the surface tension of soap films 297 
few centimetres long, at right angles to the main portion BC which 
is about one metre in length. This bent tube is surrounded by a 
second tube FG which is used as a steam jacket for heating the 
tube ABCD. The parts AB and CD are horizontal and the tube 
may, if desired, be rotated about CD as a horizontal axis. The 
Inner tube passes out of the steam jacket through rubber bungs. 
The opening D is connected by a horizontal rubber tube DE to 
the cup K on which a bubble is to be formed; the same bubble 
holder is used as in § 8. The end A remains open. 
Fig. 9. 
Let the temperature of the atmosphere in the neighbourhood 
of the apparatus be ¢,° K., i.e. t,° on the Kelvin or “absolute” scale, 
and let the temperature of the air in the tube BC bet, K. Let 
P dynes cm.~ be the atmospheric pressure at the level of A. Let 
the density of air at the normal pressure p, and the normal tem- 
perature t, K. be p, grm. cm.~, let the density of air in the atmo- 
sphere at the level of A be p, and that at Bin the tube be p,. Let 
the height of D above A be zcm. Then, if we neglect the very 
small changes of density due to differences of level, the pressure 
in the atmosphere at the level of CD is P—gp,z*, while the 
pressure in the tube at Cis P—gp,z. If the horizontal tube DE 
is long enough to ensure that the bubble stand XK is not heated by 
the steam jacket, the fall of pressure between # and the bubble 
is, to all the accuracy required, the same as that which occurs in 
* The exact expression is Pe 9:2/P, 
