In these ways the mass of Mercury has been found, with reasonable belief in its accuracy, to be about three one-hundredths that of the earth. Yet there are, indeed, considerable differences regarding it among astronomers. The exact figures are not important to any but the close student. It is certain that the mass of Mercury is very small--so small that the planet probably never had much atmosphere, and almost undoubtedly has none to speak of now. The planet could not hold any molecule moving faster than two and forty-five one-hundredths miles a second, and few gases move as slowly as this. The proportion of light that Mercury reflects to that which he receives also points to a probable scarcity of atmosphere. If he had an atmosphere, it would have clouds. Clouds have a very high reflecting power, giving out about seventy-two per cent. of the light that falls upon them. Mercury reflects only fourteen per cent. of the light he receives, which shows at least a lack of clouds, and something more. It indicates a hard, dark, almost metallic surface, and a very considerable density. Density, however, is the only quality in the possession of which Mercury seems to occupy a middle ground among the planets, being slightly less dense than either Venus, or Mars, or the earth. The earth is the densest of all the planets, and it is about one-third more dense than Mercury. Density is simply the closeness with which the particles composing a body are packed together. A piece of gold, for example, is denser than a piece of iron of the same size. WHAT THE SUN DOES FOR MERCURY It is probable that Mercury has no alternations of light and darkness, causing day and night such as we know them. That is, the planet does not rotate on its axis in such a way as to turn first one side and then the other toward the sun as the earth does. In this, as in some other things, Mercury must accept the fate that overtakes many other small bodies which revolve around large ones--that of our moon, for instance, and the satellites of some of the other planets. Working under the law of gravitation, which gives such power to the large bodies, the sun has so retarded the rotation of Mercury that the planet now makes but one rotation on its axis during one circuit around that central body, and so keeps always the same face toward the sun. Some astronomers do not regard this as having been wholly proved; but all the later observations of Mercury strongly indicate that it is the fact, and it is coming to be more and more regarded as established.