498

position of the specified element in the complete scheme of classification.

The periodic law seeks to connect the changes in the properties of the elements, and in the compositions and properties of compounds, with the changes in the atomic weights of the elements; and it endeavours to make this connexion definite and to present it in accurate terms.

If the atomic weight of an element is known, the place of the element in the classificatory scheme is determined, and therefore the properties of the element and its compounds can be stated in a general way. If the properties of an element and its compounds are determined, the position of this element in the orderly sequence of elements can be found, and thereby an approximately correct value can be deduced for the atomic weight of the element. We have had examples of the application of the periodic law both to the classification of elements and to the determination of the best values of the atomic weights of elements.

The valencies of the atoms of the elements are undoubtedly important factors determining the compositions of compounds. These valencies probably vary periodically with variations in the atomic weights of the elements; but the valencies of so few elements have been definitely determined that we are not at present able to state the connexions between changes of atomic valencies and of atomic weights.

The maximum valencies of all the elements in Series 2, except lithium, have been determined by considering the compositions of gaseous molecules of compounds of these elements with monovalent atoms. The valencies are as follows;—

The atom of lithium is probably monovalent; assuming this atom to be monovalent, we see that the maximum valency of the atoms of the elements of Series 2 increases from the first to the middle member, and then decreases to the last member.

We cannot assert that maximum atomic valency varies in all the series exactly as it does in Series 2; nevertheless, the assumption that it does thus vary would certainly in some cases lead to results which are confirmed by observation.

Thus, the assumption requires the atoms of the elements in Group IV. to be tetravalent; the maximum valencies of the atoms of all the members of this group, except cerium, have been determined and the atoms have been found to be tetravalent. On the other hand, the assumption requires the atoms of the elements in Group VI. to be divalent; but the existence of the gaseous molecules MoCl., TeCl,, WC, UCI, proves that the maximum valency of some at least of these atoms is greater than two.

The conception of atomic valency-the conception, that is, that every atom in a molecule directly interacts with a limited number of other atoms-has been deduced from the study of gaseous molecules, and is strictly applicable to gaseous molecules only. But the greater number of the compounds of inorganic chemistry are non-gasifiable bodies; the conception of atomic valency cannot therefore, at present, be made use of, otherwise than in a broad and general way, in questions regarding the conditions which determine the compositions of compound molecules.

The periodic law asserts that the compositions of com- 499 pounds vary periodically with variations in the atomic weights of the elements. If we are content to use the expression composition of a non-gasifiable compound as meaning the ratio of the numbers of atoms forming the chemically reacting weight of the compound, then we can express the composi tions of classes of similar compounds by general formulae, and we can trace connexions between these formulae and the atomic weights of the elements.

со CO 3

2

CIO2,

OS SO

Thus, let X = F, Cl, Br, I, OH, NO,, C103, 2' 2' 2

4

&c., then the compositions of a great many important compounds of the elements of the different groups may be thus expressed ;

Examples Cr2O2; TeC,UCU (SO4)2; CrO2, SO2(OH)2, WC16, UO2(SO4).

500

The compositions of the well marked compounds of the elements of a group become more varied as we pass from the lower to the higher groups.

It would however be out of place in an elementary book to attempt to generalise the connexion between the changes in the forms of compounds and the variations in the atomic weights of the elements. Let it suffice to note that the arrangement of the elements in accordance with the periodic law indicates the existence of such a connexion, and points the way by which the nature of this connexion may be elucidated.

If the properties of the elements and their compounds vary periodically with variations in the atomic weights of the elements, accurate determinations of the atomic weights of all the known elements must be demanded in chemistry. The

values given for these constants in the tables on pp. 57, 59, and 60, are given in round numbers only. The atomic weights of many elements have been determined with great accuracy, those of other elements only with a fair degree of accuracy, and those of a few elements with but little accuracy. The following table presents the most trustworthy results of the various determinations.

CHAPTER XXVII.

CONCLUSION.

WE have been endeavouring to connect the changes of composition with the changes of properties exhibited by certain kinds of matter called elements and compounds. We have divided the elements and their compounds into classes, each of which is more or less distinctly marked off from the others. The compositions of compounds have been represented by formulae which exhibit the number of combining weights of each element combined in one reacting weight of a compound; or, in the language of the only theory of the structure of matter which has been found capable of explaining observed facts, the formulae of compounds exhibit the number of atoms of each element combined in one molecule of a compound. Some of the formulae of compounds also suggest reactions by which these compounds are formed, or reactions which occur between them and other substances; such formulae not only state the compositions of the compounds but also indicate certain of their properties.

Our study of changes of properties and changes of composition has shewn that the connexions between these cannot be wholly perceived or understood so long as we look only to the compositions and properties of the substances forming a chemical system at the beginning of a reaction, and at the compositions and properties of the substances forming the system at the close of the reaction. It became necessary for us to pay some regard to the changing systems during the process of change. By doing this we were led to picture to ourselves many apparently simple chemical occurrences as consisting of two or more parts, and to regard the state of equilibrium finally attained by a system of chemically interacting substances as frequently the result of direct and