33

\pi \lambda
33

valency; any even number of triads giving a compound of any even valency not exceeding the number of triads by more than two, and any odd number giving away odd valency under the same restriction.
This shows that there are five natural classes of forms of graphs; namely, medads, monads, dyads, triads and higher perissids (odd-valents), tetrads and higher artiads (even-valents), and that is the reason why no chemical element has a minimun valency greater than 4.
In classifications generally, it may fairly be said to be established, if it ever was doubted, that form, in the sense of structure is [] for higher significance than material valency is the basis of all external structure; and where indecomposability precludes internal structure, as in the classification of elementary concepts, valency ought to be made the first consideration