Radium Emanation at Low Temperatures. 965 



wires at a temperature a few degrees higher than that at 

 which the emanation volatilizes from glass. 



In an) r case it can be said that the temperature differences 

 in the present experiments were small. The emanation could 

 not have volatilized from the glass surfaces at temperatures 

 as low as — 177° C, for then the volatilized emanation would 

 have shown its presence in- the ionization chamber imme- 

 diately after opening the stopcocks, and without any appre- 

 ciable wait for the temperature to rise. Moreover, in some 

 experiments condensation only took place at — 177° or 

 - 178° C. 



Mention has been made before of attempts to get an idea 

 of the temperature distribution in the bath by means of a 

 second thermocouple. It was found that no matter how the 

 temperatures throughout the rest of the bath varied, the 

 bottom temperature at maximum volatilization of the emana- 

 tion was about the same in all cases. The temperatures at 

 the top and middle parts of the bath would of course depend 

 on the depth of pentane in the bath, an average value of the 

 temperature gradient being about 2° 0. per centimetre depth. 



It was not considered necessary to experiment with iron, 

 tin, and other metals, since M. Laborde obtained the same 

 result with these as with copper and silver. 



No doubt the above volatilization temperatures could have 

 been fixed with greater precision if a bath which remained 

 liquid over the required range of temperature could have 

 been used. But it was not feasible to procure the large 

 quantities of a liquefied gas (such as ethylene) and of liquid 

 air, which would be necessary for such a large number of 

 experiments. The experiments of M. Laborde must have 

 suffered from this same disadvantage, for it is not likely that 

 a metallic bath of granulated copper would be of uniform 

 temperature throughout. Some trials by the writer, dupli- 

 cating as nearly as possible the conditions, have shown that 

 after the process of warming from the liquid-air temperature 

 sets in, there is, as one would expect, a continuous and large 

 temperature gradient from the bottom of the bath to the top. 

 A metallic bath also is of a discontinuous structure, and 

 allows the penetration of air down to the bottom as soon as 

 the liquid air has evaporated. For these reasons the use of 

 metallic copper as a temperature bath was abandoned iii 

 favour of pentane. 



Conclusion. 

 In the series of experiments just described only small, and 

 almost inappreciable differences (I°G.) can be found in the 



