When Dr. William Macbeth poisoned two of his sons in 1927 his wife and sister hid the murders in the intensely private realm of family secrets. But secrets can be as corrosive as poison and as time passed the story of Dr William Macbeth well-dressed poisoner haunted and divided his descendants. Macbeths granddaughter Gail Bell who grew up with the story spent ten years reading about poisons in order to read and understand Macbeths life. A chemist herself she listened for echoes in the great poison cases in history in myths fiction and poison lore. As well as Bells own story here too are the stories of Cleopatra and Snow White Madame Bovary and Napoleons doctor Dr Crippen and Harold Shipman. Intricate and beautifully written with a stunning twist at its end The Poison Principle is both a fascinating detective story and a moving memoir of death and deceit.