In the course of investigations on argon, some clue was sought for, which would lead to the selection of one out of the almost innumerable compounds with which chemists are acquainted, with which to attempt to induce argon to combine. A paper by W. F. Hillebrand, “On the Occurrence of Nitrogen in Uraninite, &c.” (‘Bull, of the U. S. Geological Survey,’ No. 78, p. 43), to which M r. Miers kindly directed my attention, gave the desired clue. In spite of Hillebrand’s positive proof that the gas he obtained by boiling various samples of uraninite with weak sulphuric acid was nitrogen (p. 55)— such as formation of ammonia on sparking with hydrogen, analysis of the platinichloride, vacuum-tube spectrum , &c.— I was sceptical enough to doubt that any compound of nitrogen, when boiled with acid, would yield free nitrogen. The result has justified the scepticism.