Copy letter from Francis Alexander Towle, Assistant Secretary of the Royal Society; to T. R. Parsons Esqurie; Physiological Laboratory Returns Mr. T. R. Parsons' paper, 'The theory of the Barcroft differential blood-gas apparatus' [not enclosed]. Notes, however, that by the Standing Orders either the original or a copy of any paper submitted has to remain in the possession of the Society. 19 January 1921