Dormitive

But "imitation" throws no light upon why they so act; it repeats the fact as an explanation of itself. It is an explanation of the same order as the famous saying that opium puts men to sleep because of its dormitive power.

Quare Opium facit dormire: ... Quia est in eo Virtus dormitiva. (Why Opium produces sleep: ... Because there is in it a dormitive power.)