Testing an analgesic inhaler. This inhaler (lower right) contained trichloroethylene, an analgesic commonly used by midwives assisting in childbirth. Inhalers like this were analysed by researchers to ensure that when the trichloroethylene liquid evaporated into gas, it was of the correct concentration to relieve pain. If the gas was too concentrated, it could lead to unconsciousness and death. Because of this risk, trichloroethylene was withdrawn from medical practices and replaced by methyl chloroform in 1968. Photographed at the National Physical Laboratory, Teddington, UK, in 1955.