Professor Andrews, in the “Philosophical Magazine” for 1852, recalled the attention of physicists to the method originally devised by Davy of making a vacuum so perfect, that the residual gas exercised no appreciable pressure as registered by the depression of a barometric column. This he effected by filling the vessel to be exhausted with carbonic acid gas, having previously inserted a cup containing a concentrated solution of caustic potash. On rapidly exhausting with an air-pump, and leaving time for the absorption of the residual carbonic acid by the caustic potash, he obtained a vacuum as perfect as a Torricellian. Andrews' method was afterwards employed by Gassiot in his well-known investigations on the passage of electricity through attenuated media.