1917] 



on Soa.p Bubbles of Long Duration 



195 



in a black bubble of less than about 1 cm. diameter would be 

 sufficient to cause the transference of a layer of gas of a thickness 

 equal to the length of the mean free path. The rate of transference 

 in bubble ii. is only about three-quarters of that in bubble i. 



A black spherical air bubble about 42 cm. diameter, supported 

 by a nozzle above and a small fixed horizontal glass ring below : 

 thus keeping its vertical diameter constant, altered to an ovoid shape, 

 33 cm. horizontal diameter, in 109 days. The lessened internal air 

 pressure caused by the ovoid form reduced the rate of the air trans- 

 ference to about one quarter that in bubble i. (Table 6). 



Gas Transference through Black Bubbles in Hydrogen. 



A few hydrogen bubbles blown in hydrogen were also observed. 

 A suitable vessel for these bubbles is shown in Fig. 7. An oxidised 

 iron-wire ring is seen supporting a bubble, not yet completely black, 

 within the flat-bottomed vessel A (about 3 litres capacity). The 

 filling of the vessel A with hydrogen is attended with considerable 

 difficulty, and requires great care in order to obtain " optical 

 emptiness " in the vessel. The nozzle B should be pushed down 

 nearly to the bottom, while the vessel A is being filled with hydrogen 

 by the tube (used later as an outlet when expanding the bubble). 

 No Tyndall cone will be visible if the vessel is properly filled. The 

 bubble is then blown on B in the usual way, and expanded and let 

 down to rest on the wetted ring already prepared for it ; or it may 

 be left suspended from the nozzle. Both inlet and outlet tubes are 

 protected by glycerined cotton-wool filters. 



The contractions observed in hydrogen bulbs were generally more 

 rapid than those of similar-sized air-bubbles, as shown in Tables 7, 

 8, 9, where the units are the same as in the last Tables. 



Table 7. — 50 per cent, glycerine, 5 per cent. soap. 



Hanging from glass nozzle. Standing on iron-wire ring 



Table 8. — 33 per cent, glycerine, 3 per cent, soap 

 glass nozzles. 



jing from 



Diameter .... 

 Internal pressure . 

 Rate of gas transference 



4 6 8 10 



I . ' 



0-45 j 0-30 0-23 | 018 



31 i 0-25 0-20 I 0-17 



2 



