blades of grass; but when I admit the air, they suddenly disappear.

Emma. This proves what you told us a day or two ago, that substances in general contain a great deal of air.

Father. Instead of bodies of this kind, I will plunge in some vegetable substances, a piece or two of the stem of beet-root, angelica, &c, and now observe, when I have exhausted the receiver, what a quantity of air is forced out of the little vessels of these plants by means of its elasticity.

Charles. From this experiment we may conclude that air makes no small part of all vegetable substances.

Father. To this piece of cork, which of itself would swim on the surface of wa ter, I have tied some lead, just enough to make it sink. But by taking off the external pressure, the cork will bring the lead up to the surface.

Emma. Is that because when the pres sure is taken off, the substance of the cork

expands, and becomes specifically lighter than it was before?

Father. It is: this experiment is varied by using a bladder, in which is tied up a very small quantity of air, and sunk in water; for when the external pressure is removed, the spring of air within the bladder will expand it, make it specifically lighter than water, and bring it to the surface.

The next experiment shows that the ascent of smoke and vapours depends on the air. I will blow out this candle, and put t under the receiver; the smoke now ises to the top, but as soon as the air s exhausted to a certain degree, the smoke descends, like all other heavy boHies.

Charles. Do smoke and vapours rise because they are lighter than the surroundng air?

Father. That is the reason: sometimes you see smoke from a chimney rise very perpendicularly in a long column; the air VOL. II.

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then is very heavy: at other times you may see it descend, which is a proof that the density of the atmosphere is very much diminished, and is, in fact, less than that of the smoke. And at all times the smoke can ascend no higher than where it meets with air of a density equal to itself, and there it will spread about like a cloud.

This figure (Plate vr. Fig. 20.) is usually called the lungs-glass: a bladder is tied close about the little pipe a, which is screwed into the bottle A. I introduce it under the receiver A, B, and begin to exhaust the air of the receiver, and that in the bladder communicating with it, will also be withdrawn; the elastic force of the air in the bottle a will now press the bladder to the shrivelled state represented in the figure: I will admit the air, which expands the bladder; and thus by alternately exhausting and re-admitting the air, I show the action of the lungs in breathing. But perhaps the following experiment will give a better idea of the subject (Plate vi. Figs

21 and 22.) ▲ represents the lungs, B the windpipe leading to them, which is closely fixed in the neck of the bottle, from which the air cannot escape: p is a bladder tied to the bottom, and in its distended state (Fig. 21.) will, with the internal cavity of the bottle, represent the cavity of the body which surrounds the lungs at the moment you have taken in breath: I force up p (as in Fig. 22.) and now the bladder is shri velled by the pressure of the external air in the bottle, and represents the lungs just at the moment of expiration.

Emma. Does Fig. 21. show the state of the lungs after I have drawn in my breath, and Fig. 22. when I have thrown it out forcibly?

Father. That is what the figures are intended to represent, and they are well adapted to show the elevation and compression of the lungs, although I do not mean to assert, that the action of the lungs in breathing depends upon air in the same manner as that in the bladder does upon

the air which is contained in the cavity of

the bottle.

I have exactly balanced on this scalebeam a piece of lead and a piece of cork: in this state I will introduce them under the receiver, and exhaust the air.

Charles. The cork now seems to be hea vier than the lead.

Father. In air each body lost a weight proportional to its bulk, but when the air is taken away, the weight lost will be res tored but as the lead lost least, it will now retrieve the least, consequently the cork will preponderate with the difference of the weights restored by taking away the air.

Thus you see that in vacuo, a pound of cork, or feathers, would be heavier than o pound of lead.