13 



buoyancy of the bubbles (i.e., it equals the volume of the bubbles times the density of the 

 liquid). The change in balance reading was always at least 0.2 gm, resulting in an accuracy 

 of measurement of volume of 1 percent. The volume of the individual bubble was obtained by 

 dividing the total volume by the number of bubbles collected in the funnel. A comparison of 

 photographs of different bubbles showed that the bubble size did not vary if the frequency of 

 bubble generation remained constant. Large bubbles from the dumping cup were weighed 

 individually. The volume of the bubble was adjusted for the change in pressure due to differ- 

 ence in depth between the level at which the rate of rise is determined and the level of the 

 inverted funnel. This was done by use of the general gas law at constant temperature, taking 

 into account the partial pressure of the saturated vapor at test temperature. 



Tiny spherical bubbles could not be generated at a frequency to allow a sufficient 

 number to be collected in the funnel, hence their size was determined from the photographic 

 record. No correction for change in depth is then needed. 



OTHER EXPERIMENTAL TECHNIQUES 



To avoid any changes in the volume of the bubble due to air interchange with the 

 liquid, the latter was saturated with air prior to actual testing. This was accomplished by 

 stirring the liquid and by blowing air through it. 



The temperature of the liquids, with the exception of cold and hot water, was room 

 temperature, which varied little throughout the day. Water was cooled by circulation through 

 a water cooler; it was heated with immersion heaters or obtained directly from the hot water 

 faucet. Both filtered and tap water were used in the tests. 



Uniformity of liquid temperature was achieved by means of mechanical stirring before 

 each test. Frequent checks of temperature at various locations inside the tank were made by 

 means of immersion thermometers. In the process of stirring, small bubbles appeared in the 

 liquid. The irregularity of motion of these small bubbles, which were still present after com- 

 pletion of the stirring, served as an indication of the presence of residual turbulence in the 

 liquid. In sufficient time, the motion of the small bubbles always became regular and hence 

 indicated that the residual turbulence, if still present, was not large enough to affect the 



♦Details of this correction are as follows: 



v JjLLLv 



1 ' 



P1 -p 



where V. is the volume of the bubble at the camera level, 



V- is the volume of the bubble as determined by weighing, 



p is the absolute pressure at the funnel, 



p. is the absolute pressure at the camera level, and 



p is the vapor pressure of the liquid at the test temperature. 



