The experiments here described were made on behalf of the Oxygen Research Committee of the Scientific and Industrial Research Department, and the paper is given by permission of the Department. The fullest acknowledgment is due to Dr J. A. Harker, F.R.S., and to his co-workers, Professor G. W. Todd and Mr S. H. Groom, who have, in a series of able memoirs presented to the Oxygen Committee, analysed the nature of the heat-transfer from the outer atmosphere to the interior of metal vacuum bottles; but for their memoirs the writer's experiments would not have suggested themselves.