text: Henry Cavendish, despite not being the first to replace water in the trough with mercury, was among the first to observe that fixed air was insoluble over mercury and therefore could be collected more efficiently using the adapted instrument. He also characterized fixed air (CO) and inflammable air (H). Inflammable air was one of the first gases isolated and discovered using the pneumatic trough. However, he did not exploit his own idea to its limit, and therefore did not use the mercury pneumatic trough to its full extent.  Cavendish is credited with nearly correctly analyzing the content of gases in the atmosphere.  Cavendish also showed that inflammable air and atmospheric air could be combined and heated to produce water in 1784.