The anaesthetic potencies of binary mixtures of the gases argon (Ar), nitrous oxide (N2O) and sulphur hexafluoride (SF6) have been measured using mice. The mixtures SF6-N2O and N2O-Ar showed additive behaviour, whereas the constituents of the mixture SF6-Ar were non-additive, having a smaller total potency than expected. Further experiments on this mixture with Italian Great Newts and on the carbon tetrafluoride mixtures CF4-Ar and CF4-SF6 with mice suggested that the anomalous potencies may arise from specific pulmonary effects associated with the breathing of SF6 accompanied by a high pressure of some other gas.

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