Much of the early experimental work in thermodynamics was concerned with gases, because chemical equipment could offer new data that could help to identify their properties. James Prescott Joule and William Thomson (later to become 1st Baron Kelvin) set up an experiment which forced gas through a valve, to test the temperature change, and they found that gas cools as it expands. This process, known as 'throttling', has had vital implications for all thermal machines, including fridges and air conditioning.
Editors. "James Joule and Lord Kelvin demonstrate that a rapidly expanding gas cools". The Literary Encyclopedia. First published 30 August 2013
[http://www.litencyc.com/php/stopics.php?rec=true&UID=6309, accessed 19 April 2014.]