"Let us observe, my friends," continued Barbicane, "that if in the actual state of the moon its long nights and long days created differences of temperature insupportable to organization, it was not so at the historical period of time. The atmosphere enveloped the disc with a fluid mantle; vapour deposited itself in the shape of clouds; this natural screen tempered the ardour of the solar rays, and retained the nocturnal radiation. Light, like heat, can diffuse itself in the air; hence an equality between the influences which no longer exists, now that that atmosphere has almost entirely disappeared. And now I am going to astonish you." "Astonish us?" said Michel Ardan. "I firmly believe that at the period when the moon was inhabited, the nights and days did not last 354 hours!" "And why?" asked Nicholl quickly. "Because most probably then the rotary motion of the moon upon her axis was not equal to her revolution, an equality which presents each part of her disc during fifteen days to the action of the solar rays." "Granted," replied Nicholl, "but why should not these two motions have been equal, as they are really so?" "Because that equality has only been determined by terrestrial attraction. And who can say that this attraction was powerful enough to alter the motion of the moon at that period when the earth was still fluid?" "Just so," replied Nicholl; "and who can say that the moon has always been a satellite of the earth?" "And who can say," exclaimed Michel Ardan, "that the moon did not exist before the earth?" Their imaginations carried them away into an indefinite field of hypothesis. Barbicane sought to restrain them. "Those speculations are too high," said he; "problems utterly insoluble. Do not let us enter upon them. Let us only admit the insufficiency of the primordial attraction; and then by the inequality of the two motions of rotation and revolution, the days and nights could have succeeded each other on the moon as they succeed each other on the earth. Besides, even without these conditions, life was possible." "And so," asked Michel Ardan, "humanity has disappeared from the moon?" "Yes," replied Barbicane, "after having doubtless remained persistently for millions of centuries; by degrees the atmosphere becoming rarefied, the disc became uninhabitable, as the terrestrial globe will one day become by cooling." "By cooling?" "Certainly," replied Barbicane; "as the internal fires became extinguished, and the incandescent matter concentrated itself, the lunar crust cooled. By degrees the consequences of these phenomena showed themselves in the disappearance of organized beings, and by the disappearance of vegetation. Soon the atmosphere was rarefied, probably withdrawn by terrestrial attraction; then aerial departure of respirable air, and disappearance of water by means of evaporation. At this period the moon becoming uninhabitable, was no longer inhabited. It was a dead world, such as we see it to-day."