In letters written in 1776-77 the Italian physicist Alessandro Volta (1745-1827) described a new form of Eudiometer, which is a device for testing the "goodness" of air. In its most useful scientific form, this was a stout glass tube of constant inner diameter closed at the top, where two electrodes passed through the glass and formed a spark gap. The lower end of the tube was placed in a dish of water, and the air to be tested was introduced into the tube and its volume noted. A known volume of hydrogen was then let into the tube, and the mixture ignited by static electricity. The experiment could then determine the goodness of the air (the oxygen content) from measurements of the volume of the remaining gas. 

   The instrument at the near right is in the collection of historical instruments at Transylvania University, and is the form invented by Volta. The mid-to-late 19th century instrument at the far right is at St. Patrick's College in Maynooth, County Kildare, Ireland, and is the form devised by Henry Cavendish (1731-1810). 

   At the left is the version developed ca. 1835 by Robert Hare of the University of Pennsylvania. It is at Transylvania University in Lexington, Kentucky.

The Eudiometer is the basis for the Volta's Pistol demonstration.

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