always divisible into four parts. What is to be the unit of chemical work? the mass of matter fixed by a given atom? where then is the equivalency between one atom of oxygen with the mass 16 and two atoms of chlorine with the mass 71? Let a carbon atom combine with four hydrogen atoms, the total chemical energy of the atoms disappears; let a carbon atom combine with two atoms of oxygen, the total chemical energy of the atoms again disappears: but if the carbon atom possesses four units of affinity,' the oxygen atom two units of affinity,' and the hydrogen atom one unit of affinity,' the heats of formation of the two compound molecules ought to be equal. But the differences between the heats of formation of carbon compounds shew that the expression 'the carbon atom 'has four units of affinity' cannot mean that the chemical energy of the carbon atom is divisible into four parts, unless indeed the unit of affinity is variable, and is varied for each combination of carbon with other atoms1.

(4) The carbon atom has four equivalencies. Can this mean that the atom exerts force in four directions? A socalled 'valency' is then a direction. But there is no force exerted till the mutual atomic transaction begins; the carbon atom considered alone has therefore no 'valencies.' Take the molecule CO, force is exerted by the carbon on the oxygen atom; the remaining 'valencies' are sometimes said to be 'mutually satisfied,' i.e. on the present hypothesis, the carbon atom in the molecule CO exerts force in two directions on itself; but here again we have the hypothesis of the nonhomogeneity of the carbon atom, and the existence of active and inactive parts in that atom.

(5) In the vibration of a carbon atom there are four points, at each of which mutual action can occur between this atom and another atom. On this supposition, a 'double link' would mean that mutual action occurs between the two atoms thus linked at two of these positions; e.g. the formula O=C=O means, that in performing a vibration the carbon atom acts twice on (and is twice acted on by) each oxygen

1 For a view analogous to this see appendix to Section 4 of the present chapter, par. 98.

atom. But if so, surely a 'double link' would imply molecular stability, whereas it frequently means the reverse'.

The theory of units of affinity, or valencies, or bonds, has been carried too far. It appears at first sight to give a dynamical explanation of the structure of molecules, but it has forgotten the two-sidedness of atomic transactions; it apparently affords a means of measuring atomic forces, but it has used a unit, undefined except as a quantity changeable at pleasure; it appears to simplify chemical formulæ, but it has really made them harder to understand by grafting on to the definite conception of atom the vague and unnecessary notion of 'bond.' When the molecule has been analysed and shewn to be an atomic structure, the theory of bonds has attempted to reconstruct the building, not by putting together the parts into which it had been separated, but by the use of new untried material called 'bonds,' the properties of which -if it has any are unknown.

The theory of 'valencies' has gone too far because it has not gone far enough; it has not clearly distinguished the atoms of Dalton from the equivalents of Wollaston. In 1858 Kekulé recalled chemists to the consideration of elementary atoms as the fundamental units of which chemical compounds are built up; twenty-two years later Lossen has recalled the followers of Kekulé to the same all-important fact.

60. Let us turn back to the facts on which was based a classification of many elementary atoms into monovalent, divalent, tri, tetra, penta, and hexvalent groups (p. 121).

The atom of tin is divalent in the molecule SnCl, the atom of tin is tetravalent in the molecule SnCl, these statements are more shortly expressed by the graphic formula

1 On the subject of ‘double-bonds' see also appendix to Section 4 of this chapter. 2 It is important to distinguish between the expressions 'valency' and ‘a valency.'

respectively; a line (−) joining two atoms is used to denote direct action and reaction between these atoms.

As thus interpreted, the statement, 'a given atom is mono-, 'di-, tri-, &c. valent in this or that molecule,' has a definite and defined meaning.

Lossen (loc. cit.) insists on the necessity of naming the molecule in which a given atom occurs, when the valency of that atom is stated. Such a general statement as 'the atom of carbon is tetravalent' must be taken as meaning 'one atom 'of carbon, so far as we know at present, is never directly 'combined with more than four other atoms,' or 'four is the 'maximum number of atoms which can come within the ""binding-sphere" of a carbon atom in any molecule' (Lossen). The special statements, 'in the molecule of carbon dioxide 'the carbon atom is divalent,' in the molecule of carbon 'monoxide the carbon atom is monovalent,' mean, that in one molecule the carbon atom acts directly on-and is acted on by-two other atoms, and in the other molecule on one other atom; or, in the first molecule there are two atoms, and in the second molecule one atom, within the binding-sphere of the carbon atom.

As illustrations of this-Lossen's-way of regarding valency, let us take the molecule POCI,. 'In this molecule the ' phosphorus atom is trivalent:' the formula

expresses this statement completely; there is direct action, and reaction, between the phosphorus atom and three other atoms, and as the chlorine atom is always monovalent, one of the three atoms must be oxygen. But it is sometimes said, ' in phosphoryl chloride the phosphorus atom is pentavalent,' and the formula

is used as expressing this statement. But this is equivalent to saying, the phosphorus atom has five 'bonds,' three of which are satisfied' by chlorine atoms, and two by an oxygen atom. The objections to such a statement have been already considered. Assuming that there is direct action, and reaction, between the phosphorus atom and each of the other atoms comprising the molecule POCI,, Lossen's formula would express the structure of this molecule thus

This formula would be translated into the corresponding system of nomenclature by saying, 'in this molecule the 'phosphorus atom is tetravalent.'

Again, the formulæ of carbon monoxide and dioxide are generally written <C=O and O=C=O respectively, and the carbon atom is said to be tetravalent in both, i.e. in each the carbon atom has four 'bonds,' in CO two are satisfied by oxygen and two satisfy one another, in CO, on the other hand each oxygen atom satisfies a pair of bonds. Lossen would write the formulæ of these molecules as C-O and O-C-O, and say, the carbon atom is monovalent (i.e. acts directly on a single atom) in the first, and divalent (i.e. acts directly on two atoms) in the second'. The first pair of formulæ almost necessarily implies that the force between the carbon and the oxygen atoms in CO is equal to that between the carbon and each oxygen atom in CO,, and this dynamical conception is strengthened by the use of such expressions as the 'bonds are satisfied,' &c. No such assumption is made by Lossen's formulæ. Most probably the force between any pair of atoms varies in different molecules in which this pair of atoms is present; whether this is so or not, and if it is so, whether the force is greater in molecule a than

1 Of course CO, may be written O-C-O; whether there is or is not direct action between the oxygen atoms must be determined by a general study of the chemical habitude of the molecule.

in molecule b (or vice versa), is a dynamical question which cannot be solved, at present, by the theory of valency; it is a question outside of this theory; and it is surely better to recognise this, and, especially in view of the masses of new facts and new hypotheses which are showered on chemists, to make the theory of valency definite, even if this be done by narrowing its scope.

61. But it is said, CO is an unsaturated, CO, a saturated molecule. What then it may be asked is a saturated molecule? A saturated molecule, it is usually answered, is one which exhibits no tendency to combine directly with other molecules, or atoms; an unsaturated molecule on the other hand is ready to form additive compounds. Now the molecule CO readily combines with Cl, to form the new molecule COCI,', therefore CO is an unsaturated molecule: to this it may be answered, with Lossen, that as Cl, readily combines with CO, Cl, is an unsaturated molecule. Definitions so indefinite as 'readiness or unreadiness to form additive compounds' do not help us to understand the apparently precise formulæ, e.g. <C=O and O=C=O, in which these definitions are expressed. The expressions 'unsaturated molecule' and 'molecule with free bonds' are frequently used as synonymous; if we can attach a precise meaning to the latter expression we shall have gained the definition we are seeking. The molecule CH, very easily combines with bromine to form CH,Br2, that is to say, CH, acts as an unsaturated molecule, and therefore contains 'free bonds'; but the generally adopted

CH2

formula,|| represents the two carbon atoms as joined

CH2

by a double bond; we should expect this molecule to be very unready to form an additive compound. Moreover molecules supposed to contain 'free bonds' are sometimes very easily produced from others containing only 'satisfied bonds'; e.g. NO is formed by the action of water on 1 This reaction is usually represented thus:

< C=O+Cl - CI =

Cl-C=O
1

M. C.

Cl.

9