relation of Radiant Heat to Aqueous Vapour," Phil. Trans., 1863. 



