In the early part of the sixteenth century the practice of the black art was carried on throughout England, mostly by old women, who also sold charms and love philtres. Shakespeare’s description of the witches’ incantations in _Macbeth_ presents some idea of a seance, and the gruesome articles in which they dealt. To know the properties of the most poisonous herbs (often quite fictitious) was part of their trade. “Round about the cauldron go; In the poison’d entrails throw. Toad, that under coldest stone, Days and nights hast thirty-one Swelter’d venom sleeping got, Boil thou first i’ the charmed pot. Double, double, toil and trouble; Fire burn, and cauldron bubble. “Fillet of a fenny snake, In the cauldron boil and bake; Eye of newt and toe of frog, Wool of bat and tongue of dog, Adder’s fork, and blind-worm’s sting, Lizard’s leg, and owlet’s wing, For a charm of powerful trouble; Like a hell-broth boil and bubble. Double, double, toil and trouble; Fire burn, and cauldron bubble. “Scale of dragon, tooth of wolf, Witch’s mummy, maw and gulf Of the ravin’d salt-sea shark; Root of hemlock, digg’d i’ the dark, Liver of blaspheming Jew; Gall of goat, and slips of yew, Sliver’d in the moon’s eclipse; Nose of Turk and Tartar’s lips; Finger of birth-strangled babe, Ditch-delivered by a drab, Make the gruel thick and slab; Add thereto a tiger’s chaudron, For the ingredients of our cauldron. Double, double, toil and trouble; Fire burn, and cauldron bubble.” The method here used by the witches to measure the time that the cauldron should boil by singing their incantation is, according to Dr. Lauder Brunton, an ancient mode of calculating time still used in some parts of the country at the present day. By thus repeating several verses they could regulate the time of boiling fairly well. The old apothecaries used the moon as a method of calculating the time certain processes should take, and the word “menstruum,” still commonly used, was employed because certain drugs were allowed to macerate a month in the liquid to extract the active constituents. In the toad that had been lying under a stone for thirty-one days and nights, we have another curious instance how the empirical practitioners of mediæval times acted on a certain traditional knowledge, which modern science has since proved to be correct.