Now, plants obtain their nitrogen mainly from the minute proportion of ammonia that exists in the atmosphere, and this ammonia is formed by the union of the nitrogen of the air with the hydrogen of the water-vapour under the influence of electric discharges--that is, of thunderstorms. It is evident, then, that the required amount of this essential compound will depend upon a due adjustment of the quantities of nitrogen and aqueous vapour always present; while the electric discharges seem to be due to the friction of various strata of air with each other and with the earth’s surface, due to the winds and storms; and winds are due to highly complex causes, involving the rate of the earth’s rotation, the rise and fall of the tide, the density of the atmosphere, the quantity of its aqueous vapour, and the amount of solar heat which it receives. Unless all these very diverse factors existed in their due proportion, some of the results might be highly prejudicial if not quite inimical to the development of life. To these various adaptations of our gaseous envelope we must add one other. Carbonic acid gas in the atmosphere is absolutely essential to vegetable life, while it is directly antagonistic to that of the higher animals. Its quantity must, therefore, be strictly proportionate to the needs of both; and that beneficial proportion must have been preserved throughout the whole period of the existence of the higher air-breathing animals. These various considerations show us that our atmosphere, consisting as it does mainly of two common gases mixed together, and therefore seeming to most people one of the simplest things possible, is really a wonderfully complex arrangement which is adapted to serve the purposes of living organisms in a great variety of ways. But this by no means exhausts the subject of its adaptation to support and develop organic life, because its very existence on the earth in a suitable quantity and composed of the essential elements can be shown to depend on other and deeper relations which will now be pointed out. The older writers on the subject of the habitability of the planets took no account whatever of the importance of size, distance from the sun, period of rotation, and obliquity of the ecliptic as determining the possibility of organic life, but simply assumed that, because the earth possessed an abundant life-development, all the other planets must also possess it. But we know that the above-mentioned factors are of very high importance, as we will proceed briefly to point out. [Sidenote: Earth’s Envelope of Gas]