The estimation of the percentage of firedamp (methane) present in the atmosphere of a mine is usually performed by observing the height of the flame cap over the lowered flame of a safety lamp. In ordinary circumstances the safety-lamp flame is reduced in size and luminosity to render the cap visible, and the estimation of the amount of inflammable gas is made by observing the height of the cap. It was pointed out by Chesneau (1892) that the visibility of the cap was increased if certain organic chlorides and copper salts were added to the fuel. Also it was shown by Briggs (1912) that strongly coloured caps were obtained if a small quantity of carbon tetrachloride was added to the fuel and a loop of copper inserted in the lamp flame itself.