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



the sides of J is filled with cotton-wool moistened with the same liquid. The 

 last tube K, which opens immediately under the cork of J, is either open to 

 the air, or connected with a gas-bag containing the gas under examination, 

 or fastened to a chloride-of- calcium tube, according to the requirements of 

 the experiment. In some experiments the little tubes G and J are sur- 

 rounded with water contained in the vessel N. The tubes G and J are 

 firmly bound to a flat piece of cork held by the heavy clamp P, which rests 

 on the bottom of N. A thermometer T is placed in the water of N. 



The apparatus is used as follows : B and F being disconnected, the 

 bottle A is nearly filled through C. The end of F is closed by the finger, 

 and, the stopcock E being opened, the siphon D is filled once for all by 

 applying the mouth to its longer end. E being then closed, the tube G 

 is filled up to the required mark with the liquid which is to serve as a 

 bubble-medium. The cotton-wool in J is moistened with the same liquid. 

 All the joints are made fast, and the tube K is connected with the gas-bag 

 L. On turning the stopcock E, water flows through the siphon D into 

 the flask M : to supply its place, gas must enter by F ; that is, gas must 

 bubble through the liquid in G. Before entering G it becomes saturated 

 with the vapour of the same liquid in J. If all the joints are tight, it 

 follows that the volume of water entering M is equal to the volume of gas 

 which bubbles through the liquid in G. It is a sufficient test of the tight- 

 ness of all the joints (as far as H), to run off a little water by D, so as to 

 bring a bubble or two of gas through h, and to allow the apparatus to rest. 

 If the tube H remains full of air to its extremity for a quarter of an hour, 

 the apparatus may be considered as air-tight. A metronome is adjusted 

 to beat to the required time. M is removed and emptied. E is turned 

 till the bubbles, passing through the liquid in G, are synchronous with the 

 beats of the metronome. This rate is maintained until the liquid in A 

 sinks to a. The flask M is then put in its place, and from that instant the 

 bubbles through G are counted. When M is filled exactly up to m, the 

 experiment is finished. The proximity between M and G enables the eye 

 to count the bubbles, and to watch without difficulty, at the same time, 

 the rise of the liquid in M. The contents of M, divided by the number of 

 bubbles, gives the mean volume of a single bubble. The use of the cotton- 

 wool in the joint between B and F is to check the flow of gas through the 

 apparatus. When this plug is absent, the considerable volume of gas in 

 the upper part of A, being in direct communication with G, causes by 

 its elasticity an irregular delivery of bubbles through G. Of course as M 

 is filled the level of the liquid in A falls, the difference between the limbs 

 of the siphon D is diminished, the flow through D is retarded, and the 

 bubbles follow one another more slowly. We shall see, however, that the 

 rate of sequence has exceedingly small, or absolutely no influence upon 

 bubble-size. In the experiments actually performed to establish this 

 fact, the metronome was allowed to continue beating throughout the expe- 

 riment, and an occasional tap on the cock E was found sufficient to regu- 

 late the rate of sequence with perfect accuracy. The great comparative 



