Mrs. B. Undoubtedly, in all essential properties.
Emily. Yet you say that it does not possess one of the general properties of bodies—attraction.
Mrs. B. The particles of air are not destitute of the power of attraction, but they are too far distant from each other to be influenced by it so as to produce cohesion: and the utmost efforts of human art have proved ineffectual in the attempt to compress them, so as to bring them within the sphere of each other's attraction, and make them cohere.
Emily. If so, how is it possible to prove that they are endowed with this power?
Mrs. B. The air is formed of particles precisely of the same nature as those which enter into the composition of liquid and solid bodies, in each of which we have a proof of their attraction.
Emily. It is then, I suppose, owing to the different degrees of cohesive attraction in different substances, that they are hard or soft, and that liquids are thick or thin.
Mrs. B. Yes; but you would express your meaning better by the term density, which denotes the degree of closeness and compactness of the particles of a body. In philosophical language, density is said to be that property of bodies by which they contain a certain quantity of matter, under a certain bulk or magnitude. Rarity is the contrary of density; it denotes the thinness and subtilty of bodies: thus you would say that mercury or quicksilver was a very dense fluid; ether, a very rare one. Those bodies which are the most dense, do not always cohere the most strongly; lead is more dense than iron, yet its particles are more easily separated.
Caroline. But how are we to judge of the quantity of matter contained in a certain bulk?
Mrs. B. By the weight: under the same bulk bodies are said to be dense in proportion as they are heavy.