"Among his papers is one on which he himself has written, "communicated to Dr. Priestley," the contents of which are referred to by the latter in his account of Experiments and Observations made in and before the year 1772... The paper... has been printed by Mr. Harcourt, and is... one of the earliest distinct accounts of . Cavendish prepared it by passing atmospheric air repeatedly through red hot , and removing the produced, by caustic potash."