The experiments of this year were unfavourable to his former hypothesis; for, whether the air had been injured by respiration, the burning of candles, or other means, it was not rendered better by vegetation, but worse and the longer the plants continued in the air, the worse it became. He tried a great variety of plants, as sprigs of mint, spinach, lettuce, and some others; but with no better success. His method was to put the roots of the plant into a phial filled with earth and water, and then to introduce it through water into the jar containing the air on which he was making the experiment. In some cases, the air was certainly meliorated, but Mr. Scheele, he adds, always found it injured and he concludes, that, upon the whole, it is probable that the vegetation of healthy plants, growing in natural situations, has a salutary effect on the air in which they grow. Thus we see, from Dr. Priestley's own representation of facts, related with his usual candour, that nothing certain can be inferred from his experiments in favour of this supposed purifying power in plants: and that his conclusion was, even at that period, in direct opposition to the experience of the celebrated Scheele. Notwithstanding, however, the uncertain, and, in many respects, contradictory evidence on which this conclusion has been shown to rest, few opinions in modern science have obtained a more general belief: and both physiologists and chemists seem, in this instance, to have satisfied themselves with contemplating at a distance the beauty of the final cause, instead of approaching to a nearer examination of the facts on which the opinion has been maintained. Accordingly, this opinion still keeps its ground, and, in no publication that we have seen, has its accuracy been even questioned. In the experiments, however, which we are about to detail, it will be seen that their results arc altogether opposed to the notion that plants, by their vegetation, purify the air; and these experiments have been so often repeated, and with such uniformity of result, as to impress us with an entire confidence in their truth. We proceed, therefore, to investigate the actual changes which atmospheric air undergoes in vegetation; and, first of all, the effects produced on its nitrogenous portion. It has been already shewn, that seeds do not in the smallest degree germinate in nitrogen gas; and Dr. Ingenhousz proved, also, by experiment, that, in pure nitrogen gas, plants do not grow. This conclusion has been more lately confirmed by the experiments of Mr. Gough. He confined several succulent plants in jars of nitrogen gas when their flowers were just ready to expand; but they died away without putting forth any of their blossoms. Others were placed in similar situations before the flower-buds formed upon them, but made not the least effort towards vegetation; and many more, which grew very well in inverted jars of atmospheric air, ceased to vegetate when transferred into jars of nitrogen gas. But this gas, although it does not aid vegetation, appears in no degree to injure the faculty of growth in plants. any more than in seeds; tor a slip of spearmint, which had remained Iwelve days in nitrogen gas, recovered upon being restored to the air; and an offset of sempervivum vegetated freely after being removed from a jar of the same gas, in which it had been kept from the 2d of April to the 2d of May. As thus it appears, that nitrogen gas exerts no direct influence on plants, so it is probable that they produce no change upon it; and we may conclude, therefore, that this gas, in its simple and uncombined form, is altogether inactive in the process of vegetation. "We have next to inquire into the necessity and use of oxygen gas, and the changes which it undergoes in vegetation. It has been shewn by Dr. Ingenhousz, that plants do not, any more than seeds, grow in any species of air unless it contain a portion of oxygen gas, and this gas have access to their leaves; that they vegetate very well in atmospheric air and in oxygen gas, and, therefore, that this latter gas is necessary to vegetation. What then are the changes which it suffers in that process? By the experiments of Dr. Woodhouse, we are taught, that plants growing in earth and confined in a glass vessel of atmospheric air render it impure, and that carbonic acid is formed: that when confined in oxygen gas, previously well washed in lime-water, carbonic acid is, in like manner, produced: and that the quicker this acid is generated, the sooner does the plant die. These facts derive confirmation from the following experiment: "Some mustard seeds, which were raised on moistened flannel, and had grown to more than an inch in height, were introduced into a jar of atmospheric air, and supported about half way up it by a small whalebone hoop, covered with netting. In a deep dish, filled with water, and appointed to receive the inverted jar, was placed a glass of lime-water, which stood directly under the hoop. In four hours, a thickish pellicle overspread the lime-water, and the inside of the jar was bedewed with moisture. At the end of twenty-four hours, the lime-water was very turbid, and the water, into which the jar was inverted, had risen nearly half an inch. These appearances increased during the two succeeding days, at which time the water stood nearly an inch high in the jar. The plants now began to look sickly, gradually drooped, and, by the ninth day, fell down in different directions against the sides of the jar, and, when withdrawn from it, emitted a putrid smell. The residual air extinguished a taper, rendered lime-water turbid, and suffered thereby a farther loss of bulk. This experiment was repeated, by confining mustard plants in the same manner in jars of air inverted over mercury. The lime-water was, in like manner, rendered turbid, and the mercury for two or three days continued to rise into the jars: after a certain time, however, it became stationary, and then began to fall, and the plants when withdrawn yielded a putrid smell.” pur The fact, then, seems to stand thus: seeds during germination, plants during vegetation, and animals during respiration, produce the same changes on the air in which they live. From the simplest plant which has been subjected to experiment for this pose, through all the intervening links of organized existence up to man, animated beings are continually changing the atmosphere by abstracting oxygen, and by affording carbonic acid. Whence does this carbonic acid come? Where does this oxygen go? Is the arbonic acid formed independently of the oxygen which disappears? Or Or if the carbonic acid be formed by a union of the oxygen which is supplied by the atmosphere, and the carbon which is supplied by the living seed, plant, or animal, does the union take place within the vessels of the living being, or exterior to it? These are the questions which seem to have particularly engaged the attention of Mr. Ellis; these are what he has attempted to answer, and these, in spite of his attempts, we fear will continue subjects of lasting disputation. It is the opinion of Mr. Ellis, that the seed, plant, or animal, emits carbon in a minute state of division, which combines with the oxygen of the air, exterior to the organized body, and is thus the cause of that appearance of carbonic acid, and that disappear ance of oxygen, which constitute the changes produced on the atmosphere by the growth of seeds, plants, and animals. The arguments by which Mr. Ellis supports this opinion are too much scattered to allow of quotation, or we should have been happy to allow him to speak for himself. We have endeavoured to collect them together, and shall state them to our readers as clearly, as concisely, and as fairly as we can. In the chapter on Germination, the chief arguments are these: This process cannot take place without the presence of oxygen, and the quantity of that gas which disappears, equals, as nearly as we can calculate, the quantity which enters into the composition of the carbonic acid which takes its place. The oxygen which disappears, cannot enter into the structure of the seed. It can enter only in two ways; it must be absorbed by the action of vessels, or it must be attracted by chemical affinity. It cannot be absorbed, for there is no structure to absorb, and afterwards to retain it. It cannot be attracted, for there is nothing to attract it but water and carbon, which are insufficient to destroy its combination with the nitrogen. Seeds soaked and immersed in hydrogen, or nitrogen, give out carbonic acid; but this is no living process, for it is not attended by the evolution of the seed. Upon these grounds Mr. Ellis denies the entrance of oxygen into the seed, and infers the formation of the carbonic acid to be exterior to it. In the chapter on Vegetation, Mr. Ellis argues thus: The atmospherical air cannot enter the plant, for there is no organization which will allow it. The parts of the plant which produce the changes on the air are the leaves, and as far as we know the anatomy of these organs, they possess only two sets of vessels which terminate by open mouths on the surface of the leaf; these are the vessels on the superior surface, whose office it is to emit, and those on the inferior, whose office it is to absorb moisture. It is obvious that neither of these sets of vessels can perform perform the function of aerial absorption. If we suppose that another set of vessels is provided for this function, the same vessels must be absorbing oxygen and emitting carbonic acid, two currents must be flowing through these vessels in contrary directions, which would constantly impede, or rather, which would instantly destroy each other; this supposition is wholly untenable. Does the sap in its passage through the vessels of the leaf, attract the oxygen by chemical affinity? No, this is impossible; for chemical affinity cannot be exerted, except between substances in actual contact; and this is so far from being the case with the sap and the air, that they are divided by the combined thickness of the vessels in which the sap flows, the green colouring matter, and over all the epidermis. Besides, how is the carbonic acid to be emitted? We cannot refer this to chemical affinity, nor can we suppose it to be emitted by any living action, for we cannot shew the structure which performs it. There is even another difficulty. How is the oxygen to be separated from the nitrogen before it is absorbed by the plant? We cannot suppose the whole undecomposed air to enter the vessels of the leaf; for of what use could be the nitrogen, if it is rejected again, and how could it be rejected in the exact bulk in which it existed before? As the carbonic acid formed contains exactly the quantity of oxygen which disappears, it is fair to infer that the latter is expended in the composition of the former, and this inference is not a little strengthened by the fact, that plants cannot vegetate without oxygen. When we come to the vermes, fish, amphibiæ, and the hot-blooded animals, the difficulties accumulate upon us. To these all the preceding arguments, and many more are applicable. The mucus which besmears the bodies of worms must be an insurmountable obstacle in the way of absorption; and in the case of fish, the air must be separated from the water before it can be taken up, either by absorption or affinity. Those animals which are torpid during part of the year, produce no change in the air during the period of their torpidity; yet at this time their blood is said to be supercharged with carbon, and ought therefore to attract the oxygen, if it is capable of attracting it at any time. But supposing the oxygen to be absorbed in some way or other, how can the carbonic acid be expelled? there is nothing in the external air or water which can attract it, and we cannot suppose it to be expelled, as an excretion. If the air passes into the blood, it must pass through the vessels and membranes of the lungs. It cannot be absorbed, because of the fineness of the absorbent vessels, the mucus which covers the pulmonary vesicles, and the elastic nature of air itself. Besides, if air did pass through the membrane of the lungs, it would rather pass into their cellular substance substance than into their blood vessels, because in the one case it has to pass only through the thickness of the membrane of the Jungs, and in the other, both through this and the coats of the vessels. But it has never yet been proved that venous blood contains air, much less carbonic acid air. Venous blood emits this air only in those gases which contain oxygen; these are the nitrous oxyd, atmospherical air, or pure oxygen itself. Dr. D. Darwin's experiment however is a positive proof that venous blood contains no air. "He enclosed a portion of the jugular vein of a sheep between two ligatures; it was cut out, stripped of its adhering cellular membrane, and then thrown into a glass of water of temperature 100°, standing under the receiver of an air pump. It at once sank to the bottom, and did not rise when the air was exhausted; nor when afterwards taken out, wiped dry, and laid on the floor of the receiver, did it exhibit any swelling under the exhaustion of the vessel." Besides, the presence of air in the blood vessels, so far from being salutary, is really fatal to life, Girtanner injected air into the jugular vein of a dog; convulsions were immediately produced, and death shortly followed. The experiments of Bichat, performed in a different way, led to a similar result. Such is the strain of argument by which Mr. Ellis attempts to prove that no air enters the seed, plant, or animal, during their appropriate living processes, and by which he is led to infer, that, during germination, vegetation, and respiration, carbon is emitted as an excretion in a state of minute division, combines with the oxygen of the atmosphere, and forms carbonic acid. If we understand him right, he supposes that the moisture which is continually evaporating from the surface of seeds, the leaves of plants, and the lungs of animals, carries up with it the carbonaceous excretion which afterwards combines with the oxygen; for ke says, that as the carbon of manure is soluble in water, it is probable that the excreted carbon is soluble also, and that the water answers the purpose only of a balloon to the carbon. To the old objection that carbonic acid cannot be formed at the temperature of the atmosphere, he answers, that even if formed during the circulation, it is formed at a much lower temperature than burning charcoal, and its production by seeds, plants, cold-blooded animals, and fermenting substances, is a sufficient reply to this objection. Having thus heard Mr. Ellis's arguments, we request the attention of our readers (for a few minutes) whilst we state our opinion of their value. Of the two modes in which oxygen is supposed to enter the blood, or the sap, that by chemical attraction seems to us the only probable one; and our author in conducting the ar gument |