194 



Professor Sir James Dewar 



[Jan. 19, 



In the case of a hanging bubble as large as 38 cm., referred to 

 in Table 6, the total reduction in the diameter amounts only to 

 1'58 cm. in five weeks. The volume of such a bubble is over 

 30 litres, and the surface has an area of 5,000 square cm., while 

 the total weight is only about half a gram. Being thinned to 

 the " black " stage, it has a maximum thckness of approximately 

 fifteen /x/x's ; that is to say, one and two-third million such fihns if 

 superposed would have a total thickness of one inch. The internal 

 excess pressure is of the order of a twentieth of a millimetre of water, 



r^tt-s 



Fig. 7. 



or about one fifteen-thousandth part of the pressure of the atmosphere. 

 Nevertheless the actual volume of air which passed out of the black 

 bubble in five weeks was 3 * 6 litres, or every minute a layer of air mole- 

 cules some ten to eleven times the thickness of the wall of a black 

 bubble of 38 cm. diameter passed through it. Hence the thickness of 

 the layer of air which at ordinary temperature passes through the film 

 in one second is 10/60ths of the thickness of the film, or 2*5 /x/z's — 

 in other words, a layer of air as thick as the film would take 

 six seconds to pass through it. Again, the mean free path in air is 

 100 ///i's, which is nearly seven times the thickness of the film, and 

 therefore some forty times the thickness nf the layer of air passing 

 through the film in one second. Theoretically the internal pressure 



