1. THE task of naming the homologues of hydro-X gen is not to be undertaken lightly. Nomenclature is of such importance that all considerations should be laid aside, except those of reasoned expediency and sound philological practice. The example set by Faraday, just a century ago, in framing names for use in describing and discussing electrochemical phenomena, may well serve us to-day. His words have stood the test of time, because of the extreme care with which they were chosen. 2. The new gases (from water) are clearly all Hydrogens (Water-stuffs), as each has its own water in water. They are to be grouped under their Atomic Number 1, as homologous members of the first term in the periodic series of elements. It would seem to be desirable to have an index name common to all members of the group. Deuterium is in no way reminiscent of water and also has no particular group significance. ‘Second to what?’ is the question that at once arises. The name would be a fit one for the first member of the second group (Atomic Number 2) in the periodic series. If we could agree so to use it, succeeding groups might well have similarly significant group names?Triterium, Tetra-terium, etc., up to 92, in addition to the familiar names.