Experiments in a Highly Rarefied Gas. 409 ; 



lead into (A), one into (B). Over these brass tubes, glass- 

 tubes with glass taps are cemented with sealing-wax. They 

 are connected as shown in the figure. Through the tap (t) 

 a drop of Hg can be brought into (A). By means of the 

 taps (t 2 ) and (t 3 ) the halves (A) and (B) can be separated 

 from the rest of the apparatus, i. e. a charcoal-liquid air- 

 tube and a Gaede pump. 



This absorption cell is placed in front of a resonance lamp 

 (comp. Wood, /. c), i. e. in our experiment a bulb made 

 of transparent quartz-glass of 3 cm. diameter, filled with 

 pure Hg-vapour of ()"001 mm. pressure. This Hg-vapour is 

 excited to resonance radiation by means of sideways illumi- 

 nation with the special Hg arc-lamp. Then the whole quartz- 

 bulb emits light rather uniformly and of great intensity in 

 the line 2536 A.E. and with very little stray light. The 

 distance between resonance lamp and absorption cell is 10 cm.. 

 The resonance light passes through the absorption cell and, 

 then falls upon a uranium-glass plate held in front of the 

 cell, exciting strong fluorescence in the uranium glass. 



To carry out the experiment, we proceed in the followino- 

 way: — The whole apparatus, as shown in fig. 4, is pumped 

 out with a Gaede pump, while all taps are open till the air- 

 pressure of about 0-00002 mm. is reached. After this, the 

 tap (Yj) is closed. Now the charcoal tube is submerged in 

 liquid air and all traces of Hg-vapour from the halves (A) 

 and (B) are completely absorbed. Then taps (t 2 ) and (/ 3 ) are 

 closed. At this stage the light of the resonance lamp passino 

 through the absorption cell produces two bright fluorescence 

 patches on the uranium glass of equal intensity. Now tap 

 (fj) is opened and a drop of mercury is allowed to enter (A).. 

 Immediately the light patch, due to the radiation through 

 the half (A), disappears entirely and only a few seconds 

 later the light patch due to (B) disappears too. But bv 

 opening now tap (t 2 ), and therefore connecting the half (B ) 

 with the charcoal-liquid air vessel, (B) immediately brightens 

 up and in a few seconds is as bright as at the beginning of 

 the experiment. But (A) remains dark. 



From this experiment we can deduce the following con- 

 clusions : — 



(i.) That a drop of Hg at room temperature is able to fill 

 a chamber immediately (practically) and is able to keep it 

 filled with saturated vapour, even if from this chamber the 

 Hg-vapour is sucked away through a slit of the given 

 dimensions by liquid air. 



(ii.) That a slit of the dimensions mentioned above allows 

 of remarkably rapid diffusion. 





