entering into combination with hydrogen; in which case, their valency is measured by the number of atoms of some other monovalent element which is capable of satisfying their combining capacity. Thus :

I atom of sodium combines with 1 atom of chlorine, forming NaCl.

In the combinations of elements with hydrogen alone, no instances are known in which a higher valency is exhibited than that of four; but with chlorine, as here seen, cases are known in which elements exhibit pentavalent and hexavalent characters.

Measured by their combining capacity for hydrogen and chlorine, elements do not, however, always exhibit the same valency: thus, the affinity of phosphorus for hydrogen is satisfied by three hydrogen atoms, whereas one atom of this element can unite with five atoms of chlorine.

As measured by hydrogen, the valency of sulphur is two, the compound that it forms with hydrogen being expressed by the formula SH2, while, as estimated by its capacity for chlorine, it becomes tetravalent, as seen in the compound SCI. As a general rule, however, the highest number of monovalent atoms with which one atom of an element is capable of combining is accepted as representing the valency of that element. Thus, one atom of phosphorus not only combines with five atoms of chlorine, but also with five atoms of fluorine; phosphorus is therefore a pentavalent element.

As measured by hydrogen alone, or by chlorine alone, nitrogen is a trivalent element, for the largest number of these atoms with which one atom of nitrogen can unite is three, as seen in the compounds having the composition NH, and NC; nevertheless, one atom of nitrogen is capable of combining with four atoms of hydrogen and one of chlorine, forming the compound NH,Cl, ammonium chloride, in which the nitrogen atom is pentavalent.

This rule, however, is not always followed; for example, one atom of iodine will unite with three atoms of chlorine, forming the

* Phosphorus also combines with hydrogen.

compound ICl, but iodine is not generally regarded as a trivalent element.*

In symbolic notation, this power possessed by an atom, of uniting to itself monovalent atoms, is often represented by lines, each line signifying the power of combination with one monovalent atom. Thus, in the symbol H-Cl, the line is intended to give a concrete expression to the fact that both hydrogen and chlorine are monovalent elements, and that the affinity of each element for the other is satisfied when one atom of the one unites with one atom of the other. The symbol H-O-H, in like manner, signifies that the oxygen atom is divalent, that its affinity for hydrogen is satisfied only when it has united with two monad atoms. In the same way we may express the facts that nitrogen and carbon, in their combinations with hydrogen, are respectively trivalent and tetravalent, H

by the symbols H-N-H, and H-C-H. These lines are merely

a convenient symbolic expression for the operation of the force of chemical affinity; their length and direction bear no meaning.t The power to combine with one monovalent atom is sometimes spoken of simply as one affinity: thus it is said that in the compound having the composition PH3, or H-P-H, three of the

H

affinities of the phosphorus atom are saturated, and that two affinities still remain unsatisfied, phosphorus, as already stated, being a pentavalent element.

* See Iodine, Compounds.

+ The student cannot be too often warned against attaching any materialistic significance to these lines. The use of this convention is always attended with the danger that the beginner is liable to fall into the error of regarding these lines as representing in some manner fixed points of attachment, or links, between the atoms. It must be remembered, therefore, that these lines not only have no materialistic signification, but they must not even be regarded as conveying any statical meaning. The atoms are undergoing rapid movements with respect to each other, which movements are in some way governed by the chemically attractive force exerted by the individual atoms upon one another; and the molecule will be more correctly considered, if we regard its atoms as being held together in a manner resembling that by which the numbers of a cosmical system are bound together. The lines simply denote that the atoms are held to each other by the attractive force which we call chemical affinity.

Compounds of this order, in which one of the elements has still unsatisfied affinities, are called unsaturated compounds.

In its power to satisfy the affinities of an element, a divalent atom is equal to two monovalent atoms: thus, when the affinities of the tetravalent carbon atom are saturated with oxygen, the molecule contains two atoms of oxygen, which may be symbolically expressed thus, O=C=O, in which the four affinities of the carbon (represented by the four lines) are satisfied by the two divalent atoms of oxygen. Carbon, however, combines with a smaller proportion of oxygen, forming the compound carbon monoxide, CO. The carbon atom in this case is divalent, as expressed by the formula C=O, and this substance is also an unsaturated compound.

The number of divalent atoms with which an element can unite cannot, however, be taken as a safe criterion or measure of the valency of that element in cases where that number is greater than I; for example, in such a compound as calcium oxide, CaO, we regard the two affinities of the divalent atom of oxygen as being satisfied by two affinities possessed by the calcium, and express this belief in the formula Ca=O, and regard the calcium as divalent. In the same way, in carbon monoxide, CO, the carbon being united with one atom of the divalent element oxygen is itself divalent in this compound; but in the case of carbon dioxide, where the carbon atom is united with two atoms of divalent oxygen, we are not justified in asserting that the atoms are united, as represented by the formula 0 = C = 0, in which the four affinities of carbon are represented as saturated with oxygen. There exists no positive proof that the carbon is not divalent in this compound, and that the molecule does not consist of three divalent atoms united,

as shown in the formula

C

From the fact, however, that car

bon forms a compound with four atoms of hydrogen, and another with four atoms of chlorine, we know that this element is tetravalent, and therefore we believe that in carbon dioxide it is also tetravalent.

Again, as measured by its compound with hydrogen, sulphur is divalent; while with chlorine it forms SCI. But sulphur unites with oxygen, forming the two compounds sulphur dioxide, SO, and sulphur trioxide, SO3. If it be assumed that in these molecules the

whole of the oxygen affinities are satisfied with sulphur, then the symbolic representation of these oxides will be O SO, and O=S=0, the sulphur being in one case tetravalent and in the

other hexavalent. There is, however, no positive proof that the affinities of one oxygen atom are not partially satisfied by union with another oxygen atom, and that the valency of the sulphur is higher than either two or four, as seen in the alternative formulæ, S S

Although there are no known compounds in which an atom of sulphur is united with six monovalent elements, sulphur is regarded by many chemists as capable of fulfilling the functions of a hexavalent element.*

It will be evident from these considerations, that in many cases the valency of an element is a variable quantity, depending partly upon the particular atoms with which it unites. It is also found that it is dependent in many instances upon temperature and upon pressure. Thus, between a certain limited range of temperature, one atom of phosphorus combines with five atoms of chlorine in the compound PC, but above that limit two atoms of chlorine leave the molecule, and the phosphorus becomes trivalent. Again, if phosphoretted hydrogen, PH3, be mixed with hydrochloric acid, HCl, and the mixed gases be subjected to increased pressure, the gases combine and form a solid crystalline compound known as phosphonium chloride, PH,Cl, in which the phosphorus atom, being united with five monovalent atoms, is pentavalent. When the pressure is released an atom of hydrogen and an atom of chlorine leave the molecule, and the phosphorus returns to its trivalent condition.

A compound, in whose molecules there is an atom which for the time being is not functioning in its highest recognised valency, often exhibits a readiness to unite with additional atoms to form new compounds: thus, ammonia combines eagerly with hydrochloric acid, forming ammonium chloride

NH2+ HCl = NH1Cl.

Quite recently, however, Moissan has obtained a perfluoride of sulphur, SF, in which, therefore, the sulphur atom must be regarded as hexavalent.

Carbon monoxide unites directly with chlorine to form carbonyl chloride

CO + Cl2 = COCI.

Carbon monoxide also combines with an additional atom of oxygen, and gives carbon dioxide, thus

2CO+O2=2CO2.

In this last action it will be seen that the molecule of carbon monoxide, in being converted into the dioxide, takes up one atom of oxygen; but as the molecule of oxygen is the smallest isolated particle, it follows that the two atoms contained in such a molecule must first separate, and each one then furnishes the requisite additional oxygen for one molecule of carbon monoxide. In the union of carbon monoxide with chlorine, and of ammonia with hydrochloric acid, are we to suppose that the same action takes place? That is to say, do the two atoms in the molecule of chlorine separate from each other and unite with carbon, thereby satisfying its tetrad valency, in the manner here expressed?

And in the case of ammonia and hydrochloric acid, do the hydrogen and chlorine atoms part, and each unite with the nitrogen atom, thereby raising it from the trivalent to the pentavalent condition? thus

Or are we to suppose that the two molecules, without losing their integrity, become held together as independent molecules, by virtue of the unsatisfied affinities of the carbon, or the nitrogen, as the case may be, in which case the compounds might be represented thus

This question would be settled by determining the vapourdensity of the compound. If, for instance, we were to find the