Experiment has shown that the temperature of the air varies with changes in the amount of carbonic acid and of water vapor that it contains. It has been suggested that in past geologic ages the earth's atmosphere was denser and more heavily charged with vapors than it is at present; yet even then forms of life suited to their environment existed, and from those forms the present inhabitants of our globe have been developed. There are several lines of reasoning which may be followed to the conclusion that Venus, as a life-bearing world, is younger than the earth, and, according to that view, we are at liberty to imagine our beautiful sister planet as now passing through some such period in its history as that at which the earth had arrived in the age of the carboniferous forests, or the age of the gigantic reptiles who ruled both land and sea. But, without making any assumptions as to the phase of evolution which life may have attained on Venus, it is also possible to think that the planet's thick shell of air, with its abundant vapors, may serve as a shield against the excessive solar radiation. Venus is extraordinarily brilliant, its reflective power being greatly in excess of Mercury's, and it has often been suggested that this may be due to the fact that a large share of the sunlight falling upon it is turned back before reaching the planet's surface, being reflected both from the atmosphere itself and from vast layers of clouds. Even when viewed with the most powerful telescopes and in the most favoring circumstances, the features of Venus's surface are difficult to see, and generally extremely difficult. They consist of faint shadowy markings, indefinite in outline, and so close to the limit of visibility that great uncertainty exists not only as to their shape and their precise location upon the planet, but even as to their actual existence. No two observers have represented them exactly alike in drawings of the planet, and, unfortunately, photography is as yet utterly unable to deal with them. Mr. Percival Lowell, in his special studies of Venus in 1896, using a 24-inch telescope of great excellence, in the clear and steady air of Arizona, found delicate spokelike streaks radiating from a rounded spot like a hub, and all of which, in his opinion, were genuine and definite markings on the planet's surface. But others, using larger telescopes, have failed to perceive the shapes and details depicted by Mr. Lowell, and some are disposed to ascribe their appearances to Venus's atmosphere. Mr. Lowell himself noticed that the markings seemed to have a kind of obscuring veil over them.