elements presently to be described.[7] [7] The hydrogen compounds, R_{2}H, in equivalency correspond with the type of the suboxides, R_{4}O. Palladium, sodium, and potassium give such hydrogen compounds, and it is worthy of remark that according to the periodic system these elements stand near to each other, and that in those groups where the hydrogen compounds R_{2}H appear, the quaternary oxides R_{4}O are also present. Not wishing to complicate the explanation, I here only touch on the general features of the relation between the hydrates and oxides and of the oxides among themselves. Thus, for instance, the conception of the ortho-acids and of the normal acids will be considered in speaking of phosphoric and phosphorous acids. As in the further explanation of the periodic law only those oxides which give salts will be considered, I think it will not be superfluous to mention here the following facts relative to the peroxides. Of the _peroxides_ corresponding with hydrogen peroxide, the following are at present known: H_{2}O_{2}, Na_{2}O_{2}, S_{2}O_{7} (as HSO_{4}?), K_{2}O_{4}, K_{2}O_{2}, CaO_{2}, TiO_{3}, Cr_{2}O_{7}, CuO_{2}(?), ZnO_{2}, Rb_{2}O_{2}, SrO_{2}, Ag_{2}O_{2}, CdO_{2}, CsO_{2}, Cs_{2}O_{2}, BaO_{2}, Mo_{2}O_{7}, SnO_{3}, W_{2}O_{7}, UO_{4}. It is probable that the number of peroxides will increase with further investigation. A periodicity is seen in those now known, for the elements (excepting Li) of the first group, which give R_{2}O, form peroxides, and then the elements of the sixth group seem also to be particularly inclined to form peroxides, R_{2}O_{7}; but at present it is too early, in my opinion, to enter upon a generalisation of this subject, not only because it is a new and but little studied matter (not investigated for all the elements), but also, and more especially, because in many instances only the hydrates are known--for instance, Mo_{2}H_{2}O_{8}--and they perhaps are only compounds of peroxide of hydrogen--for example, Mo_{2}H_{2}O_{8} = 2MoO_{3} + H_{2}O_{2}--since Prof. Schöne has shown that H_{2}O_{2} and BaO_{2} possess the property of combining together and with other oxides. Nevertheless, I have, in the general table expressing the periodic properties of the elements, endeavoured to sum up the data respecting all the known peroxide compounds whose characteristic property is seen in their capability to form peroxide of hydrogen under many circumstances.