Radium Emanation at Low Temperatures. 959 



average, 5 millimetre scale-divisions per degree centigrade. 

 A calibration curve was constructed by means of the fixed, 

 temperatures of melting ice (0° C), a mixture of solid C0 2 

 and ether (-79° C), boiling ethylene (-103° C), boiling- 

 methane ( — 164° C), and liquid air of which the percentage 

 of oxygen was determined by analysis ( — 186° C). Some 

 of the calibration curves were checked by further deter- 

 minations from the melting-points of pure ether and of 

 ethylene, but these points were difficult to fix with accuracy. 



At the beginning of the work an iron-nickel couple with 

 a Gambrell moving-coil galvanometer was employed, but 

 occasion arose to change these for couples of copper- 

 constantan with an Ayrton and Mather moving-coil gal- 

 vanometer. The wires of the thermo-couples were No. 30 

 double-cotton covered, and the junctions were bare, without 

 any protective covering, in order to avoid an appreciable tem- 

 perature lag. Most of the present results were obtained with 

 the warmer junction of the couple at the temperature of 

 melting ice. A few others were taken with this junction at 

 ordinary room temperatures, for which the necessary correc- 

 tions were obtained. 



Instead of an electrometer as detector of ionization, an 

 electroscope of low cubical capacity was employed. The 

 degree of sharpness of the volatilization temperatures can 

 be the better detected the smaller the ionization chamber. 

 In consequence, the ionization chamber was merely a small 

 air-tight brass tube, 11*0 cms. long, through which ran an 

 insulated brass rod carrying at the end outside the ionization 

 chamber a gold-leaf system. The volume of the free space 

 in this chamber was about 6 c.c. 



In some of the experiments where metal spirals were used, 

 the tube forming the spiral did not rise above the surface of 

 the pentane bath. In these cases the conducting tubes D 

 and D' (see diagram, fig. 1) were made of glass and were 

 joined to the tube of the spiral by rubber connexions, which 

 experience proved to be quite satisfactory. This arrangement 

 prevented the conduction of heat to the pentane bath along 

 metal conductors, and consequently ensured a slower rate of 

 rise of temperature. 



Discussion and Results. 

 On a close examination of this flow method of experiment, 

 it should be remembered that the temperature corresponding 

 to maximum ionization in the testing vessel is the temperature 

 at which the rate of volatilization of emanation from the con- 

 de using surface begins to decrease very rapidly to zero. 



