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the most perfectly opposite. Now Scheele determined that only one of these portions of the atmosphere, or the oxygene as it is called, undergoes any change, and that it is converted into a quantity of carbonic acid exactly equal in volume, so that the whole bulk of the air employed in the experiment remains unaltered. By the discovery of this important fact, this unrivalled chemist laid the foundation of all our knowledge of vegetation ; for succeeding experimentalists have done little more than confirm and extend this great discovery. Ingenhouz, for example, proved, by experiment, that every kind of air which was fatal to animal life, was incapable of supporting vegetable life either in the sunshine or in the shade, and that carbonic acid was produced whether the seeds grew in atmospheric air, or in pure oxygene gas. And the successive experiments of Saussure, Cruickshank, Gough, and others, have all led to the same conclusions, and confirmed the discovery of Scheele.
In experiments, however, of such great delicacy, and in which the results are liable to be deranged or influenced by cir cumstances too minute not to be easily overlooked, a want of uniformity might be expected on some points; and this has been more especially the case, in estimating the relative volumes of the air before and after the experiment. In the first part of his inquiry, Mr. Ellis concluded from a careful induction that, in the experiments in which the whole or the greater part of the oxygene had been converted into carbonic acid, a diminution of volume took place, which he estimated at about 1-10th the bulk of the oxygene. Since that period, however, the very accurate experiments of Messrs. Allen and Pepys, on the conversion of oxygene gas into carbonic acid, have been made known to the public, and on recurring to this question in his second volume, Mr. Ellis expresses his decided conviction that tie oxygene undergoes no change of volume in its conversion into carbonic acid, and that the whole bulk of the air employed in the experiments remains precisely the same after the experiment, as it was originally. One circumstance which has had a tendency to discredit this conclusion, is the spontaneous production of air from seeds, which (from confinement in an impure atmosphere, or one which, from the partial or total consumption of its oxygene has become incapable of supporting their growth,) have passed into a state of spontaneous decomposition and decay. In this case, the seeds give out carbonic acid, and carburetted hydrogene, both of which are supplied by the seed itself, and consequently the total volume of air is increased. An increase of volume, huwever, does not take place under any other circumstances ; and during germination the seed supplies carbon only, while the oxygene is supplied from without,--and the combination of these bodies, it is now known, is not attended with any diminution of bulk. It is remarkable that, though oxygene is the only part of the atmosphere which appears to be necessary to the growth of seeds, yet pure oxygene appears to be less favourable to their growth, than when it is mixed or diluted by soine other kind of air, and in the proportions in which they are found in common atmospheric air ; for any material deviation from that precise point, either of excess or deficiency, is injurious to germination in proportion to its extent, though a small deviation does not produce any perceptible effect. As for the azote which constitutes nearly 4-5ths of the volume of the atmospheric air, it is not at all necessary to the germination of the seed; and it appears to be proved by the experiments of Huber, that hydrogene may be substituted for the nitrogene without varying the result. It is probable that the only requisite condition may be, that the two gases should not act bemically on each other, at any atmospheric temperature. It has been found, too, that some seeds would germinate in an atmosphere containing a much smaller proportion of oxygene than others. Thus radish seeds began to germinate with only 1-32d part of oxygene, while lettuce seeds required 1-6th before their evolution commenced.
As the formation of carbonic acid, by the union of the oxygene of the atmosphere with the carbon of the seed, is one of the most constant and important of the phenomena of germination, particular attention has been given to it. A diversity of opinion has existed as to the probable mode by which their union is effected. The common notion has been that the oxygene is absorbed by, and enters into, the substance of the seed, and that part of it is retained, and the rest given out again in the form of carbonic acid. Mr. Ellis, however considers this opinion as entirely gratuitous, unsupported by any species of satisfactory evidence. He observes, that the structure of the seed, does not fit it in any degree for such a process; and the exact correspondence of the carbonic acid formed, with the quantity of oxygen originally present, renders it highly improbable that any portion of the oxygen can have been absorbed and permanently retained by the seed. It appears besides, from the experiments of Mr. Gough, as well as of Mr. Ellis himself, that seeds do not acquire any increase of weight during germination, which of course they must do, if oxygene was absorbed and retained during the process. Indeed as there is pretty strong evidence that carbon is given out by the seed during its germination, it must lose weight, and it appears in fact from an estimate made by Dr. Thomson, on a comparison of a considerable number of experiments, that in the process of malting, about 2 per cent. of carbon is actually given off. When these facts are connected with the actual increase of weight gained by the oxygene from its conversion into carbonic acid, there remains no doubt either as to the source of the carbonic acid, or that the combination must take place exterior to the seed.
The facts which have been already stated relative to the germination of seeds, apply with little variation to the vegetation of plants. They equally require the presence of moisture, heat or atmospheric air, or inore strictly speaking of oxygene ĝas. The necessity of moisture to vegetation is a fact, obvious even to the most careless observer; but for our knowledge of the quantity of water transpired by plants, we are indebted to our countrymun Dr. Hales. He found by experiment that a common sunflower about three feet high, transpired on an average about two pounds of water per day, in a favourable state of the weather. During rain, or a very humid state of the atmosphere, the transpiration is either diminished greatly, or ceases altogether : it is less too as might be expected during the nightand evergreens transpire much less, than plants which shed their leaves at the approach of winter. The supply of the large quantity of moisture which is thus given depends on the absorbent functions of the roots, and the under surface of the leaves, which are furnished with small points, or apertures fitted for the purpose of absorption. Stiil, however, a certain degree of temperature is necessary to the due performance of this function; for it is found to vary with every variation of temperature, and when it ceases finally at the approach of winter, it is less from a decay in the structure of the leaves, than from the gradual diminution of that warmth, which during the sum – mer clothes the earth with verdure, and diffuses delight and gladness through the animal creation.
The necessity of air to vegetation has been proved in the most decisive manner, by confining plants in the exhausted receiver of an air pump, and by the entire removal of their leaves. In the former case the plant dies speedily, and in the latter, if not destroyed, its vegetation is suspended for the season, and its fruit decays; a similar effect too is produced by coating the leaves with varnish or oil, and hence there is sufficient proof that the plant acts upon the air through the medium of its leaves. When however a plant is confined in a vessel filled with atmospheric air, it is the oxygenous portion only which at the end of the experiment is found to have experienced any change, and this is found as in germination to have been converted into carbonic acid. The azote remains precisely the same in quantity, as well as quality after the experiment, as it was before ; nor does this portion of the atmosphere appear from experiments made with that particular object, to exert any direct agency, whatever, either noxious or salutary upon the plant. With respect to the precise nature and extent of the changes produced by the
vegetation of plants upon the oxygenous part of the atmosphere, the experiments which have been made, have been too numerous, and their results too uniform to leave us under any uncertainty on the subject. The oxygene is invariably converted into carbonia acid, the volume of which is so nearly if not exactly equal to that of the oxygene, as to leave no doubt, that the whole of the oxygene is employed exclusively in this way; and when this change is complete, and no oxygene remains, the plant gradually declines and dies.
These facts prove that the presence of oxygene is essential to vegetation, but they do not carry us further, and explain in what manner the change effected upon the oxygene is brought about. The opinions generally entertained on the subject have varied but little from those already mentioned relative to germination; and the oxygene has been supposed, therefore, to be first absorbed by the plant, and afterwards exhaled in the form of carbonic acid. This view of the subject, however, Mr. Ellis decidedly rejects, as totally inconsistent with the structure of the plant, as far as it is yet known, and unsupported by any direct evidence, or any satisfactory analogy. If therefore the oxygene does not gain admission into the cellular structure of the plant, and is at the same time gradually, converted into carbonic acid, without any increase of volume, when every other source from which it might be derived is strictly excluded, and nothing but the plant and the air are present, it follows irresistibly, that the carbon must be supplied by the living plant, and that its combination with the oxygen, must take place exterior to it. The conversion of oxygen into carbonic acid is therefore the constant result of the vegetation process. But Dr. Priestly having observed in some of his early experiments, that impure air was ameliorated by the vegetation of plants confined in it, an opinion has been long prevalent, that carbunic acid was actually absorbed and decomposed by plants, the carbon being retained, and oxygen given out. Notwithstanding the obvious inconsistency of considering this a constant and necessary function of plants, with the known fact, that during vegetation they constantly convert oxygene into carbonic acid, yet the beauty of the final cause which was thus supposed to be established has attached an importance to it, to which on closer examination it is found not to be entitled. The difficulty of reconciling the views which have hitherto prevailed on this subject, with the general uniformity of the operations of nature, and the want of correspondence in the results obtained by different experimentalists, and even by Dr. Priestly himself, with whom the notion originated, at different periods, have led Mr Ellis to bestow considerable attention on this subject, and to attempt its complete investigation. He has determined that the formation of carbonic acid during vegetation is constant, and takes place as well during the day as the night, and that this function is not confined to any particular part of the plant, but is performed not only by the leaves and stem, but also by the roots, the flowers, and even by the fruit, whether ripe or unripe, in the sunshine as well as in the shade, and even in total darkness. But although the evidence upon which the formation of carbonic acid as a natural and constant effect of vegetation is perfectly conclusive, yet it is equally certain that, under some particular circumstances, plants do supply oxygene, by the decomposition of carbonic acid. This however cannot be considered as a natural function of plants, since its occurrence is occasional and takes place only under the direct influence of the sun's
rays. The mass of facts which Mr. Ellis has collected together on the agency of light on vegetables is extremely curious and interesting, and they place the subject in a very satisfactory point of view. They go to prove that the evolution of oxygene gas is carried on only by the green parts of plants, and only during their exposure to the direct action of the sun's rays; that under such circumstances the plant decomposes carbonic acid, whether in the state of gas, or combined with water ; and that the decomposition is rapid, and proportioned to the clearness of the day, and the favourable situation of the plant in relation to the sun. This decomposition, is not effected, however, either by the power of the plant alone, or by that of the light, but the concurrence of both is requisite, though it is produced as well hy leaves separated from the plant, as by the living plant daily. In the mean time the formation of carbonic acid by plants growing in the sun suffers no interruption, but goes on at least as vigorously during such exposure as under any other circumstance. That the decomposition of the carbonic acid is only an accidental function of the plant, has been proved by Saussure, who found that lime introduced into the vessel with the plant, would absorb the carbonic acid as it was formed, and thus completely prevent its decomposition by the combined action of the plant and the light; which always takes place when the carbonic acid remains in a free state. Whether plants have the power of decomposing carbonic acid in a small degree in the shade is not very susceptible of proof, but Saussure hinks it probable, and there are several facts which hardly admit a satisfactory explanation, on any other hypothesis.
With respect to the precise mode in which the agency of the plant is rendered effective in this decomposition Mr. Ellis considers it most probable that the carbonic acid is previously re