It is now believed that the amount of atmosphere possessed by a planet is due mainly, perhaps entirely, to the planet’s mass, and its consequent gravitative power. Spectrum-analysis has shown that vast masses of gaseous matter exist in the universe, and it is probable that, in a state of extreme tenuity, these are very widely diffused. Just as meteoric dust is constantly attracted to the earth, and periodically in larger quantities, so are gases, and supposing the aggregations of free gaseous matter to have been distributed with some approach to uniformity, then, as planets grew in size, they would also tend to secure a larger amount of the diffused gases, thus forming deeper atmospheres. The observed facts agree with this view. The largest planets, Jupiter and Saturn, have such a depth of atmosphere as permanently to obscure any solid interior they may possess. The only planet closely approaching the earth in size and density--Venus--has an atmosphere which appears to be loftier than ours, but it may be composed of different gases. Mars, which has only one-ninth the mass of the earth, has a lofty but very tenuous atmosphere, and probably no water, the Polar snows being due probably to the freezing of some dense gas. The climate and physical condition of Mars is, however, still a subject of much controversy, which I hope to discuss in a separate work dealing with the arguments of Professor Lowell [see page 105]. In that volume the reader will find, fully set forth my reasons, on scientific grounds, against the supposed habitability of Mars. [Sidenote: The Earth Selects and Uses Gas] But, besides attracting cosmic masses of gaseous matter to form its atmosphere, there is another equally important function of the mass of a planet--its selective power on the kind of gases it can permanently retain in a free state. The molecules of gases are in a condition of rapid motion in all directions, which explains the elastic force they exhibit. The speed of this motion has been determined for all the chief gases, and also the gravitative force necessary to prevent them from continually escaping into space from the upper limit of the atmosphere. Thus the moon, which has a mass only one-eightieth that of the earth, can retain no free gas whatever on its surface. Mars can retain only the very heavy gases, but neither hydrogen nor water-vapour. The earth, however, has force enough to retain all the gases except hydrogen, which is just beyond its limit; and this may explain why it is that there is no free hydrogen in the atmosphere, although this gas is continually produced in small quantities by submarine volcanoes, is emitted sometimes from fissures in volcanic regions, and is a product of decaying vegetation. Once united with oxygen to form water, it becomes amenable to gravity in the form of invisible aqueous vapour, and is thenceforth a permanent possession for us in its most valuable form.