_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. _Emily._ Then we may say that metals are dense bodies, wood comparatively a rare one, &c. But, Mrs. B., when the particles of a body are so near as to attract each other, the effect of this power must increase as they are brought by it closer together; so that one would suppose that the body would gradually augment in density, till it was impossible for its particles to be more closely united. Now, we know that this is not the case; for soft bodies, such as cork, sponge, or butter, never become, in consequence of the increasing attraction of their particles, as hard as iron? _Mrs. B._ In such bodies as cork and sponge, the particles which come in contact are so few as to produce but a slight degree of cohesion: they are porous bodies, which, owing to the peculiar arrangement of their particles, abound with interstices, or pores, which separate the particles. But there is also a fluid much more subtile than air, which pervades all bodies, this is _heat_. Heat insinuates itself more or less between the particles of all bodies, and forces them asunder; you may therefore consider heat, and the attraction of cohesion, as constantly acting in opposition to each other.