Fourier and Pouillet now thought that the atmosphere of our earth should be endowed with properties resembling those of glass, as regards permeability of heat. Tyndall later proved this assumption to be correct. The chief invisible constituents of the air which participate in this effect are water vapor, which is always found in a certain quantity in the air, and carbonic acid, also ozone and hydrocarbons. These latter occur in such small quantities that no allowance has been made for them so far in the calculations. Of late, however, we have been supplied with very careful observations on the permeability to heat of carbonic acid and of water vapor. With the help of these data I have calculated that if the atmosphere were deprived of all its carbonic acid—of which it contains only 0.03 per cent. by volume—the temperature of the earth’s surface would fall by about 21°. This lowering of the temperature would diminish the amount of water vapor in the atmosphere, and would cause a further almost equally strong fall of temperature. The examples, so far as they go, demonstrate that comparatively unimportant variations in the composition of the air have a very great influence. If the quantity of carbonic acid in the air should sink to one-half its present percentage, the temperature would fall by about 4°; a diminution to one-quarter would reduce the temperature by 8°. On the other hand, any doubling of the percentage of carbon dioxide in the air would raise the temperature of the earth’s surface by 4°; and if the carbon dioxide were increased fourfold, the temperature would rise by 8°. Further, a diminution of the carbonic acid percentage would accentuate the temperature differences between the different portions of the earth, while an increase in this percentage would tend to equalize the temperature.