Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained

by Mrs. Marcet; Thomas P. Jones

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.

Caroline. How strange! it grows quite plump, and looks like a fresh-gathered apple.

Mrs. B. But as soon as I let the air again into the receiver, the apple, you see, returns to its shrivelled state. When I took away the pressure of the atmosphere, the air within the apple, expanded, and swelled it out; but the instant the atmospheric air was restored, the expansion of the internal air, was checked and repressed, and the apple shrunk to its former dimensions.

You may make a similar experiment with this little bladder, which you see is perfectly flaccid, and appears to contain no air: in this state I shall tie up the neck of the bladder, so that whatever air remains within it, may not escape, and then place it under the receiver. Now observe, as I exhaust the receiver, how the bladder distends; this proceeds from the great dilatation of the small quantity of air, which was enclosed within the bladder, when I tied it up; but as soon as I let the air into the receiver, that which the bladder contains, condenses and shrinks into its small compass, within the folds of the bladder.

Emily. These experiments are extremely amusing, and they afford clear proofs, both of the weight, and elasticity of the air; but I should like to know, exactly, how much the air weighs.

Mrs. B. A column of air reaching to the top of the atmosphere, and whose base is a square inch, weighs about 15 lbs. therefore, every square inch of our bodies, sustains a weight of 15 lbs.: and if you wish to know the weight of the whole of the atmosphere, you must reckon how many square inches there are on the surface of the globe, and multiply them by 15.

Emily. But can we not ascertain the weight of a small quantity of air?

Mrs. B. With perfect ease. I shall exhaust the air from this little bottle, by means of the air pump: and having emptied the bottle of air, or, in other words, produced a vacuum within it, I secure it by turning this screw adapted to its neck: we may now find the exact weight of this bottle, by putting it into one of the scales of a balance. It weighs, you see, just two ounces; but when I turn the screw, so as to admit the air into the bottle, the scale which contains it, preponderates.

Caroline. No doubt the bottle filled with air, is heavier than the bottle void of air; and the additional weight required to bring the scales again to a balance, must be exactly that of the air which the bottle now contains.