(H2PO4)2 or H4P2O8 = 2H2O + 2PO3.[13]
So far as is known at present, the highest form of peroxides is met with in the peroxide of uranium, UO4, prepared by Fairley;[14] while OsO4 is the highest oxide giving salts. The line of argument which is inspired by the periodic law, so far from being weakened by the discovery of peroxides, is thus actually strengthened, and we must hope that a further exploration of the region under consideration will confirm the applicability to chemistry generally of the principles deduced from the periodic law.
Permit me now to conclude my rapid sketch of the oxygen compounds by the observation that the periodic law is especially brought into evidence in the case of the oxides which constitute the immense majority of bodies at our disposal on the surface of the earth.
The oxides are evidently subject to the law, both as regards their chemical and their physical properties, especially if we take into account the cases of polymerism which are so obvious when comparing CO2, with SinO2n. In order to prove this I give the densities s and the specific volumes v of the higher oxides of two short periods. To render comparison easier, the oxides are all represented as of the form R2On. In the column headed Δ the differences are given between the volume of the oxygen compound and that of the parent element, divided by n—that is, by the number of atoms of oxygen in the compound:—[15]
| s. | v. | Δ | s. | v. | Δ | ||
| Na2O | 2·6 | 24 | -22 | K2O | 2·7 | 35 | -55 |
| Mg2O2 | 3·6 | 22 | -3 | Ca2O | 3·15 | 36 | -7 |
| Al2O3 | 4·0 | 26 | +1·3 | Sc2O3 | 3·86 | 35 | 0 |
| Si2O4 | 2·65 | 45 | 5·2 | Li2O4 | 4·2 | 38 | +5 |
| P2O5 | 2·39 | 59 | 6·2 | V2O5 | 3·49 | 52 | 6·7 |
| S2O6 | 1·96 | 82 | 8·7 | Cr2O6 | 2·74 | 73 | 9·5 |
I have nothing to add to these figures, except that like relations appear in other periods as well. The above relations were precisely those which made it possible for me to be certain that the relative density of ekasilicon oxide would be about 4·7; germanium oxide, actually obtained by Winkler, proved, in fact, to have the relative density 4·703.
The foregoing account is far from being an exhaustive one of all that has already been discovered by means of the periodic law telescope in the boundless realms of chemical evolution. Still less is it an exhaustive account of all that may yet be seen, but I trust that the little which I have said will account for the philosophical interest attached in chemistry to this law. Although but a recent scientific generalisation, it has already stood the test of laboratory verification, and appears as an instrument of thought which has not yet been compelled to undergo modification; but it needs not only new applications, but also improvements, further development, and plenty of fresh energy. All this will surely come, seeing that such an assembly of men of science as the Chemical Society of Great Britain has expressed the desire to have the history of the periodic law described in a lecture dedicated to the glorious name of Faraday.
Footnotes:
[1] ‘Es ist wohl kaum anzunehmen, dass alle im Vorhergehenden hervorgehobenen Beziehungen zwischen den Atomgewichten (oder Aequivalenten) in chemischen Verhältnissen einander ähnliche Elemente bloss zufällig sind. Die Auffindung der in diesen Zahlen gesetzlichen Beziehungen müssen wir jedoch der Zukunft überlassen.’
[2] To judge from J. A. R. Newlands's work, On the Discovery of the Periodic Law, London, 1884, p. 149; ‘On the Law of Octaves’ (from the Chemical News, 12, 83, August 18, 1865).