Shoots and leaves

When a metal salt is burned, a distinctive coloured flame is seen – a property that is exploited in fireworks. If the light of the flame is viewed through a prism, it is separated out into its component colours and a pattern of different coloured lines is seen. Each element gives its own unique set of lines.

This technique of flame spectrometry was used in the mid-nineteenth century to identify many new elements, two of them thallium and indium in Group 13. Both have names inspired by the distinctive line in their flame spectrum, and my patterns are in turn derived from their names.

In 1861 Sir William Crookes was analysing the flame spectra of some residues of industrial waste. He had found selenium and was looking for tellurium when he saw a distinctive green line that didn’t correspond to any known element. He named his new element thallium from the Greek ‘thallos’ for green shoot as the colour reminded him of leaves in springtime.

Two years later, Professor Ferdinand Reich, who was colour-blind, and his assistant Hieronymous Theodor Richter, who thankfully was not, were checking a zinc ore to see if it contained any of the recently discovered thallium. There was no green line in their spectrum, but instead, Richter noticed an unusual indigo-blue line. Consequently they named their new element indium. The colour indigo takes its name from the dark blue dye produced from the indigo plant. Native to India, Indigo tinctoria is a member of the legume family and has pinnate (Orla Kiely-style) leaves.

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