| the surface tension of soap films 299 
To ensure that the interior of the tube ABCD is dry, air may 
be blown through it while it is heated. 
§ 14. Practical example. The following results were obtained 
by G. F. C. Searle and A. J. Berry. 
Barometric height = 76°53 cm. 
Temperature of atmosphere =¢, = 273 +19°5 = 292°5° K. 
_ ‘Temperature of steam =¢,=273 + 100°2=373'2° K. 
Height of heated column =z=95'8 cm. 
Density of air at normal pressure and temperature = pp = 1°293 x 1073 grm. 
yem:? 
i ©Elence 
76°53 273 
| — 1-295 = Sigi sree ae ~3 -3 
| pi=1:293 x 107° x 6-00 * 2925 1:2152 x 10-3 grm. cm. ~ 3, 
| 76°53 278 
3 (OF) -3 : -3 
p2= 1:293 x 10— X 76-00 * 373-2 0:9524 x 107-3 grm. cm. 
The pressure excess in the bubble is thus 
p= (p1— p2) = 981 x 95'8 x 0-2628 x 10° = 24°70 dyne cm.~? 
The following values of the distance, /, of the vertex of the bubble from 
_the plane of the rim were found with five different films: 
1:09, 1:10, 1:09, 1:10, 1:10 cm. 
Mean value of h=1:096 cm. 
Radius of rim=c=2°90 cm. 
Mean radius of bubble =7 = (c?+ h?)/2h=4°385 cm. 
Hence we find for the surface tension 
T=irp= { x 4385 x 24°70 =27:07 dynes per cm. 
§ 15. Comparison of results. The results given by the various 
methods are as follows: 
Methoa Surface tension 
Morsion balance ..+......... 27°22 dyne cm.— 
INGUPSENG eke Bee an eeen eee | 27-17 
Viscosity potentiometer...) 25:19 
SMOYANCY Wi iacanist ngs tnte ais 36 27:07 
