Page:Outlines of Physical Chemistry - 1899.djvu/54

 84 OUTLINES OF PHYSICAL CHEMISTEY

density would correspond approximately to the place assigned to these elements on the curve. '*

This curve renders the periodicity of the atomic volumes very evident. As in Mendel&effs classification, so also here we find two short or simple periods (1 and 2) followed by long or double periods ; 8-4, 5-6, &c. The last periods are still very incomplete.

An important physical property which thus appears as a periodic function of the atomic weight is the density ; and the same may be said of other properties of the elements.

We shall now examine some of these :

1. The Fusibility of the Elements. — The gaseous and the easily fusible elements are all to be found on the rising branches and at the summits of the curve. The descending branches and the minima of atomic volumes, on the contrary, correspond to the difficultly fusible or refractory elements.

Those elements, therefore, which have a higher atomic volume than that of the preceding element of lower atomic weight are comparatively easily fusible, and elements for which the opposite holds are difficultly fusible. The transition between these two classes is formed by elements of average fusibility.

The boiling points vary in the same way as the melting points. The gaseous or easily volatile elements occupy the ascending portions and the summits of the curve.

The general conclusion is that a large atomic volume (compared with the atomic weight) is favourable to fusibility and volatility.

2. Metallic or Non-Metallic Character. — The two simple periods begin with a series of metallic elements (summits and descending portions of the curve) and finish with a series of electronegative elements.

The following periods are double, and each of them comprises two series of metallic and ductile elements commencing respectively at the summits and the lowest

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