the Periodic Law

Similar considerations apply to the 18 elements from rubidium to the next inert gas, xenon, ten elements from yttrium to cadmium being excluded and relegated to the second transition series, leaving a typical period of eight elements, rubidium, strontium, indium, tin, antimony, tellurium, iodine and xenon. In the next period, the sixth, 32 elements occur from caesium to the inert gas radon. Nevertheless, as in the two preceding periods, only eight elements are closely related to those of the short typical periods, these eight being caesium, barium, thallium, lead, bismuth, polo nium, the unknown halogen element 85, and radon; 24 elements from lanthanum to mercury are consequently excluded and rele gated to the third transition series. In the last period, radium alone shows close resemblance to elements in the typical periods, and the remainder, actinium, thorium, protoactinium and uranium, are consequently relegated to the fourth transition series.

The typical classification of the elements, shown in Table

I., thus consists of 7 periods each consisting of eight groups, each period, except the rudimentary 1st, and the incomplete 7th, com mencing with an alkali metal and ending with an inert gas. This arrangement illustrates the extreme regularity of the periodicity of properties of the elements included in this abridged classifi cation. It further reveals that basic or alkaline character in creases steadily in each vertical group from a minimum in period one to a maximum in period seven, and decreases steadily in each horizontal period from a maximum in group I. to a minimum in group VIII. This decrease in basicity in each period, however, is not quite uniform, but occurs by a series of leaps, all the elements yielding alkaline hydroxides being in groups I. and II., those yielding amphoteric hydroxides (soluble in both acids and alka lis) being included in groups III. and IV., while all the acidic and non-basic elements are included in groups V. to VIII. The ele ments can thus be further classified into an alkaline class of two groups I. and II., and amphoteric class of two groups III. and IV., and a non-alkaline or acidic class of four groups V. to VIII. It may further be observed that, where elements exhibit variable valency, this variation occurs by two or four units. Elements of group III., for example, exhibit variable valencies of three and one, those of group IV. valencies of four and two, those of group V. valencies of five, three and one, and so forth. This variation of valency by two or four units is intimately related to the three main classes of elements, the alkaline class of two groups, the amphoteric class of two groups, and the non-alkaline class of four groups, a reduction in valency of two or four units being accom panied by an increase in basic character equivalent to a decrease in group number of two or four units. Thallium of group III., for

example, on reduction from valency three to one, exhibits the properties of an alkali metal of group I. with valency one.

The variations in alkaline character of the elements of Table I., as one proceeds either along the horizontal periods or down the vertical family series, lead, by a sort of cross-hatching, to a mani festation of resemblances between elements otherwise unrelated in the classification. For example, these variations explain the close resemblance of beryllium and aluminium, two metals which are not in the same group or period. In a similar way resem blances arise between lithium and magnesium, and between boron and silicon. So pronounced is this diagonal resemblance in the periodic table that a line, drawn between beryllium and boron and between aluminium and silicon and continued diagonally across the table, completely separates all the metallic from all the non-metallic elements, and allocates hydrogen and arsenic to the non-metals and antimony to the metals. All the known non metals are in the abridged classification (Table I.) which also includes a space for the missing halogen (eka-iodine).

The Transition Series.—The elements excluded from the abridged classification, as showing considerable divergence in prop erties from the typical elements of the first two octet periods, are ten in the 4th period, ten in the 5th period, twenty-four in the 6th period, and four of the five known elements of the 7th period.

Examination of the properties of the ten transition elements of the 4th period, reveals that these elements can be arranged in a period of eight groups, III., IV., V., VI., VII., VIII., I. and II., scan dium, the lightest element, being proper to group III., and zinc, the heaviest element, proper to group II., the triad, iron, cobalt and nickel, being included within group VIII. A similar arrange ment holds good for the ten transition elements of the 5th period, yttrium, the lightest, being proper to group III. like scandium, and cadmium, the heaviest, to group II. like zinc, the triad, ruthenium, rhodium and palladium, being included within group VIII.