N2O or NO [-N=0, from O=N-O-N=0 or O=N-0-0-N=0]: this reaction appears to be opposed to all ideas, however vague, which can be associated with the phrase 'free bonds.' But some chemists say that 'a double bond' is the same thing as 'two free bonds': very probably they are right; one does not venture much in asserting identity between two undefined and undefinable propositions.

We appear then to gain nothing by saying that an unsaturated molecule is one containing free bonds, unless indeed knowledge is advanced by explaining the unknown in terms of the unknowable1.

62. Lossen attempts to attach precise meanings to the expressions 'saturated' and 'unsaturated' molecules. A saturated molecule, he defines to be, one in which each polyvalent atom directly acts on, and is acted on by, its maximum number of monovalent atoms (see formula, p. 139, par. 70). An unsaturated molecule, he defines to be, a molecule in which one, or more, polyvalent atom acts directly on, and is acted on by, less than its maximum number of monovalent atoms. Saturated molecules, as thus defined, can combine only with polyvalent atoms, such combination being preceded by rearrangement of the mutual direct atomic actions: unsaturated molecules are able to combine directly with monovalent atoms. As an example of an unsaturated molecule, we may take the compound C.H, Granting that the carbon atom is tetravalent, it follows that the molecule CH, is unsaturated, because, whether we suppose the mutual atomic actions to be represented by the formula

1 Lossen points out that molecules described as 'containing free bonds' can usually take part in reactions wherein condensation of volume occurs, e.g. CO+Cl=COCI, 2CO+O,=2CO2, 2NO+O, tures 2NO+0=2NO2), 4NO+O1 = 2N2O3, &c. 4 of this chapter.]

NO, (or at higher tempera

[See also appendix to Section

at least one carbon atom is directly combined with less than its maximum number of monovalent atoms. CH, can combine with monovalent atoms, e.g. it forms C,H,Br. The compound CH, affords an example of a saturated molecule. As the valency of the carbon atom is four, CH, is necessarily saturated, because, however the interatomic actions are represented in a formula, each carbon atom must be regarded as combined with its maximum number of monovalent atoms. The molecule CHO can be produced from CH, but the reactions which occur in this change, and also the properties of C2HO, shew that the interatomic actions are differently arranged in the two molecules C2H, and C2H ̧O1.

63. It is evident that the valency of only a minority of the elementary atoms can be considered as fairly well established. In order to determine the valency of an elementary atom we ought to have several gasifiable compounds of that element with elements whose atoms are monovalent, the molecules of such compounds containing not more than one atom of the given element. When we know of but one such compound we are unable to fix the valency; we may however say that the valency of the atom is probably an odd or an even number according as its valency in the given

H-C-C-H, and H-C-C-O-H or H-C-O-C-H

2 Thus the valency of Al cannot be determined from the molecule AlCl ̧. The formula AlCl might be written

the valency of the aluminium atom varying from 1 to 7. The first of these is most probably correct, considering the general properties of the molecule AlCl, but the evidence is not sufficient to decide that Al belongs to the group of tetravalent atoms.

molecule is an odd or even number'; and we may also conIclude that the number which expresses the valency of the given atom in the special molecule under consideration will also express its valency in many other molecules. Although the valency of an atom has been determined from a consideration of several molecules containing that atom, it is still possible that this number does not express the true valency; but until the number is proved to be too small it must be used as the true valency in all questions regarding structural formulæ of molecules containing the given atom3.

one

When no molecule containing monovalent atoms, combined with a single atom-or even with more than atom of a given element can be obtained, any number assigned as the valency of the atom of that element must be very doubtful.

Many non-gasifiable compounds containing monovalent atoms combined with atoms of a single other element are known (e.g. many metallic haloid compounds): if the molecular weights deduced for these compounds by the aid of considerations such as those sketched on pp. 74-77 are assumed to be the true relative weights of the molecules of these solid compounds, and if those generalisations which have been made concerning the arrangement of atoms in gaseous molecules are assumed to hold good for the molecules of solids also, then the valency of many elementary atoms not included in the table on p. 121 could be determined. Thus, if we assume that the general formula MX represents the atomic structure of the molecules of the solid haloid salts of the alkali metals (M = K, Na, Li, &c. and X = F, Cl, Br, or I) then the atoms of these metals are most probably monovalent. Most of the generally accepted formulæ for salts of alkali metals may be written with the atoms of these metals represented as each in direct com

1 When 'valency of an atom' is spoken of without mention of the valency in a particular molecule, the expression is always to be understood as defined on p. 122, see also p. 127.

2 If this rule is not attended to endless confusion arises, and the whole theory of valency becomes merely an amusing exercise of fancy.

bination with only one other atom, but whenever this arrangement has become somewhat unsatisfactory chemists have not hesitated to assume that the atoms of the alkali metals may be tri- penta- or even heptavalent, i.e. may each act on, and be acted on by, 3, 5, or 7 other atoms. So with other elements; from a consideration of solid or liquid compounds only no trustworthy conclusions as to the valencies of the atoms in the molecules of these compounds can be deduced. It is so easy, after making the two fundamental assumptions stated above, to make an indefinite number of further assumptions; it becomes so pleasant to manipulate formulæ on paper, that it is certainly better-in the present state of knowledge to apply the theory of valency only to gaseous molecules. It is very probable that the valency of the elementary atoms varies periodically with variations in the relative weights of these atoms: if this general statement is thoroughly established, the exact nature of the periodic function is determined, and the true atomic weights of all the elements are fixed, we shall have in the Periodic Law a most important method for determining valencies. But a great deal of work must be done before this 'law' can be applied otherwise than generally and tentatively to questions of valency (see chap. III. par. 115).

SECTION IV. Allotropy and Isomerism.

64. Having gained the conception of a molecule as composed of atoms, each directly acting on, and being acted on by, a definite number of other atoms, we at once regard the molecule as a structure; we recognise what Frankland in 1852 happily called 'limited molecular mobility.' A structure involves arrangement of parts, subordination of less to more important parts; it supposes the existence of definite actions for fulfilling which the structure is adapted; in a word, structure means correlation of properties and material configuration'.

1 When arrangement of atoms in the molecule' is spoken of, or when a similar phrase is used, it is to be taken as implying only a rough approximation

And when we consider the properties of individual molecules the justness of thus regarding each as a definite atomic structure becomes more apparent. We find many compound molecules containing the same number of the same elementary atoms yet exhibiting markedly different chemical and physical properties, i. e. we find the phenomenon of Isomerism: how can we account for this except by assuming (1) that each molecule has a definite atomic structure, and (2) that the same atoms may be differently arranged in different molecules?

65. A knowledge of the atomic configurations of series of molecules, supposing this to be gained, must be supplemented by a knowledge of the way in which the energy of each molecule varies with variations in the configuration and motion of its constituent atoms, before a complete knowledge of the dynamical properties of these molecules is possible. But chemistry is yet far from this goal; she is obliged to be content with a very partial and sometimes very vague knowledge concerning the relative atomic configurations of a few molecules; she has hardly entered on the second part of her task.

66. Granting then that variations in the properties (chemical and physical) of molecules accompany variations in the atomic configurations of these molecules, it is conceivable, that the latter variations may consist of

(1) variations in the relative positions of the atoms,
(2) variations in the distances between the atoms, their
relative positions being constant.

To illustrate this point, let us take the molecule C2H¿O. More than one compound exists the molecules of which have the atomic composition expressed by this formula. On the first assumption, viz. that variation of properties is to be correlated with variations in the relative positions of the

to a knowledge of atomic arrangements. Structural formulæ sum up facts of formation and decomposition, and, assuming the fundamental positions of the molecular theory, exlfibit, in a rough and general way, connections between these facts and the directions of the mutual actions of the atoms in the molecules of the compounds formulated. No attempt is made in these formula to express quantitative measurements of atomic interactions.