26 CONTRACTION OF FILMS. BUBBLE SOLUTION. 



round bubbles. The tendency of the liquid film to 

 contract may be well studied by placing the rings 

 rather close together ; a bubble is blown between them 

 until it adheres to each ring, the air is then gradually 

 sucked back again and the pipe withdrawn. During 

 this operation the bubble assumes successively the 

 forms represented in fig. 31, A, B, and ; and if the 

 pellicle a, fig. 31, C, is destroyed by touching it with 

 the finger, the remaining portion of the film will assume 

 the form shown in fig. 31, D. Finally, if the last 

 plane pellicle is also destroyed, the remainder takes the 

 shape represented in fig. 31, E. 



Bubbles blown from soap-water do not last long, because the 

 small quantity of water in them evaporates very rapidly. But the 1 

 liquid prepared in the following manner evaporates very slowly, 

 and the films formed from it therefore last much longer, frequently 

 several hours, if the air around them is perfectly quiet. Take 

 400 CC of cold water that has previously been boiled, and put into 

 it 10 grammes of Castile soap, cut up fine. Put this into a wine- 

 bottle and set it in hot water in a saucepan on the hob ; let it 

 remain there an hour or so, shaking it up occasionally till the soap 

 is dissolved. Next, let the liquid stand quietly for a few hours, 

 for the impurities and colouring matter to settle ; pour off the 

 clear liquid, and add to it 270 CC of glycerine (about 335 grammes), 

 shaking the whole thoroughly well. Commercial glycerine is 

 frequently impure ; for this solution the glycerine should be colour- 

 less and have nearly the consistency of treacle. 



These experiments, especially those with the rings, do not 

 usually succeed at once ; the rings must be moistened with great 

 care, and the experiment should be attempted repeatedly. The 

 beauty and duration of the figures when obtained will, howeve^, 

 amply repay a good deal of trouble. 



The wire-frames, if purchased, should be of iron wire, to which 

 the liquid easily adheres, especially when the wire is slightly 

 oxidised. Brass wire, although inferior, has the advantage of 

 being more easily soldered by an inexperienced hand.. Solder is 

 prepared by melting 3 parts by weight of tin with 2 parts by 

 weight of lead in a melting ladle (a round ladle made of sheet iron 

 with a wooden handle). The molten mixture is stirred with a 



