Burning glases, late 18th century.
© Science Museum / Science & Society Picture Library
Burning glases were one of the few ways available until the 19th century of producing high temperatures at a specific point. They concentrated the sun's rays on a spot, and by making it very hot, mercury would be converted into mercuric oxide and at a higher temperature be decomposed back to mercury. Double lens were particularly popular and were used by Joseph Priestley and Antoine Lavoisier. The double lens burning glas shown here, dating from around 1770, was found at Chatsworth House, the home of the Cavendish family, and may well have belonged to Henry Cavendish, the pathologically shy natural philospher who discovered hydrogen. The single lens and the reproduction of Priestley's mercury experiment are modern.