As the atmosphere contained no aqueous vapor, the sky was always cloudless, and there was neither rain nor snow. The sun, less dazzling and less hot than formerly, shone with the yellowish splendor of a topaz. The color of the sky was sea-green rather than blue. The volume of the atmosphere had diminished considerably. Its oxygen and nitrogen had become in part fixed in metallic combinations, as oxides and nitrides, and its carbonic acid had slowly increased, as vegetation, deprived of water, became more and more rare and absorbed an ever decreasing amount of this gas. But the mass of the earth, owing to the constant fall of meteorites, bolides and uranolites, had increased with time; so that the atmosphere, though considerably less in volume, had retained its density and exerted nearly the same pressure. Strangely enough, the snow and ice had diminished as the earth grew cold; the cause of this low temperature was the absence of water vapor from the atmosphere, which had decreased with the superficial area of the sea. As the water penetrated the interior of the earth and the general level became more uniform, first the depth and then the area of seas had been reduced, the invisible envelope of aqueous vapor had lost its protecting power, and the day came when the return of the heat received from the sun was no longer prevented, it was radiated into space as rapidly as it was received, as if it fell upon a mirror incapable of absorbing its rays. Such was the condition of the earth. The last representatives of the human race had survived all these physical transformations solely by virtue of its genius of invention and power of adaptation. Its last efforts had been directed toward extracting nutritious substances from the air, from subterranean water, and from plants, and replacing the vanished vapor of the air by buildings and roofs of glass. It was necessary at any cost to capture these solar rays and to prevent their radiation into space. It was easy to store up this heat in large quantities, for the sun shone unobscured by any cloud and the day was long—fifty-five hours. For a long time the efforts of architects had been solely directed towards this imprisonment of the sun’s rays and the prevention of their dispersion during the fifty-five hours of the night. They had succeeded in accomplishing this by an ingenious arrangement of glass roofs, superposed one upon the other, and by movable screens. All combustible material had long before been exhausted; and even the hydrogen extracted from water was difficult to obtain. The mean temperature in the open air during the daytime was not very low, not falling below –10°.[5] Notwithstanding the changes which the ages had wrought in vegetable life, no species of plants could exist, even in this equatorial zone. Footnote 5: