_Mrs. B._ The air pump affords us the means of making a great variety of interesting experiments, on the weight, and pressure of the air: some of them you have already seen. Do you not recollect, that in a vacuum produced within the air pump, substances of various weights, fell to the bottom in the same time; why does not this happen in the atmosphere?
_Caroline._ I remember you told us it was owing to the resistance which light bodies meet with, from the air, during their fall.
_Mrs. B._ Or, in other words, to the support which they received from the air, and which prolonged the time of their fall. Now, if the air were destitute of weight, how could it support other bodies, or retard their fall?
I shall now show you some other experiments, which illustrate, in a striking manner, both the weight, and elasticity of air. I shall tie a piece of bladder over this glass receiver, which, you will observe, is open at the top as well as below.
_Caroline._ Why do you wet the bladder first?
_Mrs. B._ It expands by wetting, and contracts in drying; it is also more soft and pliable when wet, so that I can make it fit better, and when dry, it will be tighter. We must hold it to the fire in order to dry it; but not too near, lest it should burst by sudden contraction. Let us now fix it on the air pump, and exhaust the air from underneath it--you will not be alarmed if you hear a noise?
_Emily._ It was as loud as the report of a gun, and the bladder is burst! Pray explain how the air is concerned in this experiment.
_Mrs. B._ It is the effect of the weight of the atmosphere, on the upper surface of the bladder, when I had taken away the air from the under surface, so that there was no longer any reaction to counterbalance the pressure of the atmosphere, on the receiver. You observed how the bladder was pressed inwards, by the weight of the external air, in proportion as I exhausted the receiver: and before a complete vacuum was formed, the bladder, unable to sustain the violence of the pressure, burst with the explosion you have just heard.
I shall now show you an experiment, which proves the expansion of the air, contained within a body, when it is relieved from the pressure of the external air. You would not imagine that there was any air contained within this shrivelled apple, by its appearance; but take notice of it when placed within a receiver, from which I shall exhaust the air.