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such effect at noon. The same phenomenon is said also to occur in other plants, as Kleinia ficoides, Sempervivum arboreum, &c. This stain in the litmus paper could not have arisen from the presence of carbonic acid, as that gas will not alter blue paper, but it must have been caused by the oxygen inhaled at night. “ If,” says De Candolle, “two plants are exposed, one to darkness and the other to the sun, in close vessels, and in an atmosphere containing a known quantity of carbonic acid, and are removed at the end of twelve hours, we shall find that the first has diminished neither the quantity of oxygen nor of carbonic acid; and that in the second, on the contrary, the quantity of carbonic acid has diminished, while the quantity of free oxygen has increased in the same proportion. Or if we place two similar plants in closed vessels in the sun, the one in a vessel containing no carbonic acid, and the other in air which contains a known quantity of it, we shall find that the air in the first vessel has undergone no change, while that in the second will indicate an increase of oxygen proportioned to the quantity of carbonic acid which has disappeared; and, if the experiment is conducted with sufficient care, we shall discover that the plant in question has gained a proportionable quantity of carbon. Therefore, the carbonic acid which has disappeared has given its oxygen to the air and its carbon to the plant, and this has been produced solely by the action of solar light.”
It is a very curious circumstance, however, that although the direct solar rays are requisite to produce a decomposition of carbonic acid in plants under experiment, yet that the most feeble diffused light of day is sufficient to produce the result more or less in a natural state. Thus we find that plants growing in wells, in rooms partially darkened, in deep forests, on the north side of high walls, and on which not a single ray. of sunlight ever fell, become green, and often perform all their functions, without much apparent inconvenience. Yet De Candolle found the purest daylight, the brightest lamp-light, insufficient to bring about the decomposition of carbonic acid in an obvious manner.
It is not any kind of water in which oxygen will be evolved in the sunshine; neither boiled water, nor distilled water, nor that in which nitrogen, hydrogen, or even oxygen, has been dissolved, will produce the result. But if a small quantity of carbonic acid is dissolved in the water, the green parts, stimulated by the sun, disengage oxygen. Various ingenious means have been contrived to prove this fact, and to show that the quantity of oxygen given out is proportioned to the quantity of carbonic acid decomposed. One of the prettiest experiments is the following, by De Candolle:– He placed in the same cistern two inverted glasses, of which one (A), as well as the cistern itself, was filled with distilled water, and had a plant of Water Mint floating in it; the other glass (B) was filled with carbonic acid. The water of the cistern was protected from the action of the atmosphere by a deep layer of oil. The apparatus was exposed to the sun. The carbonic acid in the glass B diminished daily, as was obvious from the water rising in it; and at the same time there rose to the top of the glass A a quantity of oxygen, sensibly equal to the quantity of carbonic acid absorbed. During the twelve days that the experiment was continued, the Mint plant remained in good health; while, on the contrary, a similar plant, placed under a glass, filled with distilled water only, had disengaged no oxygen, and exhibited manifest signs of decomposition. The same experiment having been tried, only employing oxygen in the place of carbonic acid, no gas was disengaged in the glass that contained the Mint plant.
This is sufficient to show that the green parts of plants exposed to the sun decompose carbonic acid. By others, not less ingenious, it has been ascertained that the carbon which is the result becomes fixed in the plant itself. It has been found that Periwinkles, growing where carbonic acid had access to them, gained carbon; while similar plants, in a situation cut off from the access of carbonic acid, not only gained no carbon, but lost a part of what they previously possessed.
If the green parts of plants are placed in the dark, in a receiver full of atmospheric air, we find that the quantity of oxygen is perceptibly diminished. From this, and many other considerations, we are forced to conclude that oxygen is absorbed by plants at night. This gas does not, however, remain in the system of a plant in an elastic state, for neither the air-pump nor heat will separate it; but it appears to incorporate itself with the tissue, since solar light readily disengages it. The inference therefore is, that it is absorbed at night, and combines with the carbon already existing, forming carbonic acid, and that the latter is decomposed by the sun, as has before been shown.
It has been ascertained from other experiments, that a small quantity of carbonic acid is perpetually evolved by leaves both day and night. Some observations by Burnett, upon this subject, are detailed in the Journal of the Royal Institution, and have led their ingenious author to the opinion, that under the name of respiration two distinct phenomena are confounded; and that while respiration, properly so called, which consists in the extrication of carbonic acid, is incessantly in action, digestion, which is indicated by the decomposition of carbonic acid and extrication of oxygen, takes place exclusively in daylight. “Hence,” he says, “are we not justified in concluding that the production of oxygen, and its converse, the formation of carbonic acid, are the unvarying results of two different functions; viz. this of respiration, that of digestion; and that both are vegetative actions dependent upon vitality? To conclude: the formation of carbonic acid is constant both by day and night, during the life of the vegetable; it is equally carried on whether in sickness or in health; it is essential to its existence for the sustentation of its irritability ; for, if deprived of oxygen, and confined in carbonic acid gas, plants, like animals, quickly die. This function, which is performed chiefly by the leaves and petals, though also in a less degree by the stems and roots, like the respiration of animals, is attended with, and marked by, the conversion of oxygen into carbonic acid; it is the respiration of plants.
“ Again : vegetables, at certain times and under certain circumstances, decompose carbonic acid, and renovate the atmosphere by the restoration of its oxygen; but this occasional restoration is dependent, not upon the respiratory, but the digestive, system: it in part arises from the decomposition of water, but chiefly from the decomposition of carbonic acid, absorbed either in the form of gas or in combination with
water, either by the roots or leaves, or both; and here again the analogy holds good between the functions of respiration and digestion in animals and plants, for to both is carbonic acid deleterious when breathed, and to both is it invigorating to the digestive organs.” (Journal of Royal Institution, new series, vol. i. p. 99.)
As the decomposition of carbonic acid gas is thus evidently an important part of the act of respiration, it might be supposed that to supply a plant with a greater abundance of carbonic acid than the atmosphere will usually yield would be attended with beneficial consequences. To ascertain this point several experiments have been instituted; the most important of which are those of Saussure, who found that, in the sun, an atmosphere of pure carbonic acid gas, or even air containing as much as sixty per cent, was destructive of vegetable life; that fifty per cent was highly prejudicial; and that the doses became gradually less prejudicial as they were diminished. From eight to nine per cent of carbonic acid gas was found more favourable to growth than common air. This, however, was only in the sun: any addition, however small, to the quantity of carbonic acid naturally found in the air, was prejudicial to plants placed in the shade.
The life of a plant seems, then, to consist in a successive diurnal decomposition and recomposition of carbonic acid. By night it vitiates the atmosphere by robbing it of its oxygen, by day it purifies it by restoring it. It is a curious question whether, by this alternation of phenomena, the vegetable kingdom actually leaves the atmosphere in its original state, or whether it purifies it permanently, giving it more oxygen than it deprives it of. Considering the great loss of oxygen produced either by the respiration of animals, or by its combination with various mineral matters, or by other means, it is to be supposed that the atmosphere would in time become so far deprived of its oxygen as to be unfit for the maintenance of animal life, if it were not for some active compensating power. This appears to reside in the vegetable kingdom; for Professor Daubeny, of Oxford, has ascertained, by experiments communicated to the British Association, that plants undoubtedly exercise a purifying influence on the atmosphere. In a letter to myself he expresses himself
66 As the observations of Ellis left it in some doubt whether the balance was in favour of the purifying or the deteriorating influence upon the air which is exercised by plants during different portions of the day and night, I conducted my experiments in such a manner that a plant might be enclosed in a jar for several successive days and nights, whilst the quality of the air was examined at least two or three times a day, and fresh carbonic acid admitted as required. A register being kept of the proportion of oxygen each time the air was examined, as well as of the quantity of carbonic acid introduced, it was invariably found that, so long as the plant continued healthy, the oxygen went on increasing, the diminution by night being more than counterbalanced by the gain during the day. This continued until signs of unhealthiness appeared in the confined plant, when, of course, the oxygen began to decrease.
“ In a perfectly healthy and natural state, it is probable that the purifying influence of a plant is much greater; for when I introduced successively different plants into the same air, at intervals of only a few hours, the amount of oxygen was much more rapidly increased, -in one instance to more than 40 per cent of the whole, instead of 20 as in the air we breathe."
Thus, the vegetable kingdom may be considered as a special provision of nature, to consume that which would render the world uninhabitable by man, and to have been so beautifully contrived that its existence depends upon its perpetual abstraction of that, without the removal of which our own existence could not be maintained.
But although this is true of green plants, it does not appear to be so of Fungi. Marcet has shown from carefully conducted experiments, “ that Mushrooms, vegetating in atmospheric air, produce on that air very different modifications from those of green plants in analogous situations; in fact, that they vitiate the air promptly, either by absorbing its oxygen to form carbonic acid at the expense of the carbon of the vegetable, or by disengaging carbonic acid formed in various ways. That the modifications which the atmosphere experiences when in contact with growing Mushrooms are the