certain height in the heavens, exhibit

artificial rainbows *.

We will now force in a fresh supply of air, and try some of these jets. Emma. I observed, in the upright jets, that the height to which the water was thrown was continually diminishing.

Father. The reason is this; that, in proportion as the quantity of water in the fountain is lessened, the air has more room to expand, the compression is diminished, and consequently the pressure becomes less, till at length it is no greater within than it is without, and then the fountain ceases altogether.

* This phenomenon is described and explained in Vol. V, Conversation XVIII.

CONVERSATION IX.

Miscellaneous Experiments on the
Air-Pump.

FATHER. I shall, to-day, exhibit a few experiments, without any regard to the particular subjects under which they might be arranged.

In this jar of water I plunge some pieces of iron, zinc, stone, &c., and you will see that when I exhaust the external air, by bringing the jar under the receiver of the air-pump, the elastic spring of air contained in the pores of these solid substances will

force them out in a multitude of globules, and exhibit a very pleasing spectacle, like the pearly dew-drops on the 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 water, 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 pressure 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 it under the receiver; the smoke now rises to the top, but as soon as the air is exhausted to a certain degree, the smoke descends, like all other heavy bodies.

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

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