The greatest progress in modern chemistry was attained by perfecting instrumentation able to control fire, the chief element used by chemists to analyze substances. In the 18th century, the discovery that variations in temperature affect chemical reactions stimulated the systematic use of the thermometer and the pyrometer (for measuring the expansion of metals and the temperature of furnaces). Thanks to these instruments calorimetric standards were significantly raised, allowing the power of fire to be more efficiently controlled and measured.