COAL MININING SAFETY & CRUDE WAYS OF DETECTING METHANE GAS!!
In the 1700's, certain gases or the lack of oxygen were detected with various hit and miss types of detection. The candles on miners caps, or if carried by the miner, would either go out from the lack of oxygen or the flame would get larger with a different coloring of the flame if certain gases were in the area.
Of course, in some instances these open flames caused fires or explosions. By 1815, the Davy's Safety Lamp came into use in the mines. This certainly changed the way for miners to check for certain gases.
They took these canaries in small cages with them down the coal mines where they worked. The canaries were the miners alarm signal to show them when the coal-gas levels got too high. The canary stopped singing and was most likely to be laid feet up on the bottom of the cage, poisoned by the mine gas.