Radium Emanation at Low Temperatures. 963 



were always made to keep the conditions regarding (1) the 

 rate of How o£ air-current, (2) the rate of rise of temperature, 

 and (3) the quantity of emanation condensed the same for all 

 materials. Experience showed that there could be consider- 

 able latitude in all three factors without obtaining final 

 temperature results outside the error of experiment. This 

 is especially so with regard to the quantity of emanation. 



The numbers tabulated under " Maximum Ionization," 

 representing divisions per minute in the electroscope, give a 

 rough approximation of the relative amounts of emanation 

 nsed in the different experiments ; those under " Rate of 

 Rise of Temperature " refer to the bottom temperatures of 

 the bath at the time of maximum ionization. The " Rates of 

 Air-Flow " refer to a temperature of —160° C. 



The temperature correction for the time taken by the 

 volatilized emanation to pass from the condensing spiral to 

 the electroscope was negligible in comparison with the error 

 of experiment. 



It will be noticed from the diagram and table (Table I.) that 

 in the case of glass the temperature was consistently a few 

 degrees lower than in the case of metals. The difference in 

 the case of silvered glass was not so large, and in consequence, 

 the smaller heat conductivity in glass, compared with metals, 

 was not quite satisfactory as a reason for the differences 

 shown. To obtain more information on the point further 

 experiments were performed. In these, condensation of the 

 emanation took place in small glass and lead spirals of only 

 two or three coils, the planes of the coils when placed in the 

 bath being vertical instead of horizontal, and the spirals being 

 just covered with pentane. Under these circumstances there 

 could only be very little temperature variation over the spirals. 

 The thermo-couple (a single copper-constantan element) 

 was threaded through the spiral tube so that the temperature 

 on the inside of the tube was the one determined. The wires 

 of the thermo-couple were brought to the open, outside the 

 bath, through the walls of the glass conducting tubes, and 

 the holes through which the wires emerged were closed by 

 sealing-wax. Taking these precautions, the final temperatures 

 w T ith both lead and glass spirals come, within the experi- 

 mental error, to the same value, as can be seen from the table 

 which follows. 



This result, taken in conjunction with what has gone before, 

 has one of two explanations. Either the condensed emana- 

 tion volatilizes from the surfaces of glass, thermo-couple wires, 

 and of lead at the same temperature ; or, some emanation 

 remains condensed upon the surface of the thermo-couple 



