44. =Extension.=--You can not conceive of any portion of matter, however small it may be, that has not shape or figure. It may be so small as to appear only as a point to the naked eye, but seen through the microscope its shape becomes obvious. Even an atom must have length, breadth, and thickness, though it be so small that we can not measure it, nor see its shape with the most powerful microscopes. _Extension_, which is the term commonly used to express this idea, is then an _essential_ property of matter; that is, it is a property of which no form or kind of matter can be destitute. The distinction, in this respect, between this property and those which I have before noticed may be made obvious to you by an example. Hardness is not an essential quality of matter, for some kinds of matter are destitute of it; but no portion of matter, hard or soft, can be destitute of extension or shape. The air is sometimes spoken of in common language as being shapeless. This is partly because it is invisible, and partly because no portion or body of air assumes any definite shape. But air is continually forced into definite shapes by confinement in rooms, boxes, etc.; and then its extension in different directions can be measured as accurately as the extension of a solid can be. And besides, the atoms of which air is composed are undoubtedly solid, and we can not conceive of their existence without attaching to them the idea of figure or extension. 45. =Impenetrability.=--In common language one substance is said to penetrate another. Thus a needle penetrates cloth, a nail penetrates wood, etc. But this is not strictly true. The needle does not go into the cloth, but goes between the fibres of it, pushing them to the one side and the other. So the nail goes between the fibres of the wood, and not into them. It does not occupy the same room that the fibres do at the same time. So, also, no atom of matter can penetrate or go into any other atom. It can only push it out of the way, and then occupy its place. Impenetrability is therefore said to be one of the essential qualities of matter. This means simply that no portion of matter can occupy the same space with another portion of matter at the same time. [Illustration: Fig. 6.] [Illustration: Fig. 7.] [Illustration: Fig. 8.]