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particles of a specified liquid or solid is probably not the same for all the particles, nor even for the same particle at different times.

It is customary to apply the term molecule to the particles of liquids and solids, as well as to the much more rigidly defined particles of gases. But no generalisations can at present be made, in terms of the molecular theory, regarding the properties of liquids and solids comparable with those which have been made for gases, and which are known as the laws of Boyle, Charles, and Avogadro. We cannot define the term molecule as applied to a liquid or solid body; we can define the term when applied to a gas.

When therefore we speak of the molecular weight of an element or a compound we ought to mean the relative weight of the molecule of the gaseous element or compound. The expression molecular weight is not always used in chemistry in this strict meaning; it is frequently applied to what in former chapters we have called the reacting weight of a body.

To take an example. No compounds of sodium have been gasified; therefore we do not know the molecular weights of any compounds of this element. But the atomic weight of this metal has been determined by applying the method of specific heat. The simplest formulæ which can be given to the compounds of sodium, when we know that Na=23, are Na,O, NaCl, NaBr, NaNO,, Na,CO,, Na,SO,, &c. Moreover these formulæ enable us to express the chemical reactions of the compounds in a consistent and satisfactory manner. These formulæ are therefore adopted. But we must carefully observe that the formulæ do not necessarily express the atomic compositions of molecules of the compounds: indeed we cannot in strict accuracy speak of a molecule of sodium oxide, or sodium chloride, because these compounds are only known as solids and the term molecule corresponds to an accurately defined conception only when it is applied to gases.

We formerly used the term reacting weight of a compound; thus Na,O represents the composition of a reacting weight of sodium oxide. We may now, in the light thrown on chemical interactions by the molecular and atomic theory, widen the meaning we give to this term reacting weight. Although Na,O does not certainly represent the atomic composition of a molecule of sodium oxide, yet it almost certainly represents the ratio of the number of atoms which constitute the reacting

weight of this oxide. The reacting weight of a compound may now mean for us a group or collocation of atoms which interacts chemically with other groups of atoms. The formula of a solid or liquid compound does not necessarily express the number of atoms in this chemically reacting group of atomsindeed the number may vary under different circumstancesbut in all probability it does represent the ratio between the number of atoms in this reacting group. The atomic composition of the reacting weight of sodium oxide may perhaps be better represented by one of the formulæ Na,O,, Na ̧ ̧, Na,O,, than by the simpler formula Na,O; but

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CHAPTER XVI.

APPLICATIONS OF THE MOLECULAR AND ATOMIC THEORY,
CHIEFLY TO CLASSES OF FACTS AND PRINCIPLES
ALREADY CONSIDERED.

THE molecular and atomic theory asserts that a quantity of any gaseous element or compound is constituted of a very great number of minute particles, all having the same masses and the same properties, and all in constant motion. These particles, or molecules, are constituted of smaller particles which have a certain freedom of motion among themselves; these smaller particles, or atoms, are of one kind and of equal masses when the molecule formed by their union is the molecule of an element; but the atoms are of different kinds and different masses when the molecule formed by their union is the molecule of a compound.

Chemical change, according to the molecular and atomic theory, is an interaction between molecules, and it results in the formation of new molecules. In very many cases of chemical change the interacting molecules are separated into their constituent atoms, and these atoms rearrange themselves to form new molecules; but in some cases the interaction of the original molecules probably consists in the direct formation of more complex molecules. Thus the interactions of hydrogen and chlorine to produce hydrogen chloride, and of hydrogen and oxygen to produce water-gas, are represented thus by the theory of atoms and molecules :

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(1) H2+ Cl2 = 2HC1; (2) 2H,+02 = 2H2O. The symbols H., Cl, O, HCI, H2O, each represents the atomic composition of a molecule of an element or compound. But the interactions of water and cobalt chloride, or water and copper sulphate, are probably best represented by equations which assume the

change to consist in the combination of molecules of the interacting substances to produce more complex molecules: thus, (1) CoCl + 2H,O= CoCl,. 2H,0;

(2) CuSO, +5,0 = CuSO,.5,0.

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The theory regards most physical changes as changes in the 313 rates of motion, without changes in the atomic compositions, of molecules. But changes usually called physical may result in the coalescence of molecules into more or less complex aggregations which are stable under definite conditions of temperature, pressure, &c.

The theory of molecules and atoms does not therefore give us a means of sharply distinguishing between physical and chemical change. The typical chemical change results in a redistribution of the atoms of the interacting molecules so as to form new molecules; the typical physical change results in changes in the rates of motion of molecules without any redistribution of the parts of these molecules. But there are many changes which cannot be placed wholly in one or other of these classes. Every chemical change is accompanied by physical change: the portion of the change we call chemical is only one part of the complete occurrence. Even if the theory gave a sharp and clear definition of each kind of change, it could not give a means whereby we might classify all actually occurring changes into chemical on the one hand, and physical on the other.

The laws of chemical combination find a simple explana- 314 tion in terms of the molecular and atomic theory. The atom is the ultimate particle of matter of which we take cognisance in chemistry. The properties of a molecule depend, among other conditions, on the nature and number of the atoms which form it; this is the law of fixity of composition. If two or more different kinds of atoms combine to form several different molecules, each molecule must be composed of x atoms of one kind +x atoms of another kind +x" atoms of another kind +x"" atoms of another kind &c., and x, x', x', x" must be whole numbers, because the atom is, by definition, indivisible; this is the law of multiple, and the law of reciprocal, proportions.

The molecular and atomic theory throws light on the 315 conceptions of combining and reacting weights. The reacting weight of a gas is the molecular weight of that gas. The combining weight of an element, as the term was defined in Chap. VI. par. 79, is the atomic weight of that element.

Thus we found (s. Chap. vi. par. 86) that the reacting weight of water is 18; and that one reacting weight of this compound is composed of two combining weights of hydrogen united with 1 c. w. of oxygen. Translated into the language of the molecular and atomic theory this statement reads as follows:-the molecular weight of water-gas is 18, and a molecule of water-gas is formed by the union of 2 atoms of hydrogen with 1 atom of oxygen.

We know that 36.5 parts by weight of hydrogen chloride are formed by the combination of 1 part by weight of hydrogen with 35.5 parts by weight of chlorine. In order to express this fact in terms of combining and reacting weights, we say that one c. w. of hydrogen combines with one c. w. of chlorine to produce one reacting weight of hydrogen chloride. The molecular and atomic theory expresses the same fact by saying that one molecule of hydrogen interacts (not combines) with one molecule of chlorine to produce 2 molecules of hydrogen chloride.

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We formerly applied the term reacting weight to compounds only. We now apply the term molecule to elements as well as to compounds. But when we are dealing with solid bodies which have not been gasified, we cannot in strict accuracy speak of the interactions of molecules of these bodies. Thus when boron and aluminium are strongly heated together under proper conditions two compounds, AlB, and AlB1 are produced. As neither boron nor aluminium has been gasified, and as neither of the borides of aluminium has been gasified, we do not know the molecular weights of any of the bodies taking part, or formed, in this reaction: we cannot therefore say how many molecules of each element have taken part in the change nor how many molecules of each compound have been formed. Again, when solutions of barium chloride and sodium sulphate, in aqueous solutions, are mixed in the ratio BaCl, Na,SO1, the change of composition which occurs may be represented thus:

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BaCl,Aq+Na,SO,Aq= BaSO, + 2NaClAq. We may read the equation as meaning:-one reacting weight of barium chloride interacts with one reacting weight of sodium sulphate to produce one r. w. of barium sulphate and 2 r. ws. of sodium chloride; but the equation cannot be read as certainly meaning, one molecule of barium chloride interacts with one mol. of sodium sulphate to produce one mol. of barium sulphate and 2 mols. of sodium chloride. As none of the

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