are specifically lighter than water, as well as many that are not so. It was the elastic power of the air within the apple, that forced out all the shrivelled parts when the external pressure was taken away.

Here is a small glass of warm ale, from which I am going to take away the air.

Emma. It seems to boil, now you exhaust the air from the receiver.

Father. The bubbling is caused by the air endeavouring to escape from the liquor. Let the air in again, and then taste the beer.

Charles. It is flat and dead.

Father. You see of what importance air is to give to all our liquors their pleasant and brisk flavour, for the same will happen to wine and all other fermented fluids,

Emma.

How is it that the air,

when it was re-admitted, did not

penetrate the ale again?

Father. It could not insinuate itself into the pores of the beer, because it is the lighter body, and therefore will not descend through the heavier. Besides, it does not follow that it is the same sort of air which I admitted into the receiver, that was taken from the ale.

Emma. Are there more kinds of air than one?

Father. Yes, very many; as we shall show you in our Conversations on Chemistry*. That which I took from the beer, and which gives it the brisk and lively taste, is called fixed air, or carbonic acid gas, of which there is, in general, but a

* See Dialogues in Chemistry, Vol. I.

very small quantity in the atmosphere.

The elasticity, or spring of air, contained in our flesh, was clearly shown by the experiment, when I pumped the air from under your hand.

Charles. Was that the cause of its swelling downward ?

Father. It was: and it will account for the pain you felt, which was greater, and of a very different kind, than what you would have experienced by a dead weight being laid on the back of your hand, equal to the pressure of the air.

Cupping is an operation performed on this principle: the operator tells you he draws up the flesh; but if he were to speak correctly, he would say, he took away the external air from off a certain part of the body,

and then the elastic force of the air within extends, and swells out the flesh ready for his lancets.

Emma. When I saw you cupped he did not use an air-pump, but little glasses, to raise the flesh.

Father. Glasses closed at top are now generally made use of, in which the operator holds the flame of a lamp: by the heat of this the elasticity of the air in the glass is increased, and thereby a great part of it driven out. In this state the glass is put on the part to be cupped, and as the inward air cools, it contracts, and the glass adheres to the flesh by the difference of the pressures of the internal and external air.

By some persons, however, the syringe is considered as the most effectual method of performing the

operation, because by flame the air cannot be rarified more than one half, whereas by the syringe a few strokes will nearly exhaust it.

Here is another little square bottle like that before mentioned (Plate II, Fig. 15), only that it is full of air, and the mouth sealed so closely that none of it can escape. I enclose it within the wire cage B, and in this tate bring them under the receiver, and exhaust the external air.

Charles. With what a loud report it has burst!

Father. You can easily conceive now in what manner this invisible fluid endeavours continually, by its elastic force, to dilate itself.

Emma. Why did you place the wire cage over the bottle?

Father. To prevent the pieces of