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combines with the oxygen of the surrounding air ; and that this function is essentially necessary to their vital existence.
But it is also shown, that the green parts, and the green parts alone, of these plants, when placed in the light, and particularly in the direct rays of the sun, while they are exhaling carbon to form carbonic acid with the surrounding oxygen, are also giving out a portion of pure oxygen ;-that this oxygen seems to be derived from the decomposition of carbonic acid exexisting in the cellular texture of their green parts ;--that this acid appears to be decomposed, by the chemical agency of the solar light, which, penetrating into the cells containing it, resolves it into carbon and oxygen, whereof the former is retained, while the latter escapes through the external pores ;-and, finally, that this process is not a constant and invariable function necessary to life, but an effect, in a manner additional, or subordinate; for a plant does not die when this formation of oxygen has ceased; and it may be found to occur in a dead plant, as well as in one that is alive.
It was this production of oxygen by the green parts of plants exposed to light, or apparently depending on the decomposition of carbonic acid, which gave rise to the opinion, almost universally maintained since the time of Priestley, that by an adınirable provision of nature, vegetables, in all circumstances, were continually employed in purifying the air, which had been deteriorated by the respiration of animals. But it is indeed scarcely possible, as our author has observed, that this opinion could have obtained such general regard, had not physiologists and cheinists obviously satisfied themselves with contemplating, at a distance, the beauty of the Final Cause whose existence it implied, instead of carefully examining the facts by which it had been suggested. Mr Elis seems to have been the first to question their accuracy; and to show, that they were not only, even according to Priestley's own representation, imperfect and contradictory in themselves, but also in direct opposition to the experience of Priestley’s cotemporary, the celebrated Schcele. We have pleasure in quoting the sentiments with which our author has closed his exposition of this very popular, and perhaps not unnatural error. They are written with that modesty and candour, which we wish to consider as virtues inseparable from minds truly intelligent; and we value them the more, that, in physiological discussions, they are comparatively rare in their occurrence.
• We have been drawn into tliese detailed remarks, not from any desire to depreciate Dr Priestley's labours, but from the circumstance of their having first given origin to the opinion, that plants, by their vegetation, at all times purify the air ; and from a consideration of the importance which has ever since been attached to them. In the experimental sciences, it is cliefly by the successive detection of each other's errors, that we gradually advance to truth ; for rarely, indeed, does it happen, that human sagacity can at once foresee and appreciate all the possible circumstances in an experiment, which may influence and control its result. There is, therefore, no cause to wonder, that this illusirious philosopher did not discover those sources of fallacy, which thie more advanced state of science has a. lone enabled his successors to point out. And the retłection, that our apparently more correct views may, at no distant day, undergo a similar revision, ought not only to teach us becoming diffidence in our own opinions, but may serve to check that rising triumph which little minds are sometimes apt to feel, vlien they see thus exposed the mistakes of superior men.' $ 251.
Were the excellent remarks also contained in the following extract, limited in their rekrence merely to the subject under discussion, we should not probably have detained our readers by inserting them; but they are of much more extensive appliciition. There is a class of scientific inquirers, of lite years rather increasing in number, who seem disposed to measure the power of nan by the extent of their own individual exertions; who would damp the noble ardour of philosophical pursuit, and check the progress of discovery, by throwing in our path every obstacle vihich their ingenuity can construct, out of ilie crude and feble materials of vital principles, and final causes, and ultimate facts; and who scarcely scruple to ailirmi, with a conlidence which we are convinced their moie enlarged experience will induce them to regard as unreasonable, that they have already attained the utmost boundary to which the human faculties can reach. Against such narrow views, our author's observations contain a just and Joquent appeal.
• But there have been writers,' says he, who rested their views of the purification of the atmosphere by vegetation, not so much on observation and experiment, as on wiat ihey conceived to be its necessity in the general economy of Nature; and, with more, pero haps, of picty ihan of prudence, and certainly with a “ zeal not according to knowledge,” have represented the cortrary doctrine as derrig.tory to the wisdom of Providence, and a calumny against Nature herself. It is indeed true, and it is among the most gratify. ing truths in the pursuit of science, that every real step which we make in the knowledre of nature, serves to illustrate the skill and wisdom with which all its parts are contrived to advance the general purposes of tlie whole ; but of this whole it should also be recol. lected, that we, as yet, see but in part and as through a glass, darkiy.” Hence imperfect and erroneous views of the order of nature may be often taken, and false conclusions may be grounded on them;
and and if these conclusions be afterwards announced as examples of divine wisdom, and be allowed to borrow the authority of final causes for their support, the history of science abundantly testifies that the vainest conceits of fallible man may, in time, come to be worshipped as the wisest institutions of unerring nature It behoves us, therefore, to employ no ordinary portion of delicacy and caution in pronouncing, on the general plans and purposes of Providence, from the little and partial views of nature, which, at present, we are permitted to take ; lest, in the effervescence of our zeal, we de. grade the wisdom we pretend to exalt, and prevent the designs of the goodness we profess to revere. With respect also to the charge of calumniating nature, he surely who, by assiduous observation of the facts which she offers to his contemplation, seeks to discover the laws of their connexion, and proposes liis opinion of those laws as the simple rezult of his inquiries, may be regarded less as a calume miator, than he, who supplies the imperfection and deficiency in his facts, by the suggestions of imagination ; and confidently imposes upon Nature, laws and conditions, which she utterly disowns and disdains.' y 531, 532.
Since, then, it appears, that plants, as well as animals, are incessantly converting the pure part of the air into carbonic acid, and since it is also very generally known, that, notwithstanding the vast extent of this deteriorating process, the atmosphere siill maintains its uniformity of composition, at all times, and in all places; it is natural to ask, where are we to look for the means of its purification ? To this most interesting and difficult question, our author thinks that, in the present state of our chemical knowledge, no satisfactory answer can be returned.
Lastly, lir Ellis in investigating the source of the oxygen emitted by the green parts of plants when exposed to light, has been led to one of the most beautiful theories, that have been suggested in modern physiology: That the various colours which adorn the vegetable kingdom, depend on the varied proportions of alkaline and acid matter mixed with the juices of the coloured parts of plants: That green and yellow, for example, are always produced by an excess of alkali
, in the colourable juices of the leaf or flower--and all the shades of red by a predominance of acid; and that, in parts where neither acid nor alkali predominate, the colour is white: Just as, in an experiment fanuliar to everyone, we convert an infusion of green leaves into red, by pouring into it a little vinegar-or an intusion of red tlowers into green, by a few drops of potass or ammonia - or destroying the colours of both infusions entirely, by adding the alkali and acid in such proportions, as that ibey shall exactly neutralize each other. But it is well known, that the presence of light is essentially necessary to the production vi colours, in various
plants, plants. Thus, if a plant, which is naturally of a green colour, be made to grow in total darkness, the leaves and other parts, as they unfold themselves, will appear perfectly white, and remain so.
But if we now bring this etiolated plant, as it is calleci, into the light, the blanched foliage, and the young leaves, as they expand from the buds, will soon acquire a yellowish tint, which will gradually deepen into a green; and this colour, after being completely formod, may again be made to disappear, hy returning the plant into a dark place. The etiolation, or bianching of the roots of celery, and of the inner parts of cabbages and lettuces, are far iliar examples of the same kind. In like manner, if red rose trees are carefully secluded from the light, they will produce flowers almost white; or if a portion of a ripening peach or cherry be covered with a piece of tinfoil, the uncovered parts of the surface will become perfectly red, while the covered portion will exhibit only a pale, or straw-colour.
Now, in all these instances, our author has suggested, that the light contributes to the development of the colour, merely by modifying the proportions of alkali or acid matter in the fluids of the part. For it is found, that the blanched leaves of an etiolated plant abound in carbonic acid, and that they not only contain less alkali than green leaves, but that this alkali exists in a more neutralized state: and hence, according to Mr Ellis, their white colour. But as soon as the plant is brought into the sun, the chemical action of the solar beam begins, as it would seem, to decompose the carbonic acid existing in the white parts; the alkali by degrees comes to predominate ; and the colour of the leaves is observed to pass gradually into a full green. Thus, too, a reason may be assigned, why the green parts alone of plants placed in the light have been said to afford oxygen: for, in fact, the einission of oxygen, and the production of the green colour, appear both to depend on the same cause—the decomposition of carbonic acid ; so that we cannot Bo properly affirm that the green parts afford oxygen, as that they become green when that gas is expelled. Again, wlen, in the fall of the year, vegetation begins to decline, alkaline natter seems less abundantly supplied; while pentancous decomposition appears actually to increa-e the quantity of acid in the jcaves : and, according to the various proportions of acid which are developed in the leaves of difierent plants, the foliage exhibits those various shades of brown, or those bright tints of yellow and red, which so beautifully diversify an autumnal scene. On the other land, it would seem, that, when the sunbeans
either entirely produce, or only heighten the red colours of flowers or fruits, they produce these effects by some chemical action, which favours the formation of acid in the juices of the coloured part.
These very novel and ingenious views, so consonant with the u sual simplicity of Nature's operations, are unfolded at great length by our author, in two sections, which will probably appear the most generally interesting in his Inquiry. They are accompanied, also, with an able disquisition on the causes of colour in bodies in general; and on the analogy, in chemical operations, between the two kinds of electricity and the two species of invisible rays in the solar beam-the chemical and calorific. Of the former, we have only to remark, that Mr Eilis has both appreciated with judgment, and contributed to confirm, the valiable though inuch neglected views of Delaval and Bancroft. With respect to the latter, we can scarcely venture to grant to our author, that the analogy in question exists to the extent to which he has endeavoured to trace it. We do not, however, withhold our assent, from conceiving that there is any defect in the reasoning which Mr Ellis has en ployed-for, indeed, if the duta he has assumed be granted, we believe the conclusions he has deduced from them are altogether unobjectionable--but because we strongly suspect that most of the observations on the operation of galvanism in chemical decompositions, which have lately been offered to the Public under the specious names of • general laws,' and statements of fact,' are blended with inpotheses to no inconsiderable extent.
Our author does not seem to have made any observations on the respiration of aquatic plants: but we may presume that, like aquatic animals, they possess the power of producing changes on the air combined with the water in which they grow, similar to those which terrestrial vegetables effect on the air of the ato inosphere.
From all these views, therefore, we are fully disposed to conclude, that the author of the volumes before us has satisfactorily established this important general truth in philosophy,-- That the change which all animals and vegetables are continually producing on the atmosphere, or on the air of the fluids in which they live, consists simply in the conversion of a portion of oxygen into an equal bulk of carbonic acid, by the addition of carbon exhaled from the living organized body.
And now the question will naturally again sugvest itself, why is this exhalation of carbon, and conversion of oxygen into carbonic acid, essentially necessary to the occurrence of that assemblage of phenomena which we express by the terin Life?