Note 25 — Chapter XX · SULPHUR, SELENIUM, AND TELLURIUM

The formation of the polysulphides of hydrogen, H2Sn is easily understood from the law of substitution, like that of the saturated hydrocarbons, CnH2n + 2, knowing that sulphur gives H2S, because the molecule of sulphuretted hydrogen may be divided into H and HS. This radicle, HS, is equivalent to H. By substituting this radicle for hydrogen in H2S we obtain (HS)HS = H2S2, (HS)(HS)S = H2S3, &c., in general H2Sn. The homologues of CH4, CnH2n + 2 are formed in this manner from CH4, and consequently the polysulphides H2Sn are the homologues of H2S. The question arises why in H2Sn the apparent limit of n is 5—that is, why does the substitution end with the formation of H2S5? The answer appears to me to be clearly because in the molecule of sulphur, S6, there are six atoms of sulphur (Note 11). The forces in one and the other case are the same. In the one case they hold S6 together, in the other S5 and H2; and, judging from H2S, the two atoms of hydrogen are equal in power and significance to the atom of sulphur. Just as hydrogen peroxide, H2O2, expresses the composition of ozone, O3, in which O is replaced by H2, so also H2S5 corresponds with S6.