by Seubert having proved that the atomic weight of osmium is really lower than that of platinum, and that it is near to 191; and thirdly, by the investigations of Krüss, and Thorpe and Laurie proving that the atomic weight of gold exceeds that of platinum, and approximates to 197. The atomic weights which were thus found to require correction were precisely those which the periodic law had indicated as affected with errors; and it has been proved therefore that the periodic law affords a means of testing experimental results. If we succeed in discovering the exact character of the periodical relationships between the increments in atomic weights of allied elements discussed by Ridberg in 1885, and again by Bazaroff in 1887, we may expect that our instrument will give us the means of still more closely controlling the experimental data relating to atomic weights.

*

Let me next call to mind that, while disclosing the variation of chemical properties, the periodic law has also enabled us to systematically discuss many of the physical properties of elementary bodies, and to show that these properties are also subject to the law of periodicity. At the Moscow Congress of Russian Naturalists in August, 1869, I dwelt upon the relations which existed between density and the atomic weight of the elements. The following year Professor Lothar Meyer, in his well-known paper,† studied the same subject in more detail, and thus contributed to spread information about the periodic law. Later on, Carnelley, Laurie, L. Meyer, Roberts-Austen, and several others applied the periodic system to represent the order in the changes of the magnetic properties of the elements, their melting points, the heats of formation of their haloid compounds, and even of such mechanical properties as the coefficient of elasticity, the breaking stress, &c., &c. These deductions, which have received further support in the discovery of new elements endowed not only with chemical but even with physical properties which were foreseen by the law of periodicity, are well known; so I need not dwell upon the subject, and may pass to the consideration of oxides.‡

Thus, in the typical small period of

Li, Be, B, C, N, O, F,

we see at once the progression from the alkaline metals to the acid non-metals, such as are the halogens.

+ Liebig's Annalen, Erz. Bd. vii, 1870.

A distinct periodicity can also be discovered in the spectra of the elements. Thus the researches of Hartley, Ciamician, and others have disclosed, first, the homology of the spectra of analogous elements; secondly, that the alkaline metals have simpler spectra than the metals of the following groups; and thirdly, that there is a certain likeness between the complicated spectra of manganese and iron on the one hand, and the no less complicated spectra of chlorine and bromine on

In indicating that the gradual increase of the power of elements of combining with oxygen is accompanied by a corresponding decrease in their power of combining with hydrogen, the periodic law has shown that there is a limit of oxidation, just as there is a well-known limit to the capacity of elements for combining with hydrogen. A single atom of an element combines with at most four atoms of either hydrogen or oxygen: and while CH, and SiH, represent the highest hydrides, so RuO, and OsO, are the highest oxides. We are thus led to recognise types of oxides, just as we have had to recognise types of hydrides.*

:

The periodic law has demonstrated that the maximum extent to which different non-metals enter into combination with oxygen is determined by the extent to which they combine with hydrogen, and that the sum of the number of equivalents of both must be equal to 8. Thus chlorine, which combines with 1 atom, or 1 equivalent of hydrogen, cannot fix more than 7 equivalents of oxygen, giving Cl2O, while sulphur, which fixes 2 equivalents of hydrogen, cannot combine with more than 6 equivalents or 3 atoms of It thus becomes evident that we cannot recognise as a oxygen. fundamental property of the elements the atomic valencies deduced from their hydrides; and that we must modify, to a certain extent, the theory of atomicity if we desire to raise it to the dignity of a general principle capable of affording an insight into the constitution of all compound molecules. In other words, it is only to carbon, which is quadrivalent with regard both to oxygen and hydrogen, that we can apply the theory of constant valency and of bond, by means of which so many still endeavour to explain the structure of compound molecules. But I should go too far if I ventured to explain in detail the conclusions which can be drawn from the above considerations. Still, I think it necessary to dwell upon one particular fact which must be explained from the point of view of the periodic law in order to clear the way to its extension in that particular direction. the other hand, and their likeness corresponds to the degree of analogy between those elements which is indicated by the periodic law.

* Formerly it was supposed that, being a bivalent element, oxygen can enter into any grouping of the atoms, and there was no limit foreseen as to extent to which it could further enter into combination. We could not explain why bivalent sulphur, which forms compounds such as

while other elements, as, for instance, chlorine, form compounds such as—

C1-0-0-0-0-K.

The higher oxides yielding salts the formation of which was foreseen by the periodic system-for instance, in the short series beginning with sodium—

Na2O, MgO, Al2O3, SiO2, P2O5, SO3, C1207,

must be clearly distinguished from the higher degrees of oxidation which correspond to hydrogen peroxide and bear the true character of peroxides. Peroxides such as Na2O2, BaO2, and the like have long been known. Similar peroxides have also recently become known in the case of chromium, sulphur, titanium, and many other elements, and I have sometimes heard it said that discoveries of this kind weaken the conclusions of the periodic law in so far as it concerns the oxides. I do not think so in the least, and I may remark, in the first place, that all these peroxides are endowed with certain properties-obviously common to all of them, which distinguish them from the actual, higher, salt-forming oxides, especially their easy decomposition by means of simple contact agencies; their incapacity of forming salts of the common type; and their capacity of combining with other peroxides (like the faculty which hydrogen peroxide possesses of combining with barium peroxide, discovered by Schoene). Again, we remark that some groups are especially characterised by their capacity of generating peroxides. Such is, for instance, the case in the VIth group, where we find the well-known peroxides of sulphur, chromium, and uranium; so that further investigation of peroxides will probably establish a new periodic function, foreshadowing that molybdenum and wolfram will assume peroxide forms with comparative readiness. To appreciate the constitution of such peroxides, it is enough to notice that the peroxide form of sulphur (so-called persulphuric acid) stands in the same relation to sulphuric acid as hydrogen peroxide stands to water:

H(OH), or H2O, responds to (OH)(OH), or H2O2,

and so also

H(HSO1), or H2SO, responds to (HSO,)(HSO.), or H‚S2O,. Similar relations are seen everywhere, and they correspond to the principle of substitutions which I long since endeavoured to represent as one of the chemical generalisations called into life by the periodic law. So also sulphuric acid, if considered with reference to hydroxyl, and represented as follows

HO(SO,OH),

has its corresponding compound in dithionic acid

(SO2OH)(SO2OH), or H ̧S2O。.

Therefore, also, phosphoric acid, HO(POH2O2), has, in the same sense, its corresponding compound in the subphosphoric acid of Saltzer ::

(POH2O2) (POH2O2), or H¿P2O6 ;

and we must suppose that the peroxide compound corresponding to phosphoric acid, if it be discovered, will have the following structure:

(H2PO1), or H1P2O, = 2H2O + 2PO3.*

As far as is known at present, the highest form of peroxides is met with in the peroxide of uranium, UO, prepared by Fairley ;† while OsO 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 ▲ 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 :

* In this sense, oxalic acid, (COOH), also corresponds to carbonic acid, OH(COOH), in the same way that dithionic acid corresponds to sulphuric acid, and subphosphoric acid to phosphoric; therefore, if a peroxide, corresponding to carbonic acid, be obtained, it will have the structure of (HCO3)2, or H2C2O6 = H2O + C2O. So also lead must have a real peroxide, PbO5.

The compounds of uranium prepared by Fairley seem to me especially instructive in understanding the peroxides. By the action of hydrogen peroxide on uranium oxide, UO,, a peroxide of uranium, UO44H2O, is obtained (U 240) if the solution be acid; but if hydrogen peroxide act on uranium oxide in the presence of caustic soda, a crystalline deposit is obtained, which has the composition Na1UO4H2O, and evidently is a combination of sodium peroxide, NagO, with uranium peroxide, UO4. It is possible that the former peroxide, UO,4H,O, contains the elements of hydrogen peroxide and uranium peroxide, U2O7, or even U(OH)¿H2O like the peroxide of tin recently discovered by Spring, which has the constitution Sn2OHO.

A thus represents the average increase of volume for each atom of oxygen con

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 eka-silicon oxide would be about 47; germanium oxide, actually obtained by Winckler, 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.

By G. J. BURCH, B.A. and J. E. MARSH, B.A., University Laboratory,

Oxford.

THE theory of Van t' Hoff that there is a definite relationship to space of the four bonds or affinities of the carbon-atom has received such remarkable confirmation from facts, that chemists have very naturaily attempted to extend the conception to other elements, especially tained in the higher salt-forming oxide. The acid oxides give, as a rule, a higher value of ▲, while in the case of the strongly alkaline oxides its value is usually negative.