Conduction of heat through rarefied gases.—II

In a previous paper* measurements were described of the thermal conductivity of twelve different gases in what may he termed a “free-path vacuum,” that is, at pressures so low that the molecules conducting the heat from the hot to the cold surface do not, as a rule, experience mutual encounters. It was shown ,that at sufficiently low pressure the conductivity of all the gases was proportional to the pressure, and the conductivity K (defined as the calories dissipated per second, per 0·01 mm. pressure of gas per square centimetre of hot surface, per 1° difference of temperature between the latter and its surroundings) was compared with the theoretical conductivity Q, as calculated approximately from the molecular heat and mean molecular velocity of the gas, by means of the kinetic theory, on the assumption that the heat interchange between the molecule and the surface it impinges upon was perfect. For argon and neon the ratio K/Q (1·09 and 1·04 respectively) was in gratifying agreement with this assumption, but for all the other gases the ratio was less than unity. For these the ratio was greater than 0·66, except for helium and hydrogen, for which the very low values 0·5 and 0·25 respectively were found. The suggestion was hazarded that possibly for these light gases the interchange of heat is imperfect, owing to the greater velocities at which the molecules move. In the present paper some of the earlier measurements have been repeated with the original apparatus, with better provision for keeping the tempera­ture of the surrounding water jacket uniform. Then the apparatus was rebuilt to allow of the measurements of the conductivity to be taken over a wide range of temperature, both of the hot and cold surface, and the effect of variation of the temperature on the ratio K/Q was examined. Measure­ments were confined to the three gases—hydrogen, helium, and argon Incidentally the effect on the conductivity of hydrogen of using a hot palladium surface instead of one of platinum was examined. The result of these new experiments has been to negative the suggestion already referred to, that the discrepancies between the found and calculated conductivities might be due to imperfect interchange of energy on impact. The ratio K/Q appears to diminish as the temperature at which the experiments are performed decreases and to increase as the temperature is increased, and no explanation of this can at present be suggested.