Wikia

Primum ens Melissae is a substance that can rejuvenate, to some degree, the well-being and vitality of one's body by attracting and concentrating the source of spirit and soul that was found in some herbs.^[1]

At one point, during the brewing of the Polyjuice Potion, one will obtain an elixir of Primium ens Melissae, into which should be added powdered Bicorn horn and, on the 13th hour, leeches with Salpeter raspings.^[2]