In the horizontal rows of this table the elements are arranged in the order of their atomic weights. If the right side of each row is connected with the left side of the following row a single continuous series of all the elements will be produced. In this arrangement the nature and properties of the members will be represented as periodic functions of the atomic weights, changing systematically as the atomic weight increases from member to member, and returning to the beginning after a certain number of members. The periodicity may be more clearly indicated by means of the table at the end of this book. The table must be pasted on a wooden or pasteboard cylinder of suitable dimensions, so that the right and left sides meet.

The first two periods or series each embrace seven elements.

The corresponding members closely resemble each other—e.g. lithium and sodium, beryllium and magnesium.

This resemblance continues in the third and fourth periods, so far as the first members are concerned; the following members do not exhibit corresponding properties, and it is not until we reach the seventeenth member after potassium that another alkali metal, rubidium, recurs. The next alkali metal, caesium, is again the seventeenth element after rubidium. Both metals are preceded by elements which are closely related to the last members of the first and second periods. On closer consideration, these large periods are found to split up into two smaller periods, in which some of the properties of the elements recur at a shorter interval. This is seen in the following table:

The elements in the same column all have certain properties in common, but only the alternate elements bear a close resemblance to each other. After cerium, Ce, there is a gap of about forty units. This will probably be filled by the rare earth metals, which have not yet been sufficiently well investigated. Then comes ytterbium (the atomic weight of this metal has not been accurately determined) and tantalum; they are followed by elements resembling the former members, in the same order as in the preceding periods, but leaving some gaps and spaces:

The gaps in this and the preceding tables will probably be filled by elements which remain yet to be discovered. Several of the gaps which existed when the periodic system of the elements was first promulgated have been filled up by the discovery of scandium, gallium, and germanium, and by the correction of the atomic weights of indium, yttrium, cerium, and lanthanum.

It appears from the table on page 55 that a dozen elements are yet to be discovered. The majority of these gaps will probably be filled by rare earth metals with atomic weights lying between 140 and 180.

§ 36. Periodicity of the Physical Properties of the Elements.If we examine the long series of elements from member to member, we find a change in the properties, sometimes gradual and sometimes sudden, which recurs in the following periods in a similar way, so that almost every property of an element occurs again in a similar way in one or more of the later members, so that the properties of each individual element are determined by its position in the series. This is not only true of the chemical and physical properties of the elements, but also of the compounds.

This recurrence of the physical properties is very striking in the case of density, which regularly increases and decreases in each period. The connection between density and atomic weight is best exhibited by taking the atomic volume instead of the density-i.e. the volume occupied by the atomic weight instead of the weight of the unit volume. The simplest expression for the atomic volume is obtained by dividing the atomic weight by the density.

These quotients represent the volume of the atoms in the solid state compared with the volume occupied by the unit weight of liquid water as unity. As the absolute weight of an atom of hydrogen is not known, the unit of atomic weights is not known, and this unit of volume must remain an unknown quantity. But these quotients may be compared by means of another standard-e.g. the atomic weight of an element, expressed in grams, occupies in the solid state the same number of cubic centimetres as the value of the atomic volume. For example, the atomic volume of silver is 10.2, and therefore 107.66 grams of silver occupy 10.2 c.c.

In the table on p. 58 the density D and the atomic volume V are given under the symbol of each element. The density begins at a minimum with the alkali metals, lithium, sodium, rubidium, caesium, and ascends from here in the first two periods to carbon and silicon; in the following periods the density continues to increase after passing the homologues of these

elements till it reaches the metals copper, ruthenium, c and then sinks. In the atomic volumes the position is rev the maximum is at the alkali metals, and the minimum at carbon, aluminium, nickel, ruthenium, osmium.

The relation between the atomic volume and atomic weight is more clearly depicted by the accompanying graphic representation. In this table the elements are arranged on the