« AnteriorContinuar »
ceived into the vessels of the plant either in a state of combination with water, or some other substance, or even by its entering directly into its gaseous form. That it may obtain admission into the plant in some or all of these modes is proved by a reference to facts which are quite conclusive, and it seems difficult to conceive by what means its decomposition should be effected by the joint agency of the plant and light except it was actually contained in the cellular structure of the plant. And indeed the number of facts which Mr. Ellis has brought forward to prove that gaseous fluids obtain admission into plants, both in a state of combination with water and in an elastic form, establish the fact beyond the possibility of doubt. We have, in fact all the evidence which the subject adınits, to prove that the decomposition is effected by the solar rays penetrating the parenchymatous substance of the leaves, and setting the oxygene free, while the carbon is retained in the cellular structure of the leaf. The dependence of this effect upon the cellular structure of the leaf is farther proved from hence,—that so long as this structure remains entire, although the leaves are not only. detached from the plant, but even cut into shreds, the process still goes on; but when the structure of the leaf is broken down by being beaten into a pulp it ceases altogether. These facts also tend to confirm the conclusion that this decomposition of carbonic acid is independent of vegetation, and it may be observed also that Lenebier found that the presence of heat was not necessary to it, and that oxygene was supplied even when the temperature of the air was many degrees below zero. We have already observed that this singular office is confined to the green portion of plants alone: for all the other parts including even the flowers form carbonic acid under all circumstances, yielding no oxygene under the direct influence of the solar rays; and if even the green parts are deprived of their colour by etiolation, an effect which is always produced by the exclusion of light, they also cease to afford oxygene when exposed to the solar rays, or yield it very impure. There is, therefore, an evident connection between the colour of the plant, and its power of decomposing carbonic acid.
The prosecution of this subject has led Mr. Ellis to inquire into cause of the infinite variety of colours in plants, and it is but justice to him to say that he has conducted the inquiry with singular ingenuity and success, and proposed a theory of their formation remarkably simple and beautiful, as well as far more satisfactory than any which has yet been suggested.
The phenomena which attend the etiolation of a plant, as well as the subsequent restoration of its colour by exposure to
light, prove that the cause of this change is perfectly local in its action, since the interposition of any opaque substanca, as a piece of tin foil, on any part of the leaf will prevent its action on that part, while the remaining portion of the leaf, will pass through the regular changes of colour, or etiolation without any interruption. The change of colour cannot, therefore, depend on any living function of the plant, but must depend on some other cause, and most probably of a chemical nature; and this opinion is still further confirmed from the fact that' etiolated leaves will regain their colour when separated from the plant and immersed in water, provided they are fully exposed to the solar rays. Now it is well known to every one conversant with the most ordinary operations of ehemistry, that the coloured infusions of vegetables may have their colour changed at pleasure almost to any shades of green or red, by the addition of acid or alkaline matter, or that it may be discharged altogether, or nearly so, by these agents being made to neutralize each other, Precisely the same thing Mr. Ellis conceives takes place in the living plant, and is productive of corresponding effects. If the alcali predominates the colour will be green or some of its shades, if the acid is in excess, it will be red or some of its varieties; but if the acid and alcali mutually saturate each other, then the plant will be white,-or colourless. This is exactly what is found to occur during etiolation, the carbonic acid of the plant being no longer decomposed by the solar rays, its alcali is completely saturated by the acid, and it consequently loses its green colour and becomes white; but when the light is readmitted, the carbonic acid is again decomposed as usual, and the green colour is restored, as the alcali becomes uncom- ( bined, and free to exert its specific action.,
This is a very brief statement of Mr. Ellis's theory, the operation of which he has traced with a minuteness of detail, and confirmed by a variety of illustration, and a combination of facts, which make this one of the most pleasing portions of his work, to general readers. The admirable simplicity of this theory strongly recommends its adoption, from its accerdance with our knowledge of many of the operations of nature, in which the most extraordinary eilecis, are often brought about by the most natural and inartificial means. But it is moreover in perfect harmony with all the facts hitherto observed in relation to the subject, though the true explanation of them appears first to have occurred to Mr. Ellis. It would far exceed our limits to abstract the facts and analogies which Mr. Ellis has adduced in support of his theory. We may just observe that the green part, or the leaves of plants are found to contain a much larger proportion of alcaline matter, than any other
portion of their structure, and in a more free states that acids and alcalis are found to act not only upon the colourable juices of the leaves, but also upon the whole of their substance; that an etiolated leaf contains much less alcali than a green one of the same plant, and in a state of more complete saturation, or even supersaturation in the carbonic acid; and that its alcali again predominates as the green colour is restored. It is therefore to the agency of the solar rays in decomposing the juices of plants, that the infinite variety of colours with which the vegetable world is adorned is primarily to be attributed, By their agency,
the carbonic acid in the cellular structure of the leaves is decomposed, and the alcaline matter which appears to be abundantly supplied to every vegetable is enabled to produce its proper effect on the colourable juices of the plant, But when this acid predominates, either naturally, or from the declining state of vegetation supplying alcaline matter less abundantly (as happens on the approach of winter,), then the various tints of yellow and red come to prevail ; so that to the operation of these very limited means, modified and adjusted by the most delicate operations of natural chemistry, all the rich verdure of suinmer and the mellow tints of autumų are exclųsively owing:
After having thus traced the various colours exhibited by plants to the variations in their chemical constitution produced by the solar rays, Mr. Ellis has introduced a disquisition of considerable extent on the physical and chemical agency of light in promoting the coloration of plants, or in other words on the mode by which these effects of light are produced. The object of this essay is to point out the insufficiency of the theories of colorification hitherto proposed; to explain all the phenomena ; and to give more complete developement to a view of the subject proposed by Dr. Bancroft, which attrihutes the permanent colours of bodies to a degree of affinity, or elective attraction, between the particles of the coloured body, and the rays of light which are absorbed and made to disappear, while the remaining rays are reflected or transmitted. And, further, as experiments have proved that in the solar spectrum there are not only coloured or luminous rays, but also two species of invisible rays, differing in their properties, and exhibiting the maximum. of their respective powers at the opposite extremities of the spectrum, and beyond the limits of the luminous rays, Mr. Ellis has attempted to trace an analogy between these effects, and the agency of galvanic electricity as it is exhibited at the two opposite poles. As the positive wire of the galvanic battery attracting oxygene, and occasioning its combination with the wire, repels the hydrogene, disengaged in the decomposition of water, while the negative vire attracts hydrogen, and repele Vol. X.
oxygene without effecting any new combination ; so in the solar spectrum the maximum of power in producing chemical combination and sensible heat exists beyond the extremity of the red or least refrangible rays, while the maximum of power in effecting chemical decomposition, exists at the opposite extremity beyond the limits of the violet rays, and without the power of altering the thermometer in any sensible degree. The resemblance between the extremities of the spectrum, and the positive and negative poles of the battery in their chemical powers, is sufficiently striking, and Mr. Ellis has traced it to a considerable extent : but in a subject so replete with difficulty from the very subtile nature of the agents, and of which our knowledge is probably extremely imperfect, he has cautiously forborne to infer an identity of nature, where the facts go only to establish a resemblance of effects. Since however it appears to be clearly proved, that plants subjected to the action of galvanic electricity have the same changes produced in the chemical composition of their juices, as is effected by the agency of light, and with similar phenomena, he bas very fairly inferred, that from the similarity of their effects, light and electricity must exert a common operation, on the saline compounds contained in the juices of plants.
Having thus by series of the most satisfactory and conclusive evidence demonstrated, that plants and seeds, in every period of their growth and during all seasons, and as well during the day as the night, in sunshine'as well as in the shade, deteriorate the atmosphere by converting its oxygen into carbonic acid, Mr. Ellis, before finally quitting this part of his inquiry, stops to consider the commonly received opinions relative to the purification of the atmosphere, by the functions of vegetable growth, but of which his own admirable researches have pointed out the insufficiency, by proving the evolution of oxygene to be only a 'subordinate and occasional function of vegetable structure. His observations on this interesting subject are so excellent and judicious, and manifest so much of that sobriety and seriousness of reflection, which inquiry into the hidden, and to us inscrutable operations of nature are calculatert to inspire, when properly pursued, that we cannot bet-, ter conclude this first portion of our 'analysis, than by presenting our readers with the following extract.
• But if the operation of vegetables in purifying our atmosphere be, even under the most favcurable circunstances, merely negative, and if, upon the whole, they must like animals be considered greatly to deprave it, where, it may be asked, are we to look for those causes of puritication, by the operation of which the uniformity of compo sition in the atmosphere is, at all times, and in all situations main. gained? To this most interesting but difficult question, no satisface
tory answer can we think, be returned, in the present state of chee mical kņowledge. The modes in which the atmosphere is depraved by the living functions of animals and vegetables, by cornbustion, and by various other processes, in which its oxygen is withdrawn, and made to enter into new combinations, are pretty well known, and to a certain extent may be appreciated with tolerable accuracy; but the various measures by which this oxygen is released from its combinations, in the diversified mode of decomposition which are per. petually taking place, have been much less regarded, and cannot therefore, with equal accuracy be traced ; until this department of chemistry attain to greater perfection, it is therefore, impossible to present a tolerably accurate view of this subject. We may however, be certain of the general fact, that, as oxygen is withdrawn from our atmosphere, in order to enter into new combinations, so it can be again restored to it only by decompositions which shall set it free; and these decompositions must be as numerous, and to an extent as great, as the combinations to which they succeed. To follow however, this circle of actions through all its round, may demand the persevering industry of ages; and it is only when this shall be accomplished, that chemistry will have advanced our knowledge of the indivi. dual relations of our globe, in a degree corresponding with that to which physical astronomy has carried its generale onnection with the universe.
6. But there liave been writers, who rested their views of the puri. fication of the atmosphere by vegetation, not so much on observation and experience, as on what they conceived to be its necessity in the general economy of nature; and with more perhaps of piety than prudence, and certainly with a' zeal not according to knowledge, they have represented the contrary doctrine, as derogatory to the wisdom of Providence, and a calumny against nature herself. It is indeed true, and it is among the most gratifying truths in the pursuit of science, that every real step which we make in the knowledge of Dature, serves to illustrate the wisdom and skill with which aħl its parts are contrived, to advance the general purposes of the whole. But of this whole, it should also be recollected, that we," as yet, but see in part, and, as through a glass, darkly." Hence imperfect and erroneous views of the order of nature may be often taken ; and false conclusions may be grounded on them; and if these conolusions 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 mạn 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, sest in the effervescence of our zeal, we degrade the wisdom we pretend to exalt, and prevent the designs of the goodness we professio revere.' Vol. I. p 247.