Molecule of the Month: The poison prescribed by Agatha Christie: Correction

In 'The poison prescribed by Agatha Christie' (Molecule of the Month, 20 July), Dr John Emsley described how, in 1971, Graham Young used thallium sulphate to poison workmates at a Hertfordshire photographic equipment factory. The article erroneously stated that the photographic company used thallium to make lenses. This is not the case: the company does not and has never used thallium in any form. The thallium that Graham Young used was purchased from a chemist's in London.