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 affected by changes of temperature, although the total amount was less; but this important fact lay dormant and unused.

Mercury thermometers were also known in Paris; Ismael Boulliau is said to have made them in 1659; and a letter dated 28th May, 1684, written in Paris to the Royal Society, London, by Mr. Musgrave, describes one three inches long and five lines in diameter, that was used for taking the temperature of fever patients.

Christian Huyghens, in 1665, and Dalencé, in 1668, are also credited with the invention of mercury thermometers. In the same year that Dalencé's book appeared, Edmund Halley, the eminent English mathematician and astronomer, was studying experimentally the relative expansion of water, alcohol and mercury, and he stated that mercury would make a good thermometrical liquid if its coefficient of expansion was greater; he did not actually recommend the liquid metal, although he perceived that its expansion was large enough to influence the readings of the barometer. He observed that mercury heated in boiling water ceased to expand on long continuing the operation, and according to Momber he proposed taking the boiling-point of water as a fixed