not possess the quality of selective absorption.” This view, which was founded on too wide a use of Newton's law of cooling, must be abandoned, as Langley himself in a later memoir showed that the full Moon, which certainly does not possess any sensible heat-absorbing atmosphere, has a “mean effective temperature” of about 45° C.
The air retains heat (light or dark) in two different ways. On the one hand, the heat suffers a selective diffusion on its passage through the air; on the other hand, some of the atmospheric gases absorb considerable quantities of heat. These two actions are very different. The selective diffusion is extraordinarily great for the ultra-violet rays, and diminishes continuously with increasing wave-length of the light, so that it is insensible for the rays that form the chief part of the radiation from a body of the mean temperature of the Earth.
The selective absorption of the atmosphere is * * * of a wholly different kind. It is not exerted by the chief mass of the air, but in a high degree by aqueous vapor and carbonic acid, which are present in the air in small quantities. * * * The influence of this absorption is comparatively small on the heat from the Sun, but must be of great importance in the transmission of rays from the Earth. * * *
II. The Total Absorption by Atmospheres of Varying Composition.
*********
III. Thermal Equilibrium on the Surface and in the Atmosphere of the Earth.
*********
IV. Calculation of the Variation of Temperature that would ensue in consequence of a given variation of the Carbonic Acid in the Air.
If the quantity of carbonic acid increases in geometric progression, the augmentation of the temperature will increase nearly in arithmetical progression. This rule—which naturally holds good only in the part investigated—will be useful for the following summary estimations.
V. Geological Consequences.
I should certainly not have undertaken these tedious calculations if an extraordinary interest had not been connected with
