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from the law of volumes or the law of specific heats. In the determination of molecular and atomic weights the latter laws give more efficient aid to chemistry than the law of isomorphism, although the enunciation of these laws may not be strictly accurate from a physical point of view. We have already made this remark in connection with the law of specific heats, which is an incomplete law (p. 125). The same remark applies, though in a less degree, to the law of volumes. In fact, the laws of Gay-Lussac and that of Avogadro and Ampère are dependent upon the law of Mariotte, and are in a manner forced to follow its variations.

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CHAPTER VI.

THE NEW SYSTEM OF ATOMIC WEIGHTS RESPECTS AND RENDERS EVIDENT THE ANALOGIES WHICH EXIST BETWEEN BODIES.

DUMAS-MENDELEJEFF.

I.

THE new system of atomic weights renders evident numerous analogies which have been discovered in chemistry, between either the elements themselves or between their compounds or reactions, thus dealing with the most varied and the most profound questions of science. It is a vast subject, which might be developed to a great length, but of which we shall here only endeavour to give a sketch.

Chemistry is not merely an immense collection of facts, but more exactly the science which teaches us to classify and arrange them, and this classification should begin with the elements themselves. Attempts have, we know, for some time been made in this direction.

The first, which was the most satisfactory, is due to Dumas. Admitting the distinction between the metals and the metalloids, Dumas proposed to divide the latter bodies into five families-namely, those of hydrogen, chlorine, sulphur, phosphorus, and boron. The principle, moreover, of this classification-that of comparing bodies which form similar compounds-agrees with the natural method. We shall not now dwell upon this point, which will be developed presently, but merely give Dumas' division of the metalloids into five families.1

1st family: hydrogen.

2nd family: fluorine, chlorine, bromine, iodine.
3rd family: selenium, sulphur. Appendix: oxygen.
4th family phosphorus, arsenic. Appendix: nitro-
gen.

5th family: boron, silicon. Appendix: carbon.

Time has made little alteration in this attempt at classification. The bodies added as appendices have become the heads of their respective families. The only change that has been made has been to separate boron from carbon and silicon. Attempts have long been made to form the metals into similar groups. But here the problem becomes much more complicated, because, in the case of a great number of metals, the analogies are much less strongly marked, and the extreme

1 Traité de Chimie appliquée aux Arts, t. i., Introduction, p. lxxvii. We owe the term metalloids to Simon, who proposed it in 1808 to designate the metals of the alkalis and of the earths resembling the metals properly so called. In 1811 Berzelius applied the term to the non-metallic elements. (H. Kopp, Geschichte der Chemie, t. iii. p. 96.)

terms of each group disagree to some extent in their properties and in the nature of their compounds. Thus certain metals constitute a transition between the several groups, and these intermediary terms only serve to put difficulties in the way of classification. Nevertheless, several groups of metals have been established. We will mention, in the first place, the alkaline metals, to which may be added, as an appendix, silver, thallium, and, to a certain point, copper and gold.

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Cobalt, nickel, iron, and manganese are connected with the preceding series through zinc. But this group throws some difficulties in the way of classification, as iron and manganese, which offer some analogies with the metals of the magnesium series, are connected, from another point of view, with both chromium and aluminium.

Another group of metals is connected with silicon, and comprises

Titanium

Zirconium

The following metals are connected with the family of nitrogen, phosphorus, and arsenic :—

Vanadium

Antimony

Bismuth

Niobium

Tantalum.

Molybdenum and tungsten present many mutual analogies, and resemble chromium and uranium.

Copper is difficult to classify. In the nature of its compounds it is not unlike mercury, but it also presents analogies with silver, and from this point of view resembles the alkaline metals.

Finally, the metals which accompany platinum have always been grouped into one family, which may be subdivided into three classes-i.e. ruthenium-osmium, rhodium-iridium, palladium-platinum.

The metals which form part of these families or classes are characterised by the analogy of the compounds which they form with the metalloids, particularly with oxygen and chlorine; for here, unfortunately, hydrogen compounds are wanting. Similar formulæ are accorded to a given group of the compounds in question, if appropriate atomic weights are assigned to the latter.

Each group of metals differs from the rest in the nature of its compounds. This is an established fact, and will be developed presently. But it was formerly unknown, having only recently been discovered, that the characteristic properties of the elements, which

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