Plants play a very important part in the economy of the creation; for, independent of their relation to animals as the real source of all food, they are essential to the purity of the air. We are told by chemists, that the composition of the atmosphere is quite constant and uniform; that air, from whatever place it is collected, is always found to consist of the same proportions of oxygen and nitrogen and carbonic acid. This fact has now been established by a number of careful experiments, made by different observers, in various parts of the globe, and extending over a considerable period of time, so that it may in fact be considered as proved, not only that the composition of the air is uniform throughout, but also that it does not undergo any appreciable variation in nature from year to year. There is certainly something very wonderful in this, and indeed it seems difficult to understand how it really can be the case, when we observe the great chemical changes which are at all times going on in the atmosphere, and the enormous quantities of certain substances which are constantly being poured into it in various ways.

Night and day all living animals are consuming the oxygen of the air, converting it into carbonic acid, and the same effect is also constantly being produced all over the earth, by fires, furnaces, lamps, candles, and in short by combustion in all its forms. Decay, putrefaction, and fermentation, are also giving rise to the evolution of carbonic acid; and lastly, besides all these sources, immense quantities of the gas in question are evolved from fissures in the earth, especially in what are termed volcanic districts. There are then, to say nothing of smaller or more local causes, at least half a dozen great natural operations, all tending to vitiate the air, diminishing the quantity of oxygen which it contains, and increasing the proportion of carbonic acid. All these causes, we are told, are exactly balanced and neutralised by the decomposing influence of plants, and the wonderful power which they have of absorbing the carbon of carbonic acid. We owe this remarkable fact to the researches of Drs. Priestley and Ingenhousz, extended, repeated, and confirmed by a host of other observers.

In Dr. Priestley's first experiments on carbonic acid or fixed air, it is evident that he regarded it as highly poisonous to plants; for in the first volume of his very interesting Essays on Air, he says - "Fixed air is presently fatal to vegetable life;" and he goes on to show that plants, in many cases, were very soon killed, when exposed to the influence that he did not repeat the experiment with pure carbonic acid artificially prepared, and regrets that he had not yet had leisure to do so. These experiments were repeated and extended by his friend Dr. Percival, who published an account of his results in Hunter's "Surgical Essays." The conclusion to which he arrived, was just the contrary of what might have been expected from Dr. Priestley's experiments; for he states, "that fixed air not only retards decay, but actually continues the vegetation of plants, and affords them a pabulum, which is adequate to the support of life and vigor in them for a considerable length of time." So satisfied was he of the truth of this, and the important influence that the discovery would have upon the art of gardening, that he goes on to point out how the florist who is anxious to obtain a prize at some great meeting held for the encouragement of horticulture, might perhaps avail himself of the powers of "fixed air," either to quicken the growth of his plant if still immature, or to preserve it in all its beauty, and retards its decay if fully developed; and he accordingly gives several directions to gardeners who are willing to try the effect of carbonic acid gas on their flowers.

Some years before this time, however, Dr. Priestley had observed that plants grew well in air which had been vitiated by the breathing of animals, and that they soon restored such air to its original state of purity; it seems that he did not at that time very distinctly connect the two phenomena together, for it was not till after Dr. Perciyal's experiments had been published, and after Ingenhousz had promulgated his views upon the subject, that carbonic acid began to be regarded as essential to the growth of plants. It is not easy now, neither in fact is it very important, to decide exactly what degree of merit belongs to each of these three philosophers. The experiments of Ingenhousz were the most numerous and minute, and his views were the most strongly criticised by contem porary naturalists; at the same time he distinctly states that his experiments arose out of those of Dr. Priestley.

The circumstance which prevented many observers from believing in the importance of carbonic acid to plants, arose from the fact, that as in nature they are only supplied with it very gradually, and in small quantities, and as their organs are only fitted to absorb it when largely diluted with common air, so when placed in an atmosphere containing a considerable proportion of this gas, they are always more or less injured; and it was difficult to understand how, if carbonic acid in small quantities was food and nourishment, that in rather large quantities it would be hurtful and even poisonous.

Dr. Percival's idea of the probable value of carbonic acid as a powerful agent in the hands of a skillful gardener, is one which naturally leads to several interesting considerations, and it has accordingly produced a number of curious experiments, which, though they certainly do not justify his prediction, or, indeed, give much prospect of any really important direct improvement in practical gardening, are nevertheless, very curious and interesting. The primary question of course is, are plants always able to absorb as much carbonic acid from the air as they require, and would benefit be derived by increasing the proportion of it, in the air of a forcing-house for example? When Dr. Priestley found that pure carbonic acid was hurtful to plants, he next tried mixtures of it with common air, and he found that even one-eighth of it rendered the air poisonous. Ingenhousz, after a great number of experiments, and, though quite satisfied that plants really decompose carbonic acid, speaks with great doubt of the effects which it produces when artificially added to the air.

He says, "plants grow tolerably well in air to which a small quantity of carbonic acid has been added, especially if they are exposed to the sunshine; sometimes even they seemed to grow all the better for it;" but he adds, "that it is doubtful if this effect was really due to the presence of the carbonic acid." A second, and very remarkable aeries of experiments is described by DE Saussure, the result of which showed that the addition of carbonic acid, where more than a tenth was added, always did harm, whilst a twelfth produced a decidedly beneficial effect, if the plants were exposed at the same time to the influence of light; there are, however, unfortunately, several interfering causes in these experiments, which Saussure overlooked, and which diminish their real value. Three years since some valuable observations on this subject were made by Dr. Daubeny, who conducted a careful series of experiments, with the object of ascertaining what proportion of carbonic acid in the air, healthy plants are able to endure, without inconvenience. The result of his investigations proves that ten per cent of carbonic acid is not at all injurious to plants, and that it is sometime before even rather larger proportions of that gas begin to produce bad effects.

Even as much as twenty per cent produced no injury in ten days.

In all considerations of this sort, it is obviously the best way to commence by studying the natural conditions of plants, before we begin to place them in new and unnatural circumstances. In nature, plants are supplied with an almost infinitely small quantity of carbonic acid, and the effect of their growth is to deprive the air which surrounds them of that carbonic acid, and to replace it by an equal volume of oxygen.

The chief difficulty which exists in comprehending the true bearings of this great fact, depends on the enormous magnitude of the entire atmosphere, and the almost infinitely large quantities which we have to consider. It is easy to form some idea of the quantity of carbonic acid which is produced in a given time on a square mile of the ^earths's surface, and which the plants growing on it consequently may be supposed to have to decompose in the same period.. But it is very difficult to form any just idea of the whole atmosphere, or to comprehend the relation which exists between the bulk of the carbonic acid and that of the entire atmosphere. Large as the former appears to us, and enormous as its quantity is, when compared to the largest tree, yet it is a mere trifle, a thing of very little account, when contrasted with the bulk of the whole atmosphere.

If, for example, we assume, for the mere purpose of comparison, that on an average each human being on the surface of the earth gives out, in the course of twenty-four hours, twenty-four ounces of carbonic acid gas, a proportion probably not very far from the truth, the volume of the gas thus expired would be about twelve and a half cubic feet; and the entire quantity given out during a whole year at this rate, would be 4744 cubic feet of carbonic acid gas; having a weight of about 547 lbs., very nearly a quarter of a ton! If then we go a little further, and calculate what would be the bulk and weight of the quantity of carbonic acid that would be given out by the entire population of the globe, during a whole year, assuming its number to be 600 millions, and supposing that every individual required the same quantity of carbonic acid, we should then arrive at the result of very nearly three million of millions of cubic feet, or 316,266 million cubic yards of carbonic acid. This quantity is so large that it is difficult to conceive any just idea of its real volume, and the consideration is not rendered much simpler, if we reduce it into cubic miles; we then find that it would amount to about 58 cubic miles.

On the other hand, if we calculate the weight of such a quantity of carbonic acid, we are are nearly as much bewildered with high numbers, for we find that it amounts to 326,200 million pounds, or 145 million tons! and this, be it remembered, is but a very small portion of the carbonic acid which plants must absorb every year.