heptad

Seven, or the heptad, is a number very powerful for good or for evil.

Thus one of the Meta molecules of iron, containing seven atoms, is identical in appearance with a Hyper heptad, but the latter dissociates into seven atoms, the former into two triads and a single atom.

Colours may best be expressed by a heptad, the largest possible formula for things finite, as the pentad is the smallest possible form.

The word pentad at once recalls to you the way in which the chemist speaks of a monad, triad, heptad, when he deals with elements.

Indeed, the heptad of things finite is in all cases reducible to the pentad.