the bottle from breaking the receiver, an accident that would be liable to happen without this precaution.

Take a new-laid egg, and make a small hole in the little end of it, then, with that end downwards, place it in an ale-glass under the receiver, and exhaust the air; the whole contents of the egg will be forced out into the glass, by the elastic spring of the small bubble of air which is always to be found in the large end of a new-laid egg.

CONVERSATION VIII.

Of the Compression of the Air.

FATHER. I have already alluded to the compressibility of air, which it is proper to describe here, it being a consequence of its elasticity: for whatever is elastic is capable of being forced into a smaller space. In this respect air differs very materially from other fluids.

Charles.. You told us that water was compressible in a very small degree.

Father. I did so; but the compression, which can be effected with

the greatest power, is so very small, that, without the greatest attention and nicety in conducting the experiments, it would never have been discovered. Air, however, is capable of being compressed into a very small space compared with what it naturally possesses.

Emma. made by plunging an ale-glass with its mouth downwards, clearly proved that the air which it contained was capable of being reduced into a smaller space,

The experiment you

Father. This bended tube A B C

(Plate 11, Fig.

and open at c.

18) is closed at A

It is in the common state, full of air. I first pour into it a little quicksilver, just sufficient to cover the bottom a b: now the air in each leg is of the same density,

pour

and, as that contained in a B cannot escape, because the lighter fluid will be always uppermost, when I more quicksilver in at c, its weight will condense the air in the leg A B; for the air, which filled the whole length of the leg, is, by the weight of the quicksilver in c B, pressed into the smaller space A x, which space will be diminished as the weight is increased so that, by increasing the length of the column of mercury in C B, the air in the other leg will be more and more condensed. Hence we learn that the elastic spring of air is always, and under all circumstances, equal to the force which compresses it.

Charles. How is that proved? Father. If the spring, with which the air endeavours to expand itself

when it is compressed, were less than the compressing force, it must yield still farther to that force; that is, if the spring of the air in a x were less than equal to the weight of the mercury in the other leg, it would be forced into a yet smaller space; but if the spring were greater than the weight pressing upon it, it would not have yielded so much; for you are well aware that action and re-action are equal, and act in opposite directions.

You can now easily understand why the lower regions of the atmosphere are more dense than those higher.

Emma. Because they are pressed upon by all the air that is above them, and therefore condensed into a smaller space.