Further, it is apparent that the materials of which Mercury's globe is composed reflect light very feebly. It has been calculated that the planet reflects only 17 per cent. of the light which falls upon it, 83 per cent. being absorbed; and this fact obviously carries with it the conclusion that the atmosphere of this little world cannot be of any great density. For clouds in full sunlight are almost as brilliantly white as new-fallen snow, and if Mercury were surrounded with a heavily cloud-laden atmosphere, he would reflect nearly five times the amount of light which he at present sends out into space. As his orbit falls entirely within that of our own earth, Mercury, like his neighbour Venus, exhibits phases. When nearest to us the planet is 'new,' when furthest from us it is 'full,' while at the stages intermediate between these points it presents an aspect like that of the moon at its first and third quarters. It may thus be seen going through the complete series from a thin crescent up to a completely rounded disc. The smallness of its apparent diameter, and the poor conditions under which it is generally seen, make the observation of these phases by no means so easy as in the case of Venus; yet a small instrument will show them fairly well. Observers seem generally to agree that the surface has a dull rosy tint, and a few faint markings have, by patient observation, been detected upon it (Fig. 20); but these are far beyond the power of small telescopes. Careful attention to them and to the rate of their apparent motion across the disc has led to the remarkable conclusion that Mercury takes as long to rotate upon his axis as he does to complete his annual revolution in his orbit; in other words, that his day and his year are of the same length--namely, eighty-eight of our days. This conclusion, when announced in 1882 by the well-known Italian observer Schiaparelli, was received with considerable hesitation. It has, however, been confirmed by many observers, notably by Lowell at Flagstaff Observatory, Arizona, and is now generally received, though some eminent astronomers still maintain that really nothing is certainly known as to the period of rotation. [Illustration: FIG. 20.--MERCURY AS A MORNING STAR. W. F. DENNING, 10-INCH REFLECTOR.] If the long period be accepted, it follows that Mercury must always turn the same face to the sun--that one of his hemispheres must always be scorching under intense heat, and the other held in the grasp of an unrelenting cold of which we can have no conception. 'The effects of these arrangements upon climate,' says Miss Agnes Clerke, 'must be exceedingly peculiar.... Except in a few favoured localities, the existence of liquid water must be impossible in either hemisphere. Mercurian oceans, could they ever have been formed, should long ago have been boiled off from the hot side, and condensed in "thick-ribbed ice" on the cold side.'