As the gases held by water in solution sup ply the means of aquatic respiration to many animals and plants, a knowledge of the quan tity and composition of these gases is also necessary for the full comprehension of the function of respiration. Humboldt and Gay Lussac state that the water of rivers, and dis tilled water well aired, hold in solution about 1-13.th of their volume of air composed of about 32 of oxygen and 68 of azote, by volume.f Morren t concludes from his experiments that sea-water contains in solution between Ath and .,16th of its volume of air, a quantity sen sibly less than that obtained from fresh-water, which usually contains from .5A,-,th to or even rkath of its volume.§ He found that the air obtained from fresh-water under ordinary circumstances, whether distilled and -again perfectly aerated, or the limpid water of a moderately rapid stream, contains 32 parts of oxygen, and from 2 to 4 of carbonic acid, by volurne, in the 100 ; while the air obtained from sea-water yielded 33 of oxygen and from 9 to 10 of carbonic acid in the 100. The relative proportion of the gases obtained both from fresh and sea-water varies considerably under certain conditions. In fresh-water ponds abounding in plants or green animalculre, and in shallow parts of the sea, where numerous algm flourish, the proportion of oxygen gas may be considerably increased during sun shine, especially if the water be at the same time still. Morren analyzed, in a bright day in July, the gas dissolved in the water of a fish-pond of a green colour, chiefly from the numerous animalculm it contained, and found in that procured in the morning 25, at mid-day 48, and in the evening as much as 61 of oxygen in the 100 parts.* Similar changes, but to a less extent, were detected by Morren in the air of sea-water, and they are chiefly dependant upon the action of the algw. In one experiment, performed on a fine sunny day, when the sea was at the same time calm, the air obtained from the water yielded 40 per cent. of oxygen in the early part of the day-, and 53.6 in the evening. The total quantity of air obtained from both kinds of water varied at different times of the day ; and its increase was chiefly dependant upon the addition of oxygen, the carbonic acid at the same time suffering a decrease, but not in the same pro portion, while the nitrogen f seemed to suffer little change. This increase of oxygen will partly contribute to the supply required for the respiration of the numerous aquatic ani mals which usually frequent the localities where it is evolved, and be partly given off to the superincumbent air, and thus assist in maintaining the purity of the atmosphere.
Notwithstanding the large quantity of oxy gen daily removed from the atmosphere by the respiration of animals and other causes, yet from the great extent of the atmosphere, and the rapid mixture of its different parts, a long period of time must necessarily elapse before it suffers any marked deterioration, even were there no compensating operation in the vege table kingdom. The oxygen gas in the atmo sphere is equal in weight to a column of 7.8 feet of water pressing upon every part of the earth's surface : and it has been stated that it would require 10,000 years, supposing the earth peopled with 1,000,000,000 of men to produce a perceptible effect upon the eudio meter of Volta, even though vegetable life was annihilated ; and that to suppose all the animals on the surface of the earth could by their respiration deteriorate the air to the ex tent of removing in a century the 8000th part of the oxygen in the atmosphere, is to make a supposition very much beyond the truth.* Respiration of plants.— The results of the chemical actions between the atmospheric air and the vegetable kingdom, are chiefly influ enced by the presence or absence of light, and the condition of the plants at the time. When a plant is surrounded by the ordinary atmospheric air, and exposed to the sunshine, the green parts of the plant, and especially the leaves, decompose the carbonic acid contained in the atmosphere, seize upon the carbon, and liberate the oxygen ; while the same plant in the dark, not only ceases to decompose car bonic acid, but actually exhales into the sur rounding atmosphere a portion of this gas.
A quantity- of nitrogen gas is also given off by plants along with the oxygen.t Plants, therefore, during exposure to light, purify the air by removing carbonic acid and adding oxygen, while during the night they, like animals, deteriorate the air by exhaling car bonic acid gas. As, however, the quantity of oxygen gas liberated during the day from the decomposed carbonic acid is more than suffi cient to counterbalance the quantity of car bonic acid formed during the night, plants on the whole must counteract, either entirely or in part, the accumulation in the atmosphere of the carbonic acid gas formed by the respi ration of animals, and in various chemical processes going on at the earth's surface. Indeed, nearly the whole of the carbon which enters so largely into the formation of the ve getable tissues, appears to be obtained through the decomposition of the carbonic acid of the atmosphere.
The parts of a plant which are not of a green colour, such as the roots, &c., absorb oxygen from the atinosphere, and give out carbonic acid gas even in the sunshine ; and this process seems essential to the vigorous growth of the plant. The flowers of a plant also absorb oxygen, and exhale carbonic acid, and the quantity of the latter gas evolved during inflorescence is considerable. The seeds of plants during germination also absorb oxygen and give out carbonic acid.t. The Fungi evolve carbonic acid gas in large quan tity from all parts of their structure, and at all periods of their growth, even when exposed to a bright sunshine, and these plants derive their supply of carbon from the soil in which they grow.* It is also maintained that a quantity of oxygen is absorbed by the surface of plants during spring and summer, to assist in the elaboration of their acids, resins, and volatile oils. We thus perceive that the che mical actions between the atmospheric air and plants are varied, and differ in some important respects from those that occur in animals. Attempts have been made to show that the respiratory function is essentially the same in these two great divisions of the or ganic kingdom ; that the fixation of carbon and the liberation of the oxygen gas by the leaves, and other green parts of plants during their exposure to the rays of the sun, form a part of their digestive process; while the evo lution of carbonic acid, which proceeds during the day as well as during the night, from seeds during germination, from the flowers, from the surfaces not coloured green, and also, it is asserted, partly from the leaves, is their true respiratory process.t According to others, if the actions of the juices upon the atmospheric air, by which they are changed from the crude to the fully elaborated sap, and rendered fit for the nutrition of the plant, constitute the function of respiration, then the green surfaces, and especially the leaves, are the true respiratory organs of plants.* Besides, it has been alleged that the evolution of the carbonic acid gas from the leaves during the night is not attended by an absorption of oxygen, as in the respiration of animals ; that it is a mechanical process, having no con nexion with the nutrition of the plant ; and that it depends upon the carbonic acid ab sorbed along with the water by the roots and leaves, escaping into the air along with the water evaporated during the periods when the plant, as in the absence of sunshine, is incapable of fixing the carbon.t As the re spiratory process in animals forms a part of the great Nutritive Function, for preparing, elaborating, and assimilating the nutritious juices, and as the two functions performed respectively by the digestive and respiratory organs in the higher animals are not definitely separated in the vegetable kingdom, we can readily understand that the same structures in the vegetable kingdom which carry on the process of respiration, may also at the same time assist in the performance of other parts of the nutritive function.