ONE of the most noticeable features of the periodic classification of the elements is the connexion between valency and atomic weight. As is well known, the constitution of the higher oxides shows that valency rises regularly throughout the groups, whilst the composition of the, hydrides indicates a rise to a maximum in the fourth group followed by a decline to zero in the eighth. I n view of the compleh development of oxygen valency, the question arism as to the cause of this peculiar

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... of hydrogen valency, which follows so regular a path thatl it seems some definite explanation should be forthcoming. Clearly the phenomenon must be due t o some property of (a) the hydrogen atoms, o r ( b ) the atoms to which they are united. Taking the two cases in order: (a) The decline of hydrogen valency cannot be due t o any property peculiar tol the hydrogen atom, because the alkides are of