During the course of some work on Dewar vessels, which was carried out by one of us (B. L.) and S. F. Gates for the Oxygen Research Committee, a curious anomaly was noticed in the behaviour of an all-metal Dewar vessel. This was a commercial copper vessel of the usual spherical type with a long narrow neck of an alloy of low heat-conductivity; its capacity was two litres. The rate of evaporation of liquid oxygen stored in this vessel was approximately double that of liquid oxygen stored in a silvered glass flask of like capacity; but, when equal weights of hot water were put into each of the vessels, it was found that the rate of cooling of the water in the copper vessel was actually slower than in the silvered glass vessel. It appeared, then, that the copper vessel was only half as efficient as a silvered glass one of like capacity for the storage of liquid oxygen, whereas its efficiency for the storage of hot water was greater than that of the silvered glass vessel. This investigation arose out of a desire to explain the apparent anomaly. Previou work on the factors which influence the efficiency of Dewar vessels has been carried out by Dewar ('Proc. Roy. Inst.,' 1898, p. 815), Banneitz, Rhein and Kurze ('Ann. d. Phys.,' 1920, vol. 61, p. 113), and Briggs ('Proc. Roy. Soc. Edin.,' 1920, vol. 51, p. 97). These investigations have dealt with the efficiency of Dewar vessels considered only as containers for liquid air or oxygen, and the above-mentioned anomaly has therefore not been noticed. Briggs (
loc. cit
.) worked with vessels with the vacuum-adjacent surfaces of polished gilding metal (95 per cent. copper). From his results on the rates of evaporation of liquid oxygen from these vessels, he calculated a value for the emissivity of the polished surfaces which was considerably greater than that anticipated from the usually accepted value for copper. This observation is intimately connected with that of Lambert and Gates and will be referred to later.
On parabolic subgroups of Artin–Tits groups of spherical type