Radium Emanation at Low Temperatures. 723 



we Lave concerning the process of volatilization o£ condensed 

 emanation is that given by the flow method of experiment 

 originally devised by Rutherford and Soddy *. This method 

 is best adapted, and so far has been used, for experiments 

 with small quantities of emanation. Under the circum- 

 stances of its use condensation of the emanation can only 

 result in a very sparse distribution of emanation molecules 

 over a considerable area of coo'ed surface, so that the con- 

 densed "layer" will be of much less than molecular thickness. 

 In these cases it is probable that the phenomenon is entirely 

 one of surface adhesion or occlusion. 



The object of the present paper is to describe briefly some 

 experiments which were performed to seek further informa- 

 tion on the process of volatilization at low temperatures. 

 The conditions in the experiments were quite different from 

 those in the flow method. The emanation was contained in 

 sealed glass tubes which were as free as possible from all 

 other gases ; condensation and volatilization were confined 

 to the point where the minute volume of condensed emanation 

 was situated ; and no current of air or other gas was 

 required. 



Apparatus and Method of Experiment. 



The tubes containing the emanation were of the shape 

 ABC shown in fig. 1 (p 724). The wall of the part AB was 

 2\5 mm. thick, and of the part BO 1 mm. thick. The bore of the 

 tube was usually about 2*5 mm. The end of the tube at A 

 was closed by a very thin sheet of mica, which was secured 

 to the wall by a special kind of marine glue. The thickness 

 of the mica was equivalent to 1'9 cm. of air in its stoppage 

 of a-particles ; nevertheless the sheet was strong enough to 

 support the full atmospheric pressure over the opening, and 

 thus maintain a vacuum inside the tube. 



For an experiment the glass tube was first evacuated to a 

 charcoal vacuum. The required amount of purified emanation 

 received from Prof. Rutherford was then introduced, and 

 the tube was sealed at C. Four hours after admittino- the 



m o 



emanation the amount present was determined by the 7-ray 

 method. The tube was then fitted to a small ionization 

 vessel MNO in the manner shown in the diagram. The 

 vessel and fittings were made air-tight so that the pressure 

 of air in the ionization chamber could be adjusted to any 

 value. With such an arrangement the ionization was 

 usually very intense, and it was often very difficult to obtain 

 saturation. 



* Phil. Mag. [6] v. p. 561 (1003). 



