This celebrated author has taken a great deal of notice of those air bubbles which cover the leaves when plunged under water. He says, p. 26, that the leaves draw these bubbles from the water. He is the more persuaded that this is the case, because he found these bubbles did not appear when the water had been boiled some time, and appeared more when the water is impregnated with air, by blowing in it. He had also observed, that they did not appear after sunset. Page 31, he explains his opinion farther upon this head: he says, that these air bubbles are produced by common air adhering to the external surface of the leaves, which swells up into bubbles by the heat of the sun; and that the cold of the night is the reason why these air bubbles do not make their appearance at that time. As he found that dry leaves put under water gather such bubbles also upon their surface, he concludes, p. 33, that the appearance of these bubbles cannot be owing to any vital action in the leaves.
I took some pains to disclose the cause of these bubbles, which, I think, are of more importance than Mr. Bonnet at that time imagined them to be; and found the fact to be generally this:
The most part of leaves gather these bubbles upon their surface, when plunged in any water in the sun-shine or by day-time in the open air; but infinitely more in fresh pump water than in any other. In clear river water they appear later, less in number and in size; less so in rain water, and the least of all in boiled water, in stagnating, and in distilled water.
They are not produced by the warmth of the sun rarifying the air adhering to the leaves; for many kinds of leaves produce them almost as soon as plunged under water, though the water be very cold, and the leaves warm from the sun-shine be plunged in it.
They do not appear after sun-set, at least not in any considerable number; but those that already exist do not shrink in or disappear by the cold of the night.
As soon as the sun begins to diffuse its warmth over the surface of the earth in the spring, and to promote that general tendency to corruption which all dead bodies of the animal and vegetable kingdom, and many other substances, are so liable to, the trees display in a few days, the most wonderful scene that can be imagined. Contracted as they were in the state of stupor and inactivity in which they remain during the winter, exposing to the air no other surface than that of their trunk and branches, as if they wanted to have as little to do as possible with the external air, they all at once increase, perhaps more than a thousand times, their surface by displaying those kind of numberless fans which we call leaves. Some of them produce their leaves a long while before any flowers appear upon them; others a good while after the flowers are formed, and the fructification is already in an advanced state; and keep their leaves in the best condition, and even push out continually new ones, long after the whole fructification is finishes; which seems to indicate, that the chief use of these fans is not to assist the fructification and propagation of their species. These fans, when compleated, seem to compose or arrange themselves in such a manner as to expose their upper and varnished surface to the direct influence of the sun, and to hide as much as they can their under surface from the direct influence of this luminary. It seems as if they required rather the light of the sun than the influence of its heat, as their polished surfaces must reflect some of the rays of the sun, and thus moderate the degree of heat.
It will, perhaps, appear probable, that one of the great laboratories of nature for cleansing and purifying the air of our atmosphere is placed in the substance of the leaves, and put in action by the influence of the light, and that the air thus purified, but in this state grown useless or noxious to the plant, is thrown out for the greatest part by the excretory ducts, placed chiefly, at least in far the most part of plants, on the under side of the leaf.
Is there not some probability that the under part of the leaves may have been chiefly destined for this purpose; because in this way the dephlogisticated air, gushing continually out of this surface, is inclined to fall rather downwards, as a beneficial shower for the use of the animals who all breathe in a region of the air inferior to the leaves of trees? Does not this conjecture get some weight, if we consider that dephlogisticated air is in reality specifically heavier than common air, and thus tends rather to fall downwards?
If we add to these reflexions another of no less importance, viz. that most sorts of foul air are specifically lighter than common air, we shall be inclined to believe that the difference of the specifical gravity of that beneficial air of which I treat, and that which is become hurtful to our constitution by corruption, breathing, and other causes, indicates one of those special blessings designed by the hand of God: for by this arrangement we get soon rid, in a great measure, of that air which is become hurtful to us, as it rises soon up out of our reach; whereas the dephlogisticated air, being heavier than common air, is rather inclined to settle on the surface of the earth among animal creation.
But, as animals spoil equally as much air in the winter as in the summer by the act of respiration, it might seem somewhat surprizing, that this great laboratory ceases entirely by the decay of the leaves. Is this defect supplied by some other means equally powerful? Though we are very far from being able to trace all the active causes which contribute their share in keeping up the wholesomeness of our atmosphere, yet we have already traced some of them, and therefore must not despair of discovering some more. The shaking of foul air in water will in great measure correct it. Water itself has a power of yielding dephlogisticated air, as Dr. Priestley discovered. Plants have a power to correct bad air, and to improve good air. Winds will blow away the noxious particles of the air, and bring on air corrected by the waters of the seas, lakes, rivers, and forests. All these causes exist equally in the winter as in the summer, or at least nearly so. The influence of the vegetable creation alone ceases in the winter: but the loss of this influence is, perhaps, more than amply counterbalanced by the diminution of the general promoting cause of corruption, viz. heat. Every body knows, that warm weather hastens in a great degree putrefaction. In the summer time numberless insects are produced, which did not exist in the winter: these insects infect the air by the corruption of their bodies. That immense quantity of animal substances, and many others, which undergo a putrefaction by the warmth of the weather, seems to require an additional power or agent to counter-act it; and this office is destined to the leaves. In frosty weather no animal substance is subject to putrefaction, which cannot go on without a proper degree of heat. The perspiration of animals is less offensive in the winter than in the summer, and of consequence must corrupt the atmosphere less. It seems therefore probable, that, if we are deprived of one way by which air is corrected in the winter, we have also at that time less causes which tend to contaminate our element.
It is not very rare to see these bubbles so quickly succeeding one another, that they rise from the same spot almost in a continual stream: I saw this more than once, principally in the nymphaea alba. ...
But as this dephlogisticated air is not produced immediately from the pump water, but only when this green matter is formed, it is clear, that the air obtained from the leaves, as soon as they are put in the water, is by no means air form the water, but air continuing to be produced by a special operation carried on in a living leaf exposed to the day-light, and forming bubbles, because the surrounding water prevents this air from being diffused through the atmosphere.
It is true, that pump water, placed in the sun-shine, will soon yield some small air bubbles, settling at the bottom of the jar, and every where at the sides; but this air is very far from being the same as that contained in the air bubbles of the leaves.
I placed, in a warm sun-shine, a great number of inverted jars, full of pump water, and collected carefully from them all the air yielded by these bubbles, which proved to be much worse than the common air.
I boiled some pump water in a pot, in which I had placed a long cylindrical jar, quite full of the same water: a good deal of air was collected at the top of the inverted jar, which was by the heat disengaged from the water. This air proved to be much worse than common air, and entirely unfit for respiration.
Abbé Fontana has made, some years ago, a great many experiments, tending to investigate the nature of air contained in different waters.
I squeezed a handful or two of potatoe [sic] leaves under water, and kept an inverted jar full of water above it, to receive the air. A great deal of it was instantly obtained, which proved to be nearly as good as common air.
I squeezed, in the same way, some air out of leaves of sage, salvia, which proved to be somewhat worse than the former. A potatoe plant was shook under water, so as not to hurt it: a good deal of air was immediately disengaged, which, by the nitrous test, proved to be worse than common air.
A plant of lamium album was treated in the same way, and in like manner a good deal of air was obtained, which was nearly of the same quality with the former.
Some leaves of an apple tree were put in a cylindrical jar full of pump water. The jar was then inverted in a vessel full of the same water, and placed upon the fire. As soon as the water grew warm, the leaves were covered with air bubbles, just as in the sun. After the water had boiled a little while, it was put by to cool. A great deal of air was obtained, which proved to be so bad as to extinguish flame.
Some of the same leaves were put into a jar, inverted in a pot full of water, and only placed near the fire: a great deal of air was obtained, but as poisonous as the former.
If it was the warmth of the sun, and not its light, that produced this operation, it would follow, that, by warming the water near the fire about as much as it would have been in the sun, this very air would be produced; but this is far from being the case.
I placed some leaves in pump water, inverted the jar, and kept it near the fire as was required to received [sic] a moderate warmth, near as much as a similar jar, filled with leaves of the same plant, and placed in the open air, at the same time received from the sun. The result was, that the air obtained by the fire was very bad, and that obtained in the sun was dephlogisticated air.
A jar full of walnut tree leaves was placed under the shade of other plants, and near a wall, so that no rays of the sun could reach it. It stood there the whole day, so that the water in the jar had received there about the same degree of warmth as the surrounding air (the thermometer being then at 76°); the air obtained was worse than common air, whereas the air obtained from other jars kept in the sun-shine during such a little time that the water had by no means received a degree of warmth approaching that of the atmosphere, was fine dephlogisticated air.
No dephlogisticated air is obtained in a warm room, if the sun does not shine upon the jar containing the leaves. ...
I cannot consider the plants kept thus under water to be in a situation so contrary to their nature as to derange their usual operation. Water, even more than they want, is not hurtful to plants, if it is not applied too considerable a time. The water only cuts off the communication with the external air; and we know, that plants may live a long while without this free communication. Besides, water plants, as persicaria urens, becabunga, and others, which I have employed in my experiments, are often found a long while quite covered by the water in which they grow.
By bending a living plant (the root remaining in its own earth) in an inverted jar full of water, you only surprize nature upon the fact in the middle of its operation, by shutting at once all communication with the free air. In such a situation no air can be absorbed by the leaves, or by any parts of the plant under water; but any air may freely come out of it.
Without covering the leaves or the plant entirely with water, it is impossible to know what quantity of air oozes out of the plant, and of what quality this air is; for any air issuing out of a plant incorporates immediately with the surrounding air, and makes a compound whose constituent parts are an intimate mixture of air from the plant and common air; and it would be as difficult to judge accurately how much dephlogisticated air such a plant has communicated to the ordinary air which was already in the jar, as it would be for a chymist to judge accurately what quantity of distilled water was mixed with a certain quantity of common water, if some of it was really added to it on purpose to puzzle him. It may, however, be ascertained, in an inaccurate way, what quantity of this beneficial air a plant, placed in a jar full of common air, has communicated to it, by computing the degree of superior goodness the air is found to possess.
As plants yield in a few hours such a considerable quantity of dephlogisticated air, though their situation seems rather unfavourable for it when they are kept under water; may it not with some degree of probability be conjectured, that they yield much more of it when remaining in their natural situation; for then, being continually supplied by new common air, their stock of dephlogisticated air cannot be exhausted. It is an unfavourable circumstance, that air is not an object of our sight; if it was, we should perhaps see that plants have a kind of respiration as animals have; that leaves are the organs of it; that, perhaps, they have pores which absorb air, and others which throw it out by way of excretion, as are the excretory ducts of animals; that the air secreted, being dephlogisticated air, is thrown out as noxious to the plant (which article is clearly demonstrated by Dr. Priestley and Mr. Scheele); that in the most part of plants, principally trees, the greatest part of inhaling pores are placed upon the upper side of the leaf, and the excretory ducts principally on the under side.
If these conjectures were well grounded, it would throw a great deal of new light upon the arrangement of the different parts of the globe, and the harmony between all its parts would become more conspicuous. We might find, that partial tempests and hurricanes, by shaking the air and the waters, produce some partial evils for the universal benefit of nature; that, by these powerful agitations, the septic and noxious particles of the air are blown away, and rendered of no effect, by being thus diluted with the body of air, and partly buried in the waters. We might conceive a little more of the deep designs of the Supreme Wisdom in the different arrangement of sublunary beings. The stubborn atheist would, perhaps, find reason to humiliate himself before that Almighty Being, whose existence he denies because his limited senses represent to him nothing but a confused chaos of miseries and disorders in this world.