106. Before giving a detailed explanation of this table let us meanwhile gather together some of the best established generalisations concerning the periodic connection of properties and atomic weights of the elements.

A phenomenon is said to be periodic when, if the conditioning circumstances vary continuously, it repeats itself at definite intervals. The variable under consideration is the atomic weight, the phenomenon to be examined is the nature of the chemical element and its compounds. Although it is not as yet possible to state quantitatively the nature of the periodic function which connects the atomic weights and general properties of the elements, it may nevertheless be established that the function in question is periodic. For this purpose it will be necessary to break up the phenomenon nature of the chemical element and its compounds' and to endeavour to shew that the malleability, ductility, atomic volume, power of forming oxides (or chlorides) of definite composition, position in electrical series, &c. of the elements do vary periodically with variations in the atomic weights of these elements1.

107. Atomic volume. The quotient obtained by dividing the specific gravity of an element, in the solid form, by its atomic weight is called the atomic volume of that element. This quotient expresses the volume, in cubic centimetres, occupied by an amount of the solid element, in grams, pro*portional to the atomic weight of that element.

Arranging the elements in order of increasing atomic weights it is found that the value for atomic volume reaches its first maximum at lithium, that it then diminishes through beryllium, boron, &c. and again increases through carbon, &c. reaching a second maximum at sodium; the other maxima occur at potassium, rubidium, and cæsium.

The periodic nature of the connection between atomic volumes and atomic weights becomes very apparent when the

1 For greater details on this point see L. Meyer, Die Modernen Theorien, 4th Ed. pp. 139-173, of which this and the few following pages must be regarded as an abstract.

710 20 30

40

50 60 70 80 90 100 no 120 130 140 150 160 170 180 190 200 210 220 230 240 Thick line curve shews atomic volumes.

magnitudes of those quantities are graphically represented as is done on the plate facing this page'.

The maximum points on the curve are seen to be occupied by metals of low specific gravity, while the minimum points are occupied by heavy metals.

The position of an element on the curve, with reference to the preceding and succeeding elements, appears to exert a marked influence on the properties of the element in question. Thus phosphorus and magnesium on the one hand, and calcium and chlorine on the other, have nearly equal atomic volumes; phosphorus and chlorine are followed by elements the atomic volumes of which are larger than their own (i.e. are situated on ascending portions of the curve), whereas magnesium and calcium are followed by elements having atomic volumes smaller than their own (i.e. are situated on descending portions of the curve).

The ductile metals are placed at or near to maximum and minimum points on the curve; those of low specific gravity occurring at, and immediately after maximum points, and those of high specific gravity at, and immediately after minimum points. The brittle heavy metals occur in sections 4, 5, and 7 immediately before the minimum points.

The elements on the descending parts of sections 2 and 3 of the atomic volume curve are electropositive and form basic hydroxides; those on the ascending portions of the same sections are electronegative and form acid hydroxides. Sections 4 and 5 each contain four groups of elements arranged in accordance with their negative or positive character. Electropositive elements occur on the first portions of the descending curve in each of these sections (K, Ca; Rb, Sr); these are

1 Only those elements the specific gravities of which in the solid state have been directly determined are included in the curve; want of data is indicated by a broken line.

"A section of the curve means the part situated between two maxima; section I includes hydrogen only, section 2 extends from lithium to sodium, &c. There are probably several unknown elements with atomic weights greater than that of didymium and smaller than that of tantalum; the curve, if complete, would probably be marked by a sixth maximum point between cæsium and thorium, this part of the curve is therefore said to comprise two sections (6 and 7).