The above-mentioned peculiarity of the laws of chemistry respecting definite compounds and the atomic weights leads one to think that the time has not yet come for their full explanation, and I do not think that it will come before the explanation of such a primary law of nature as the law of gravity. It will not be out of place here to turn our attention to the many-sided correlation existing between the undecomposable _elements and the compound carbon radicles_, which has long been remarked (Pettenkofer, Dumas, and others), and reconsidered in recent times by Carnelley (1886), and most originally in Pelopidas's work (1883) on the principles of the periodic system. Pelopidas compares the series containing eight hydrocarbon radicles, C_{_n_}H_{2_n_ + 1}, C_{_n_}H_{2_n_} &c., for instance, C_{6}H_{13}, C_{6}H_{12}, C_{6}H_{11}, C_{6}H_{10}, C_{6}H_{9}, C_{6}H_{8}, C_{6}H_{7}, and C_{6}H_{6}--with the series of the elements arranged in eight groups. The analogy is particularly clear owing to the property of C_{_n_}H_{2_n_+1} to combine with X, thus reaching saturation, and of the following members with X_{2}, X_{3} ... X_{8}, and especially because these are followed by an aromatic radicle--for example, C_{6}H_{5}--in which, as is well known, many of the properties of the saturated radicle C_{6}H_{13} are repeated, and in particular the power of forming a univalent radicle again appears. Pelopidas shows a confirmation of the parallel in the property of the above radicles of giving oxygen compounds corresponding with the groups in the periodic system. Thus the hydrocarbon radicles of the first group--for instance, C_{6}H_{13} or C_{6}H_{5}--give oxides of the form R_{2}O and hydroxides RHO, like the metals of the alkalis; and in the third group they form oxides R_{2}O_{3} and hydrates RO_{2}H. For example, in the series CH_{3} the corresponding compounds of the third group will be the oxide (CH)_{2}O_{3} or C_{2}H_{2}O_{3}--that is, formic anhydride and hydrate, CHO_{2}H, or formic acid. In the sixth group, with a composition of C_{2}, the oxide RO_{3} will be C_{2}O_{3}, and hydrate C_{2}H_{2}O_{4}--that is, also a bibasic acid (oxalic) resembling sulphuric, among the inorganic acids. After applying his views to a number of organic compounds, Pelopidas dwells more particularly on the radicles corresponding with ammonium.