They are not so close together.

When air is thus expanded, what is said of it?

It is said to be rare.

When pressure is put upon air in a vessel, what becomes of the air?

It is forced or pressed into a smaller space, and, therefore, occupies less room.

When pressure is put upon air, are its particles closer together?

They are closer together.

When air is thus compressed, what is said of it?

It is said to be dense.

Can air be much condensed by pressure?