assume that matter is composed of other particles, called molecules. Only by some such assumption can they explain (for the present, at least) the properties of gases as expressed in the laws of Boyle and Charles (cf. §§ 38 and 40). We reconcile the molecular hypothesis with the atomic hypothesis by assuming that the molecules are usually more complex than the atoms; that they are, in fact, composed of atoms. Atoms are thus thought of as the smallest possible particles of the elements (see, however, § 441) that can take part in a chemical reaction, while the molecule is the physical unit of matter, i. e., it is the smallest portion of any substance, simple or compound, that can exist by itself.

To illustrate: The physical properties of water are determined by the properties of the water molecules; it is only when we subject water to a chemical change that we divide the molecule. Then it is that the properties of the oxygen and hydrogen atoms come into play.

The molecules of elements consist of atoms of only one kind; while the molecules of compound substances contain atoms of two or more kinds.

99. Properties of Molecules.

Molecules are very small: Lord Kelvin estimates that 1 c.c. of a gas, under standard conditions, contains not less than 100 million million million of them (100,000,000,000,000,000,000). The distances between the molecules of gases are about 1,000 times as great as the diameters of the molecules. When gases are compressed, it is the distance between molecules that is made smaller, not the molecules themselves. By "distance

THE MOLECULAR THEORY.

85

between molecules " we mean average distances, for molecules are not at rest, but in motion. Molecular motion is heat. The rapidity of the motion determines the temperature. The cause of the motion is the inherent energy of the molecules. The direction of the motion is along straight lines, except as the molecules collide with one another, or with the walls of the containing vessel. Molecules " occupy space" much as a person "occupies " a room, not by filling it completely, but by moving about in it.

100. The Molecular Theory and Physical State. The theory of molecules explains the differences between the gaseous, liquid, and solid states. The energy of the particles of a gas is so great that the gas "fills" any space presented to it, and exerts pressure upon the walls of a containing vessel. The attraction between molecules (cohesion) is slight in gases. Because of molecular motion, gases do not remain in layers according to their densities, but mix in all proportions, giving a mixture of uniform composition. The air is such a mixture (cf. § 200).

In the case of liquids, the cohesion between molecules is greater than with gases, yet the molecules move about freely enough to permit the liquid to take the form of the containing vessel. Because the molecular energy is less, diffusion is more slow than with gases, and the extent to which it can take place is often limited. Thus, ether dissolves in water, and water in ether, up to a certain limit (cf. § 82).

In solids, cohesion predominates, and the molecules do not usually alter their relative positions. As a result, a solid has a definite form. Molecular motion has not ceased, however, although solids diffuse into other solids very slowly. A case of

"solid diffusion was the formation of the alloy, solder (cf. §§ 512 and 515), from its constituents without melting. The metals were subjected to a pressure of about 6,000 atmospheres, in steel molds, and diffused into each other, as gases do at ordinary pressure. The solder had the same properties as that made by melting the constituents together.

IOI. Exercises.

1. Give the origin and meaning of triturate, deliquesce, effloresce, conservation, marsh gas, atom, molecule, hypothesis, theory, cohesion, adhesion.

2. In which of the cases given in this chapter does an element have more than one equivalent weight?

3. What theory accounts for the effects of temperature and pressure upon the volumes of gases? How does this theory explain the evaporation of liquids? The dissolving of solids? Why, on the basis of this theory, must crystals be allowed to form slowly if they are to be large and perfect (cf. § 86)? Why do solids generally dissolve more readily in hot water than in cold, while gases do not?

4. Look up, in the Appendix, the atomic weights of sodium, potassium, chlorine, oxygen, hydrogen, carbon. Why are the figures somewhat different in the two columns?

CHAPTER VIII.

EQUATIONS AND NOMENCLATURE.

In

102. Symbols, Formulas, and Equations. place of the names of substances chemists use symbols and formulas. A symbol is a capital letter, or a capital and a small letter, that stands for an element. The letter used is the initial letter of the name of the element. When two names begin with the same letter a second letter is added to distinguish them. Thus the symbols C, Cl, and Ca stand for carbon, chlorine, and calcium respectively. Some symbols are derived from the Latin or Greek, e. g., Hg for mercury.

A compound is represented by the symbols of its elements written side by side. The resulting expression is a formula. Thus HgO stands for mercuric oxide, and HCl for hydrogen chloride.

An equation stands for a reaction. It is made up of the symbols and formulas of the factors and the products (cf. § 5).

is,

Thus, the equation for the reaction between sodium and water

Na+H2O→→→ NaOH+H.

Na is the symbol of sodium (from natrium); H, that of hydrogen; O, that of oxygen. The formula H2O, for water, shows that water is composed of hydrogen and oxygen; while the formula NaOH, for sodium hydroxide, shows that sodium hydroxide is

a compound of sodium, oxygen, and hydrogen. The sign is read "give," or " produce," and the sign + is read "and." The equation is read in the direction of the arrow. Two arrows are generally used in an equation showing replacement or double decomposition (cf. § 92); one to show the direction in which the equation is to be read, and the other, what becomes of the products. The equation given above thus becomes,—

The arrow pointing upward indicates a gas; one pointing downward, a precipitate. When nothing is removed, as when the products formed by mixing two solutions are soluble, the double arrow is often used (cf. § 242).

103. Quantitative Meaning of Equations. - An equation stands for a reaction not only qualitatively, but quantitatively, that is, it represents not only what substances are taken and produced, but also their proportions, by weight. We give this quantitative meaning to the equation by letting the symbol represent not only the element in general, but also an atom of it. Then, since each element has a definite atomic weight, the symbol means, also, an atomic weight of the element. Finally, since our common unit of weight, in Chemistry, is the gram, the symbol is allowed to represent a gramatomic weight of the element, i. e., as many grams of it as there are units in its atomic weight. Thus, the symbol O stands for oxygen, for an atom of oxygen, for an atomic weight of oxygen, and for a gramatomic weight of oxygen, . e., 16 grams.

i.