_Mrs. B._ The air is a fluid differing in its nature from liquids, but no less impenetrable. If I endeavour to fill this phial by plunging it into this bason of water, the air, you see, rushes out of the phial in bubbles, in order to make way for the water, for the air and the water can not exist together in the same space, any more than two hard bodies; and if I reverse this goblet, and plunge it perpendicularly into the water, so that the air will not be able to escape, the water will no longer be able to fill the goblet. _Emily._ But it rises some way into the glass. _Mrs. B._ Because the water compresses or squeezes the air into a smaller space in the upper part of the glass; but, as long as it remains there, no other body can occupy the same place. _Emily._ A difficulty has just occurred to me, with regard to the impenetrability of solid bodies; if a nail is driven into a piece of wood, it penetrates it, and both the wood and the nail occupy the same space that the wood alone did before? _Mrs. B._ The nail penetrates between the particles of the wood, by forcing them to make way for it; for you know that not a single atom of wood can remain in the space which the nail occupies; and if the wood is not increased in size by the addition of the nail, it is because wood is a porous substance, like sponge, the particles of which may be compressed or squeezed closer together; and it is thus that they make way for the nail. We may now proceed to the next general property of bodies, _extension_. A body which occupies a certain space must necessarily have extension; that is to say, _length_, _breadth_ and _depth_ or thickness; these are called the dimensions of extension: can you form an idea of any body without them? _Emily._ No; certainly I can not; though these dimensions must, of course vary extremely in different bodies. The length, breadth and depth of a box, or of a thimble, are very different from those of a walking stick, or of a hair. But is not height also a dimension of extension? _Mrs B._ Height and depth are the same dimension, considered in different points of view; if you measure a body, or a space, from the top to the bottom, you call it depth; if from the bottom upwards, you call it height; thus the depth and height of a box are, in fact, the same thing. _Emily._ Very true; a moment's consideration would have enabled me to discover that; and breadth and width are also the same dimension. _Mrs. B._ Yes; the limits of extension constitute _figure_ or shape. You conceive that a body having length, breadth and depth, can not be without form, either symmetrical or irregular? _Emily._ Undoubtedly; and this property admits of almost an infinite variety.