Orsat Tester Used to Test Flue Gases, circa 1935

Developed by Louis Orsat in the late 1800s, Orsat testers measured carbon monoxide, carbon dioxide, and oxygen gases during combustion. They were often used to monitor fuel efficiency in factory fuel boilers -- inefficient combustion meant higher costs! Orsat testers, like this one from the 1930s, were eventually replaced by modern gas analyzers with digital readouts.