21. =Space in Gaseous Substances.=--We can have some idea of the great amount of space in a gaseous or aeriform substance by observing the difference between water in its liquid and in its aeriform state. A cubic inch of water, when it becomes steam, occupies 1696 times as much room as it did when it was water. The difference in proportion is exhibited in Fig. 1, the inner circle representing the water, and the outer the steam into which it is converted. Now the water is not altered at all in its nature by being changed into steam. The particles are simply put farther apart by the heat, and as soon as the heat is withdrawn they come together again to form water, or, in other words, the steam is condensed into water. It is plain, therefore, that the space between the particles is 1696 times as great in steam as it is in the water from which the steam is made. 22. =Solutions.=--When any substance, as sugar or salt, is dissolved in water, its particles are diffused through the spaces that exist between the particles of the water. So also when water evaporates (§ 12), the particles of water are diffused through the spaces between the particles of the air. In like manner are the particles from an odorous substance diffused in these spaces, and thus mingled with the particles of the air they are carried into the nostrils, and strike upon the minute extremities of the nerve of smell. 23. =Relation of Heat to the Spaces of Matter.=--The variation in the amount of space between the particles of matter in any substance generally depends on the variation of the amount of heat present. Thus heat expands iron; that is, it increases the spaces between the particles of the iron. So also heat increases the spaces between the particles of mercury, and thus makes it occupy more room in the thermometer. This effect of heat will be considered more fully hereafter. The general views which I have given of the constitution of matter will throw light upon the different qualities of different substances, some of which I will notice. 24. =Density and Rarity.=--The density of a substance depends upon the quantity of matter it contains in a given space. The more dense, therefore, a substance is the greater is its weight. A piece of lead is forty times heavier than a piece of cork of the same size. Mercury is nearly fourteen times heavier than an equal bulk of water. You see, then, that density must depend on the nearness of the atoms to each other. In so dense a substance as gold the atoms are all very close together; in wood there are spaces, some of which are so large that you can see them; and in air, steam, and the gases there is a great deal of space among the particles (§ 21), so that we speak of their _rarity_ instead of their density.