28 
PEOFESSOE TYNDALL ON THE ABSOEPTION AND 
being due to the cooling of the plate and not to absorption. It is also certain that had 
I used heat from a luminous source, I should have found the absorption of 0‘33 per cent, 
considerably diminished. 
§ 8 . 
I have now to refer briefly to a point of considerable interest as regards the efiect of 
our atmosphere on solar and terrestrial heat. In examining the separate efiects of the 
air, carbonic acid, and aqueous vapour of the atmosphere on the 20th of last November, 
the following results were obtained : — 
Air sent through the system of drying tubes and through the caustic potash tube 
produced an absorption of about 
1 . 
Air direct from the laboratory, containing therefore its carbonic acid* and aqueous 
vapour, produced an absorption of 
15. 
Deducting the effect of the gaseous acids, it was found that the quantity of aqueous 
vapour diffused through the atmosphere on the day in question, produced an absorption 
at least equal to thirteen times that of the atmosphere itself. 
It is my intention to repeat and extend these experiments on a future occasion f; but 
even at present conclusions of great importance may be drawn from them. It is 
exceedingly probable that the absorption of the solar rays by the atmosphere, as esta- 
blished by M. PouiLLET, is mainly due to the watery vapour contained in the air. The 
vast difierence between the temperature of the sun at midday and in the evening, is also 
probably due in the main to that comparatively shallow stratum of aqueous vapour 
which lies close to the earth. At noon the depth of it pierced by the sunbeams is very 
small ; in the evening very great in comparison. 
The intense heat of the sun’s direct rays on high mountains is not, I believe, due to 
his beams having to penetrate only a small depth of air, but to the comparative absence 
of aqueous vapour at those great elevations. 
But this aqueous vapour, which exercises such a destructive action on the obscure 
rays, is comparatively transparent to the rays of light. Hence the differential action, as 
regards the heat coming from the sun to the earth, and that radiated from the earth 
into space, is vastly augmented by the aqueous vapour of the atmosphere. 
De Saussure, Fourier, M. Pouillet, and Mr. Hopkins regard this interception of the 
terrestrial rays as exercising the most important influence on climate. Now if, as the 
above experiments indicate, the chief influence be exercised by the aqueous vapour, every 
variation of this constituent must produce a change of climate. Similar remarks would 
apply to the carbonic acid diffused through the air; while an almost inappreciable 
admixture of any of the hydrocarbon vapours would produce great effects on the terres- 
trial rays and produce corresponding changes of climate. It is not therefore necessary 
* And a portion of sulphurous acid produced by the two gas-lamps used to heat the cubes. 
t The peculiarities of the locality in which this experiment was made render its repetition under other 
circumstances necessary. 
