If the air-bladder of a fish be pricked or broken, the fish presently sinks to the bottom, unable either to support or raise itself up again. Flat fishes, as soles, plaise, &c. which always lie grovelling at the bottom, have no air-bladder.

CONVERSATION II.

Of the Air-Pump.

EMMA. You have told us, papa, of taking away the air from vessels; will you show us how that is performed?

Father. I will; and I believe it will be the most convincing method of proving to you that the air is such a body as I have described.

Emma. Are we then to infer, that the particles of salt are smaller than those of water, and lie between them, as the small shot lie between the cannon balls; and that the particles of sugar are finer than those of salt, and, like the sand among the shot, will insinuate themselves into vacuities too small for the admission of the salt?

Father. I think the experiment fairly leads to that conclusion. Another fact respecting the particles of fluids deserving your notice is, that they are exceedingly hard, and almost incapable of compression.

Charles. What do you mean, sir, by compression?

Father. I mean the act of squeezing any thing, in order to bring its parts nearer together. Almost all substances with which we are ac

quainted may, by means of pressure, be reduced into a less space than they naturally occupy. But water, oil, spirits, quicksilver, &c., cannot by any pressure of which human art or power is capable, be reduced into a space sensibly less than they naturally

possess.

Emma. Has the trial ever been made?

Father. Yes, by some of the ablest philosophers that ever lived. And it has been found, that water will find its way through the pores of gold even, rather than suffer itself to be compressed into a smaller space.

Charles.

ment made?

How was the experi

Father. At Florence, a celebrated city in Italy, a globe made of gold was filled with water, and closed so accurately that none of it could es

cape. The globe was then put into a press, and a little flattened at the sides the consequence of which was, that the water came through the fine pores of the golden globe, and stood upon its surface like drops of dew.

Charles. Would not the globe contain as much after its sides were bent in as it did before?

Father. It would not; and as the water forced its way through the gold rather than suffer itself to be brought into a smaller space than it naturally occupied, it was concluded at that time, that water was incompressible. Later experiments have, however, shown, that those fluids which were esteemed incompressible are, in a very small degree, as, perhaps, one part in twenty thousand, capable of compression.