In the first place the atmosphere must be of a sufficient density, this being needed in order that it may be an adequate storer up of solar heat, and also in order that it may be able to supply sufficient oxygen, water-vapour, and carbonic-acid gas for the requirements of both vegetable and animal life. We have a striking example of the use of air as a storer-up and distributor of heat and moisture in the very different character of our south-west and north-east winds. The effect of the density of the air is equally well shown when we ascend lofty mountains where we find perpetual snow and ice, due simply to the fact that the air is not dense enough to retain the heat of the sun--which is actually greater than at low levels--so that at night the temperature regularly falls below the freezing point. On the other hand a very much denser atmosphere would absorb so much water vapour as probably to shut out the light of the sun, and thus have a prejudicial effect on vegetable life. Again, there is good reason to believe that the proportions of the various gases in the atmosphere are, within certain narrow limits, such as are most favourable not only for the life that actually exists, but for any life that could be developed from the elements that constitute the universe. Oxygen has properties which seem absolutely essential to organic life; but nitrogen, though only serving to dilute the oxygen so far as the higher animals are directly concerned, is yet indirectly essential for them, since it is in vegetables a constituent of that protoplasm which is the very substance of their bodies. [Sidenote: Use of Thunderstorms] [Sidenote: The Wonder of the Atmosphere]