page_title: Eudiometer
section_title: History
hierarchical_section_title: Eudiometer / History
caption_attribution_description: English: Eudiometer after J.H. Magellan, probably by W. Parker, London
context_page_description: A eudiometer is a laboratory device that measures the change in volume of a gas mixture following a physical or chemical change.
context_section_description: In 1772, Joseph Priestley began experimenting with different "airs" using his own redesigned pneumatic trough in which mercury instead of water would trap gases that were usually soluble in water. From these experiments Priestley is credited with discovering many new gases such as oxygen, hydrogen chloride, and ammonia. He also discovered a way to find the purity or "goodness" of air using "nitrous air test". The eudiometer functions on the greater solubility of NO₂ over NO, and the reaction
  2 NO  + O₂ → 2 NO₂. A quantity of air is combined with NO over water, and the more soluble compound dissolves, leaving the remaining air somewhat contracted in volume. The richer the air was in oxygen, the greater the contraction.
  It is from these experiments that Marsilio Landriani became inspired to create a more useful tool in measuring the "healthiness" of air. In 1775, Landriani invented the first eudiometer and used it to conduct his own experiments. From these experiments Landriani theorized about the temperament of certain gases and the atmosphere and later published a paper called Ricerche fisiche intorno alla salubrità dell'aria (Physical researches on the salubrity of air). Though many of his findings were incorrect, his invention was the precursor of the modern day eudiometer.
  The eudiometer with the nitrous air test was the way Jan Ingenhousz verified that the bubbles given off under water by plant leaves exposed to sunlight were oxygen bubbles. His description of photosynthesis was published in 1779, and in 1785 he wrote about eudiometers in Journal de Physique (v 26, p 339). According to a biographer, Ingenhousz indicated that "many instruments were called eudiometers although strictly speaking they didn't deserve the name ... misunderstandings could exist when not everybody was using the same instruments."
  An electrified version of the eudiometer was developed by Count Alessandro Volta (1745–1827), an Italian physicist who is well known for his contributions to the electric battery and electricity. Aside from its laboratory function, the eudiometer is also known for its part in the "Volta pistol". Volta invented this instrument in 1777 for the purpose of testing the "goodness" of air, analyzing the flammability of gases, or to demonstrate the chemical effects of electricity. Volta's Pistol had a long glass tube that was closed at the top, like a eudiometer. Two electrodes were fed through the tube and produced a spark gap inside the tube. Volta's initial use of this instrument concerned the study of swamp gases in particular. Volta's pistol was filled with oxygen and another gas. The homogeneous mixture was taped shut with a cork. A spark could be introduced into the gas chamber by electrodes, and possibly catalyze a reaction by static electricity, using Volta's electrophorus. If the gases were flammable, they would explode, and increase the pressure within the gas chamber. This pressure would be too great and eventually cause the cork to become airborne. Volta's pistol was made with either glass or brass, however due to the electricity the glass was vulnerable to exploding. Volta's extensive studies on measuring and creating high levels of electric currents caused the electrical unit, the volt, to be named after him.
  In 1785 Henry Cavendish used a eudiometer to determine the fraction of oxygen in the Earth's atmosphere.
caption: Eudiometer after J.H. de Magellan, Teylers Museum
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