Engraving by W. Bromley after J.H. Füssli's painting, Wellcome Library, London

In Shakespeare’s Macbeth, the three witches brew a potion with ingredients chosen to symbolise incredible evil. Amongst human organs and animal parts, the witches use the root of the deadly hemlock (Conium maculatum), a highly poisonous plant known to have a chemical structure and pharmacological properties similar to nicotine. Even in low doses, the plant can cause respiratory failure and death. The root of the plant, used by the witches in their deadly concoction is known to have the highest toxicity concentration of all of the plant.

The witches also add yew (Taxus baccata). The plant contains the highly toxic alkaloid taxine which if ingested can cause rapid death through cardiac arrest.

Though the witches potion is a magical creation, the deathly potential of the ingredients used is very much real. The plants are some of the most poisonous species found in Western Europe and it is evident that Shakespeare was aware of their lethal nature.

It is also significant that the witches were boiling their broth. Boiling has long been known to sterilise water and destroy pathogenic, disease causing micro-organisms. The increased temperature would also give the molecules more kinetic energy and the reaction would proceed faster. However, whether or not the concentration of the product would increase is dependent on the type of reaction.

The scene is one of the most evocative in all of Shakespeare, and has inspired many writers, from J K Rowling to Samuel Beckett.