As a reservoir of heat and a regulator of temperature, a rather dense atmosphere is a first necessity, in co-operation with the large quantity and wide distribution of water referred to in the last section. The very different character of our south-west from our north-east winds is a good illustration of its power of distributing heat and moisture. This it does owing to the peculiar property it possesses of allowing the sun's rays to pass freely through it to the earth which it warms, but acting like a blanket in preventing the rapid escape of the non-luminous heat so produced. But the heat stored up during the day is given out at night, and thus secures a uniformity of temperature which would not otherwise exist. This effect is strikingly seen at high altitudes, where the temperature becomes lower and lower, till at a not very great elevation, even in the tropics, snow lies on the ground all the year round. This is almost wholly due to the rarity of the air, which, on that account, has not so much capacity for heat. It also allows the heat it acquires to radiate more freely than denser air, so that the nights are much colder. At about 18,000 feet high our atmosphere is exactly half its density at the sea-level. This is considerably higher than the usual snow-line, even under the equator, whence it follows that if our atmosphere was only half its present density it would render the earth unsuitable for the higher forms of animal life. It is not easy to say exactly what would be the result as regards climate; but it seems likely that, except perhaps in limited areas in the tropics, where conditions were very favourable, the whole land-surface would become buried in snow and ice. This appears inevitable, because evaporation from the oceans by direct sun-heat would be more rapid than now; but as the vapour rose in the rare atmosphere it would rapidly become frozen, and snow would fall almost perpetually, although it might not lie permanently on the ground in the equatorial lowlands. It appears certain, therefore, that with half our present bulk of atmosphere life would be hardly possible on the earth on account of lowered temperature alone. And as there would certainly be an added difficulty in the needful supply of oxygen to animals and carbonic acid to plants, it seems highly probable that a reduction of density of even one-fourth might be sufficient to render a large portion of the globe a snow and ice-clad waste, and the remainder liable to such extremes of climate that only low forms of life could have arisen and been permanently maintained.
The Gases of the Atmosphere
Coming now to consider the constituent gases of the atmosphere, there is reason to believe that they form a mixture as nicely balanced in regard to animal and vegetable life as are the density and the temperature. At a first view of the subject we might conclude that oxygen is the one great essential for animal life, and that all else is of little importance. But further consideration shows us that nitrogen, although merely a diluent of the oxygen as regards the respiration of animals, is of the first importance to plants, which obtain it from the ammonia formed in the atmosphere and carried down into the soil by the rain. Although there is only one part of ammonia to a million of air, yet upon this minute proportion the very existence of the animal world depends, because neither animals nor plants can assimilate the free nitrogen of the air into their tissues.
Another fundamentally important gas in the atmosphere is carbonic acid, which forms about four parts in ten thousand parts of air, and, as already stated, is the source from which plants build up the great bulk of their tissues, as well as those protoplasms and proteids so absolutely necessary as food for animals. An important fact to notice here is, that carbonic acid, so essential to plants, and to animals through plants, is yet a poison to animals. When present in much more than the normal quantity, as it often is in cities and in badly ventilated buildings, it becomes highly prejudicial to health; but this is believed to be partly due to the various corporeal emanations and other impurities associated with it. Pure carbonic acid gas to the amount of even one per cent. in otherwise pure air can, it is said, be breathed for a time without bad effects, but anything more than this proportion will soon produce suffocation. It is probable, therefore, that a very much smaller proportion than one per cent., if constantly present, would be dangerous to life; though no doubt, if this had always been the proportion, life might have been developed in adaptation to it. Considering, however, that this poisonous gas is largely given out by the higher animals as a product of respiration, it would evidently be dangerous to the permanence of life if the quantity forming a constant constituent of the atmosphere were much greater than it is.
Aqueous Vapour in the Atmosphere
This water-gas, although it occurs in the atmosphere in largely varying quantities, is yet, in two distinct ways, essential to organic life. It prevents the too rapid loss of moisture from the leaves of plants when exposed to the sun, and it is also absorbed by the upper surface of the leaf and by the young shoots, which thus obtain both water and minute quantities of ammonia when the supply by the roots is insufficient. But it is of even more vital importance in supplying the hydrogen which, when united with the nitrogen of the atmosphere by electrical discharges, produces the ammonia, which is the main source of all the proteids of the plant, which proteids are the very foundation of animal life.
From this brief statement of the purposes served by the various gases forming our atmosphere, we see that they are to some extent antagonistic, and that any considerable increase of one or the other would lead to results that might be injurious either directly or in their ultimate results. And as the elements which constitute the bulk of all living matter possess properties which render them alone suitable for the purpose, we may conclude that the proportions in which they exist in our atmosphere cannot be very widely departed from wherever organic forms are developed.
The Alternation of Day and Night
Although it is difficult to decide positively whether alternations of light and darkness at short intervals are absolutely essential for the development of the various higher forms of life, or whether a world in which light was constant might do as well, yet on the whole it seems probable that day and night are really important factors. All nature is full of rhythmic movements of infinitely varied kinds, degrees, and durations. All the motions and functions of living things are periodic; growth and repair, assimilation and waste, go on alternately. All our organs are subject to fatigue and require rest. All kinds of stimulus must be of short duration or injurious results follow. Hence the advantage of darkness, when the stimuli of light and heat are partially removed, and we welcome 'tired nature's sweet restorer, balmy sleep'—giving rest to all the senses and faculties of body and mind, and endowing us with renewed vigour for another period of activity and enjoyment of life.
Plants as well as animals are invigorated by this nightly repose; and all alike benefit by these longer periods of greater and less amounts of work caused by summer and winter, dry and wet seasons. It is a suggestive fact, that where the influence of heat and light is greatest—within the tropics—the days and nights are of equal length, giving equal periods of activity and rest. But in cold and Arctic regions where, during the short summer, light is nearly perpetual, and all the functions of life, in vegetation especially, go on with extreme rapidity, this is followed by the long rest of winter, with its short days and greatly lengthened periods of darkness.