became the stronghold of Protestantism in Hainaut, but was conquered by the Spaniards, who committed all sorts of excesses. In 1656 the Spaniards under Condé made a successful defence against the French under Turenne; but in 1677 Louis XIV. took the town after an eight days' siege, and Vauban constructed the citadel. Valenciennes, which then became the capital of Hainaut, has since always belonged to France. In 1793, after forty-three days' bombardment, the garrison, reduced to 3000 men, surrendered to the allied forces numbering some 140,000 or 150,000 men, with 400 cannon. In 1815 it defended itself successfully.
VALENCY. The doctrine of valency, in chemistry, may
be defined as the doctrine of the combining power of the
atoms or elementary radicles of which compound molecules
consist. The conception that each elementary atom has a definite
atom-fixing power, enunciated by Frankland in 1852, is
the foundation of the system of rational or structural formulae
which now plays so great a part in chemical science. Frankland
dealt more particularly with the valency of the metallic
elements, in which he was specially interested at the time;
but in conjunction with his co-worker Kolbe, he subsequently
applied it to compounds of carbon. At that time (1852–56),
the application of Avogadro's theorem to the determination
of atomic weights was not yet recognized; it was only when
Cannizzaro[1] made this clear that it became possible to develop
the doctrine of valency upon a consistent basis. Kekulé,
whose services in this field rank with those of Frankland, was
the first to develop the consequences of the conception that
carbon is a quadrivalent element and to apply it in a logical
manner to the explanation of the structure of carbon compounds
generally; his paper published in 1858, “ On the Constitution
and Metamorphoses of Chemical Compounds and on the Chemical
Nature of Carbon,” is admittedly the foundation of the modern
theory of the structure of these compounds.
An admirable though brief summary of the historical development of the doctrine of valency is to be found in the lecture delivered in 1898 by Professor Japp in memory of Kekulé (Journ. Chem. Soc. 73, p. 97). Several discoveries have since been made which have an important bearing on the doctrine.
Frankland held that each element has a certain maximum valency but may manifest one or more subordinate valencies, the affinities in abeyance in cases in which only the lower valency is manifest satisfying each other mutually. By a logical extension of this view, elements have been divided into those of odd and those of even valency; apart from a few exceptional compounds, elements are to be reckoned as belonging either to the one or to the other of these two classes.
Kekulé always maintained that valency could not vary and in discussing this question Professor Japp goes so far as to say: “ Of all the doctrines which we owe to Kekulé, that of fixed valency is probably the one that has met with least acceptance even among chemists of his own school. At the present day it is, so far as I am aware, without supporters.” But he adds, “ Yet Kekulé held it to the last.” And such a fact cannot be overlooked: that Kekulé went too far in asserting that valency could not vary is probably true; the essential feature in his objection—that in many cases valency was overestimated by the Franklin school—cannot be so easily disposed of.
He saw clearly that structure is the determining factor to be taken into account in all such discussions; he also considered that it was necessary always to make use of univalent or monad elements in determining valency; moreover, that the only compounds on which valid arguments could be based were those which could be volatilized without undergoing decomposition—a condition that must be fulfilled if the molecular weight of a compound is to be placed beyond question. He therefore objected to the use of compounds such as ammonium chloride and phosphorus pentachloride as criteria of valency, as they undergo decomposition when volatilized. This objection has been somewhat robbed of its force by Brereton Baker's observation that decomposition can be prevented if the utmost care be taken to exclude moisture. In objecting to the use of such compounds, however, Kekulé took the further important step of dividing compounds into two classes—that of atomic compounds, such as ammonia and hydrogen chloride, in which the components are held together by atomic affinities; and that of molecular compounds, such as ammonium chloride, containing atomic compounds held together by molecular affinities: but Kekulé never gave any very clear explanation of the difference. Notwithstanding Brereton Baker's observations, the question remains with us to-day, the only difference being that we have substituted the more precise term “residual affinity” for Kekulé's term “ molecular affinity.”
Hydrogen is the one element which at present can be affirmed to be of unvarying valency: as no compound of determinable molecular weight is known in which a single atom of this element can be supposed to be present in the molecule in association with more than a single atom of another element, the hydrogen atom may be regarded as a consistent univalent or monad radicle. As the element of unit valency, hydrogen is, therefore, the one fit atomic measure to be used in ascertaining valency; unfortunately, it cannot always be applied, as so few elements form volatile hydrides. Hydrocarbon radicles such as methyl, CH3, however, are so entirely comparable with the hydrogen radicle that they form equally efficient standards; as many elements form volatile methides, some assistance may be obtained by the use of such radicles. But in all other cases the difficulty becomes very great; indeed, it is doubtful if a trustworthy standard can then be found—we are still forced, in fact, to recognize the wisdom of Kekulé's contentions. The greatest difficulty of all that we have to meet is due to the fact that valency is a dependent variable in the case of many if not of most elements, the degree in which it is manifest depending on the reciprocal affinities of the associating elements, as well as on environmental conditions.
Among univalent elements, carbon is the only one that appears to have a determinate maximum valency; this is manifest in methane, CH4, the simplest hydride the element forms, the first parent of the mighty host of compounds numbering thousands upon thousands which are the subject-matter of organic chemistry. Carbon, it is well known, is distinguished from all other elements by forming a great variety of compounds with hydrogen—the hydrocarbons; from these, in turn, other series of compounds are formed by the displacement of hydrogen atoms in the hydrocarbons by various radicles. The chemistry of the carbon compounds is, in fact, the chemistry of substitution compounds; no other element can be said to give rise to substitution compounds. It is because of this fact—because of the simple relationship obtaining between the various series of hydrocarbons and between these and their substitution compounds—that we are able to deduce structural formulae for carbon compounds with a degree of certainty not attainable in the case of any other element; and we are consequently able to infer the valency of carbon with a degree of definiteness that cannot be approached in any other case. Several of the simpler derivatives of carbon exhibit peculiarities which may be referred to as of particular interest. as showing how dinicult it is to arrive at any understanding of the manner in which valency is exercised. Apparently the compound represented by the symbol CH; cannot exist, all attempts to isolate it having failed, the hydrocarbon ethylene, formed by the union of two such groups, being obtained in its place. This would be in no way surprising were it not that the corresponding oxygenated compound, carbon monoxide, CO, has no tendency whatever to undergo polymerization under ordinary conditions and is, ” in fact, speaking generally, a remarkably inert substance, although in certain cases it forms compounds without difficulty—yet always in a very quiet manner. A single atom of oxygen apparently has the power, if not of satisfying, at least of stilling the needs of the carbon atom. One other case which makes the behaviour of carbon monoxide still more exceptional may be referred to, that of the analogous sulphur compound carbon monsulphide, CS, recently discovered by Sir James Dewar and Mr H. O.
- ↑ Stanislao Cannizzaro, A Course of Chemical Philosophy (1858). Alembic Club Reprints, No. 18. [1910.]
