330 



Mr. H. S. Allen on the Motion of 



depended to some extent on the pressure employed in separat- 

 ing it. Frequently, with such tubes, several bubbles were 

 detached one after the other, those in front being larger than 

 those behind. Some of these bubbles were so minute as to 

 be only just visible. It is suggested that in these cases the 

 larger bubbles were produced by the practically instantaneous 

 formation of a number of the very minute bubbles. 



The behaviour of one of these fine tubes would differ con- 

 siderably at different times, no doubt in consequence of the 

 particular state of cleanliness of the tube or the deposit of 

 some minute particle. Thus a tube which had been giving a 

 a small cloud of fine bubbles would suddenly refuse to give 

 anything but a much larger bubble of fairly constant size. 



The terminal velocities of the bubbles were measured by 

 observing the times of ascent through distances marked on 

 the vertical tube. The times were taken with a stop-watch 

 or chronometer. 



It is necessary to measure the size of a bubble whose 

 velocity has been determined. The simple arrangement 

 shown in fig. 2 was constructed to catch the bubble at the 



Fisr. 2. 



top of the tube, so that it might be removed to the stage of 

 a microscope fitted with a micrometer eyepiece and measured. 

 To two microscope cover-glasses, 2^ centim. and .1^ centim. 

 in diameter respectively, were cemented with Canada balsam 

 glass rings of rather smaller diameter. 



The smaller cup thus formed was suspended, mouth down- 

 wards, in the top of the receiver by three silk fibres. When 

 the bubble to be examined had entered from below, the larger 

 cnp was slipped underneath by means of a handle of wire, 

 and the whole was lifted out and transferred to the stage of 

 the microscope. The diameter of the bubble now in contact 



