All the carbon which is taken into the animal economy passes, in the process of time, again into the atmosphere, in combination with oxygen, this being effected in the body, under the catalytic power of tissues, immediately influenced by the excitation of nervous forces, which are the direct manifestations of vital energy. The quantity of carbonic acid thus given out to the air is capable of calculation, with only a small amount of error. It appears that upwards of fifty ounces of carbonic acid must be given off from the body of a healthy man in twenty-four hours. On the lowest calculation, the population of London must add to the atmosphere daily 4,500,000 pounds of carbonic acid. It must also be remembered that in every process for artificial illumination, and in all the operations of the manufactures in which fire is used, and also in our arrangements to secure domestic comfort, immense quantities of this gas are formed. We may, indeed, fairly estimate the amount, if we ascertain the quantity of wood and coal consumed, of all the carbon which combines with oxygen while burning, and escapes into the air, either as carbonic acid or carbonic oxide. The former gas, the same as that which accumulates in deep wells and in brewers’ vats, is highly destructive to life, producing very distressing symptoms, even when mixed with atmospheric air, in but slight excess over that proportion which it commonly contains. The oppressive atmosphere of crowded rooms is in a great measure due to the increased proportion of carbonic acid given off from the lungs of those assembled, and collected in the almost stagnant air of badly ventilated apartments. It will be evident to every one, that unless some provision was made for removing this deleterious gas from the atmosphere as speedily as it formed, consequences of the most injurious character to the animal races would ensue. It is found, however, that the quantity in the atmosphere is almost constantly about one per cent. The peculiar properties of carbonic acid in part ensure its speedy removal. It is among the heaviest of gaseous bodies, and it is readily absorbed by water; consequently, floating within a short distance from the surface of the earth, a large quantity is dissolved by the waters spread over it. A considerable portion is removed by the vegetable kingdom; indeed, the whole of that produced by animals, and by the processes of combustion, eventually becomes part of the vegetable world, being absorbed with water by the roots, and separated from the air by the peculiar functions of the leaves. However, this heavy gas unites with the lighter atmospheric fluid in obedience to that law which determines the diffusion of different specific gravities through each other.
The leaves of plants may be regarded as performing similar offices to the lungs of animals. They are the breathing organs. In the animal economy a certain quantity of carbon is necessarily retained, in combination with nitrogen and other elements, to form muscle; but this is constantly undergoing change; the entire system being renewed within a comparatively limited period. The conditions with plants are somewhat different. For instance, the carbon is fixed in a tree, and remains as woody fibre until it decays, even though the life of the plant may extend over centuries.
Animals, then, are constantly supplying carbonic acid; plants are as constantly feeding on it; thus is the balance for ever maintained between the two kingdoms. Another condition is, however, required to maintain for the uses of men and animals the necessary supply of oxygen gas. This is effected by one of those wonderful operations of nature’s chemistry which must strike every reflecting mind with admiration. During the night plants absorb carbonic acid; but there is a condition of repose prevailing then in their functions, and consequently their powers of effecting the decomposition of this gas are reduced to their minimum. The plant sleeps, and vital power reposes; its repose being as necessary to the plant as to the animal. With the first gleam of the morning sun the dormant energies of the plant are awakened into full action; it decomposes this carbonic acid, secretes the carbon, to form the rings of wood which constitute so large a part of its structure, and pour out oxygen gas to the air. The plant is, therefore, an essential element in the conditions necessary for the support of animal life.
The animal produces carbonic acid in an exact proportion to the quantity of carbonaceous matter which it consumes. Fruit and herbage contain a small quantity of carbon in comparison with muscle and fat. But let us confine our attention to the human race. Man within the Tropics, where the natural temperature is high, does not require so great an amount of chemical action to go on within him for the purpose of maintaining the requisite animal heat; consequently his Maker has surrounded him with fruits and grains which constitute his food.
As we advance to the colder regions of the earth man becomes a flesh-eater, and his carnivorous appetite increases as the external temperature diminishes. Eventually we reach the coldest zones, and the human being there devours enormous quantities of fat to supply the necessities of his condition.
It must necessarily follow, that the inhabitants of the tropics do not produce so much carbonic acid as those who dwell in colder regions. In the first place, their habits of life are different, and they are not under the necessity of maintaining animal heat by the use of artificial combustion, as are the people of colder climes. The vegetation of the regions of the tropics is much more luxuriant than that of the temperate and arctic zones. Hence an additional supply of carbonic acid is required between the torrid zones, and a less quantity is produced by its animals. These cases are all met by the great aërial movements. A current of warmed air, rich in oxygen, moves from the equator towards the poles, whilst the cooler air, charged with the excess of carbonic acid, sets in a constant stream towards the equator. By this means the most perfect equalization of the atmospheric conditions is preserved.
The carbonic acid poured out from the thousand mouths of our fiery furnaces,—produced during the laborious toil of the hard-working artizan,—and exhaled from every populous town of this our island home,—is borne away by this our aërial currents to find its place in the pines of the Pacific Islands, the spice-trees of the Eastern Archipelago, and the cinchonas of Southern America. The plants of the valley of the Caucasus, and those which flourish amongst the Himalayas, equally with the less luxuriant vegetation of our temperate climes, are directly dependent upon man and the lower animals for their supply of food.
If all plants were removed from the earth, animals could not exist. How would it be if the animal kingdom was annihilated?—would it be possible for vegetation to continue? This question is not quite so easily answered; but, if we suppose all the carbon-producing machines—the animals—to be extinct, from whence would the plants draw their supply? It has been supposed that during the epoch of the coal formation a luxuriant vegetation must have gone on over the earth’s surface, when the existence of animal life was regarded as problematical. It is supposed that the air was then charged with carbonic acid, and that the calamites, lepidodendra, and sigilaria, were employed to remove it, and fit the earth for the oxygen-breathing races. The evidence upon these points is by no means satisfactory; and although at one time quite disposed to acquiesce in a conjecture which appears to account so beautifully for the observed geological phenomena of carboniferous periods, we do not regard the necessities for such a condition of the atmosphere as clearly made out.[223] Geological research, too, has shown that the immense forests from which our coal is formed teemed with life. A frog as large as an ox existed in the swamps, and the existence of insects proves the high order of organic creation at this epoch.
In all probability the same mutual dependence which now exists between the animal and vegetable kingdoms existed from the beginning of time, and will continue to do so under varying circumstances through the countless ages of the earth’s duration.