Page:Popular Science Monthly Volume 59.djvu/595

Rh a few such values have as yet been determined numerically; instances may be chosen from the magnesium group, where the numbers run: Magnesium $$= +$$ 1.2; Zinc $$= +$$ 0.5; Cadmium $$= +$$ 0.19; or from the fluorine column, where the numbers are: Fluorine $$=-$$2.0; Chlorine $$=-$$1.6; Iodine $$=-$$0.4. In each case the potential, positive or negative, is the highest for the element with smallest atomic weight, and decreases with increase of atomic weight, for elements in the same column. The order of some of the elements is: Cs Rb K Na Li Ba Sr Ca Mg Al Mn Zn Cd Fe&#8243; Co Ni Pb H Cu Ag Hg&#8242; Pt&#8244; &#8242; Au&#8242; &#8243;; and for electro-negative ions, S&#8243; O&#8243; I Br CI F; the first element, cæsium, being the most electro-positive, and the last, fluorine, the most electro-negative.

The order given above corresponds fairly well with the order in the periodic table, passing from left to right. But, as in the table, the atomic weights follow each other continuously round the cylinder or round the spiral, the abrupt change from elements of an extreme electronegative character, like fluorine to sodium, an element of highly electropositive character, or from chlorine to potassium, has always appeared remarkable. The old dictum, Natura nihil fit per saltum, if not always true (else we should have no elements at all, but a gradual and continuous transition from one kind of matter to another—a condition of affairs hardly possible to realize), has generally some spice of truth in it; and it might have been predicted (and the forecast seems to have been made obscurely by several speculators) that a series of elements should exist which should exhibit no electric polarity whatever. Such elements, too, should form no compounds, and, of course, should display no valency; they should be indifferent, inactive bodies, with no chemical properties.

The discovery of argon in 1894, followed by that of terrestrial helium in 1895, and of neon, krypton and xenon in 1898, has shown the justice of the foregoing remarks. In as much as the methods employed for the isolation of these elements illustrate their properties and confirm the views as to their inertness and lack of electric polarity, I propose to sketch shortly the history of their discovery.

An accurate investigation of the density of atmospheric nitrogen and of nitrogen prepared from its compounds led Lord Rayleigh to inquire into the cause of the discrepancy, for the density of the nitrogen of the atmosphere was found to exceed that of 'chemical nitrogen' by about one part in two hundred, whereas the accuracy of his experiments was such that it would have excluded an error of one part in five thousand. I need not here allude to the reasons which were at first put forward to account for this anomaly; suffice it to say that they offered no explanation; and that we ultimately traced the discrepancy to the presence in 'atmospheric nitrogen' of a gas nearly half as dense again