SO2{NO2 SO2{NO2 SO2{OH {Cl {OH {OH Sulphuryl nitryl Leaden chamber Sulphuric chloride. crystals. acid. These formulæ serve to show how the several substances are mutually related, and that they may be derived from one another by the substitution of atoms of chlorine for hydroxyl (OH), or nitryl (NO2), or _vice versa_. It was pointed out in 1851 by Williamson, and subsequently by Gerhardt, that these groups are characterised by differences in their power of combining with or replacing atoms of hydrogen, or of groups or elements which, like chlorine, are chemically equivalent to hydrogen. Such a radical or residue as _ethyl_ (C2H5) is chemically equivalent to _one_ atom of hydrogen, as is shown when we compare the formula for ether, as established by Williamson, with that of ordinary alcohol: C2H5}O C2H5}O H } C2H5} Alcohol. Ether. _Sulphuryl_, SO2, is chemically equivalento _two_ atoms of hydrogen; _phosphoryl_, PO, as suggested by Odling, to _three_ atoms of hydrogen. Gerhardt therefore proposed to designate these and similar groups as _monatomic_, _diatomic_, _triatomic_, according to their respective hydrogen-replacing power. This conception of the definite atom-fixing or replacing power of groups or compound radicals was extended by Frankland, in 1852, so as to include the simple radical—that is, the elements. In the memoir in which he announced the existence of the organometallic compounds he pointed out that the elements may be classified according to their combining power, or, as he expressed it, according as “their affinities are best satisfied.” This idea was independently developed by Couper and Kekulé in 1858; it is from that period that the definite introduction of the conception of _atomicity_, _atomic-value_, or _valency_, into chemical doctrine may be said to date.