THERMAL PROPERTIES OF CARBONIC ACID AT LOW TEMPERATURES. 85 



thermo-junctions and temperature could not be satisfactorily represented by the 

 empirical formula often used, viz., log E = nlog t + m. A. calibration curve was 

 therefore drawn, points on it being obtained as follows. For temperatures between 

 + 100 C. and 0" C. the junction was compared with a standard mercury thermometer 

 with N.P.L. certificate. 



At 20* C. it was compared with a mercury thermometer which had Ix'en vi-ritii-il 

 at that temperature by the N.P.L. The melting-point of mercury was then olerved. 

 The mercury was purified by dropping it through nitric acid, washing it in watT, drying 

 it at 120 C., and finally distilling it in vacuo. About l Ibs. of the purified mercury 

 was put in a glass vessel and frozen by packing it with CO a snow. The mercury 

 was then gradually melted and the temperature of the melting-point observed. There 

 was no difficulty in keeping the mercury half-melted and half-frozen for any length of 

 time desired. The melting-point was assumed to be 38'80 C.* A calibration curve 

 was drawn through these points and extrapolated to 50 C. As this extrapolation 

 was open to doubt a further point at 50 C. was afterwards obtained by comparison 

 with a platinum-resistance thermometer which had been carefully calibrated against 

 the standard thermometer. The new point obtained in this way fell within -, 1 ,, C. of 

 the curve, thus confirming it satisfactorily. The curve is believed to be correct to 



A'c. 



To maintain the other junctions of the wires at a steady known temperature they 

 were immersed in a large tin of paraffin oil, well jacketed with slag wool, fitted with 

 a calibrated thermometer and lens so that the temperature of the " cold junction " (in 

 our case usually the warmer of the two) could be read to T ,\ C. The oil was stirred 

 by blowing air into it. An incandescent lamp was placed in the oil, so that the oil 

 could be warmed to approximately atmospheric temperature. The temperature of 

 the oil was read several times during each test and rarely varied more than t\y C. 



The E.M.F. produced by the thermo-couples was measured against the standard 

 cadmium cell by a potentiometer with twenty 1-ohm coils and a gilt manganin slide- 

 wire, 1 m. long, and of just over 1-ohm resistance. Special precautions were taken 

 to avoid thermo-electric effects. The various thermo-junctions could be switched on 

 in turn to the potentiometer by a two-pole six-way selector switch, designed to avoid 

 thermo-electric effects. The potentiometer was sensitive enough to measure tempe- 

 rature differences of 7^ C. Such accuracy was of use when measuring the small rise 

 or fall of the temperature of the bath during the run ; also in Series IV. experiments, 

 which depend on small differences of temperature, and in measuring the slow 

 temperature rise during radiation tests. 



The pressure of the gas was measured by steel tube Bourdon gauges made by 

 Messrs. Schaeffer and Budenberg with specially fine needles and fine scale-divisions. 

 They were calibrated by means of a dead-weight testing machine, in which a dead- 



* CHAPI-UIS, 1900, quoted KAYE and LABY, p. 48. Dr. J. A. BARKER, of the N.P.L., has informed 

 the authors that 38 -86 C. is probably a more accurate figure. 



