FUSIBILITY AND ATOMIC WEIGHT

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horizontal axis of the abscissae at distances from zero proportional to their atomic weights. The position of each element is denoted by the corresponding symbol, and from each of these points an ordinate is drawn, which is proportional in length to the atomic volume of the element. By connecting the summits of all the ordinates by lines, we obtain a curve which clearly illustrates the relation of the atomic volume to the atomic weight. The alkali metals occupy a striking position at the five maxima. Those portions of the curve between the maxima resemble chains, which are broken at certain points, because the elements which should occupy these positions are either unknown or have not yet been sufficiently investigated. The great regularity in the course of the curve indicates that the curve will, without doubt, closely follow the dotted course. On this account the probable values, in round numbers, of the density and atomic volume of these elements have been inserted in the

preceding table. In order to distinguish the real from the hypothetical values, notes of interrogation are affixed to the latter.

It is very remarkable that the properties of all the elements appear to be determined by their position on this curve. The descending portions of the curve from the maximum to the minimum, and a little beyond, are entirely occupied by difficultlyfusible and non-volatile elements, and, as a rule, the lower the element is on the curve the less fusible it is found to be. Only easily-fusible, and as a rule volatile elements occur on the ascending portions of the curve.

In the first period, nitrogen, oxygen, and fluorine are gases, in the second period chlorine; but phosphorus and sulphur are easily fusible and volatile. In the next period the series of volatile elements begins with zinc (or perhaps even with copper). In the following period, the volatile elements begin with silver, which can be distilled by means of the oxyhydrogen blowpipe, and in the last incomplete period mercury is the first volatile element. This relation may be expressed in general terms by saying that when the atomic volume decreases with an increasing atomic weight the elements are refractory and non-volatile; but when the atomic volume increases as the atomic weight increases the elements are easily fusible and volatile.

Other properties change twice in these larger periods. This is

the case with the metallic nature of the elements as exhibited by their malleability. The elements at the maximum of the curve and their immediate successors are metals. They are followed by brittle elements down to the minimum. These are succeeded by malleable metals, which are separated from the metals at the maximum by brittle non-metals and semi-metals-e.g.

K, Ca, (Sc?), malleable;

Ti, V, Cr, Mn, brittle;

Fe, Co, Ni, Cu, Zn, Ga, malleable;

Ge, As, Se, Br, brittle or non-metallic.

The malleability is the same in the following periods. Other physical properties change in a similar way, but some of these properties have not yet been sufficiently investigated. The elements on ascending portions of the curve are without exception diamagnetic, those on the falling part of the curve are magnetic. The optical properties, crystalline form, and expansion by heat exhibit similar regularities.

§ 37. Periodicity of the Electro-chemical Properties.-The intimate connection between the chemical properties and the atomic weight proceeds from the fact that the whole system of arrangement consists in bringing together the natural families of elements which have been developed from Döbereiner's triads.

The chemical elements and their compounds exhibit certain contrasts in their nature. These are indicated by the terms 'positive' and ' negative.' The use of these terms arises from the close relation between chemical and electrical properties. As a rule, when two or more bodies of different composition are brought in contact with each other, both are electrified-one becoming positive, the other negative. The greater the difference between the composition of the two substances the greater the electrical excitement will be. The difference which exists between the two bodies is called the electro-chemical difference. The direct measurement of the electric charge produced by the contact of heterogenous bodies is difficult. But the electro-chemical nature of the substances can be determined by another method. Many liquids and some solids are decomposed by the electric current.

ELECTRO-CHEMICAL PROPERTIES

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In this act of electrolysis, those constituents which are positive on contact are liberated with the positive electricity and those which become negative with the negative electricity, so that the electro-chemical nature of the constituents is easily recognised. The difference in the electro-chemical nature of a substance is not absolute, but merely relative, so that one element can be positive with regard to a second element and negative to a third. It has also been observed that a positive element in a compound can generally be replaced by a more positive and a negative by a more negative. This replacement of one element by another, affords another means of ascertaining the electrochemical nature of an element. The oxides and hydroxides of the positive elements have basic properties (i.e. they neutralise acids); the oxides, hydroxides, and some of the hydrides of the negative elements are acids.

If the elements are divided into the two classes-electronegative and electro-positive-these properties are regularly divided in the periods. In the atomic volume table, the positive elements are denoted by* and the negative by -. The positive nature changes in the same way as the metallic nature and malleability, i.e. twice in the large periods of atomic volumes.

The first family in the table on page 55 consists of positive elements, the alkali metals, Li, Na, K, Rb, Cs; the positive character increases with the atomic weight, and caesium is not only the most electro-positive metal of this group but of all the elements. The metals Be, Mg, Ca, Sr, Ba, in the second family, closely resemble these metals in their electro-positive nature; again, we find the metal with the highest atomic weight, barium, is the most electro-positive. In the third family, containing Bo, Al, Sc, Y, La, Yb, the electro-positive nature is much feebler. The hydroxide of boron is a feeble acid, and aluminium hydroxide exhibits the properties of a weak acid as well as of a strong base: here again the negative character grows feebler and the positive stronger, as the atomic weight increases. In the fourth family C, Si, and Ti yield acids, but the higher members Zr, Ce and Th have a more positive character. The elements in all these four families have one property in common: they form very stable compounds with oxygen, and consequently their oxides are difficult to reduce.