32 Prof. Guthrie on Bubbles. [Jan. 19, 



These liquids, which were purposely taken the same as those whose 

 drop- sizes were examined, are arranged in Table e in the order of the 

 magnitude of the bubble-size. We see that the order is not the same in 

 the two cases. The difference is due to the elimination in Table e of the 

 influence of gravitation. In fact the only forces which influence bubble- 

 size, as shown in Table e, are the retentive and stubborn cohesions of the 

 liquid* ; for the first of these seeks to diminish, the second to increase the 

 bubble-size. If RC be the retentive, and SC the stubborn cohesion, the 



liquids are arranged in Table e in the same order of magnitude as are the 



SP 

 values of = T . The density of a liquid seems therefore to vary with its 



IvC 



stubborn rather than with its retentive cohesion ; for there is an evident 

 general tendency in the above Table e for the liquids to arrange themselves 

 in the order of their specific gravities. Water once more distinguishes 

 itself, taking a higher place in the scale than its density would point to : 

 this must arise either from its exceptionally great stubborn, or from its ex- 

 ceptionally small retentive cohesion. 



Acetic ether and alcohol are also exceptional the former taking a lower, 

 the latter a higher place in the scale than would be the case if the same state 

 of quantity of matter in a given space (which is usually measured by means 

 of gravity) affected also the cohesion of the liquid so as alone to determine 

 the bubble-size of a gas passing through it. Perhaps also the gas having 

 different degrees of solubility in the different liquids may affect their cohe- 

 sions unequally. This source of variation, however, is probably very small, 

 as we have seen to be the case when the gas varies and the liquid remains 

 the same. A few experiments with a mixture of benzol and turpentol, and 

 with alcohol and water, showed that in all cases the mixed liquid gives rise 

 to a bubble intermediate in size between those caused by the single liquids. 



By measuring the volume of a greater number of bubbles, the actual dif- 

 ferences of bubble-size due to various liquids would of course become more 

 apparent. 



Throughout the examination of drops and bubbles in the present and 

 previous communications, I have sought to direct attention to the main influ- 

 ences which fix the size of a drop or bubble, rather than to pursue any one 

 branch of the inquiry into its minute ramifications. Further, the subject 

 has been treated wholly from a statical point of view ; that is, the bubble 

 and drop have been considered at that period of their being when the con- 

 tending forces which act upon them have brought them into a state of un- 

 stable equilibrium or incipient motion. It is in fact only at this point, the 

 instant of their ripeness, that they have a definite size ; for their size increases 

 until the contending forces themselves withdraw the drop or bubble from 

 the sphere of the action which determines their volume. 



Knowing now the direction and ^approximately the relative amounts of 

 the effects due to the various conditions under which the drop and bubble 

 * ^or the meaning of these terms see Paper "On Drops," p. 469. 



