Soap Bubbles
and the Forces which Mould Them

170 SOAP-BUBBLES, AND end of the tube should be slightly spread out when hot by rapidly pushing into it the cold tang of a file, and twisting it at the same time, so that it may be lightly held by the teeth without fear of slipping. If a light T-piece or so great a length of small india-rubber tube cannot be obtained, then the mouth must be removed from the pipe and the india-rubber tube slipped in when air is to be changed for gas. This makes the manipulation more difficult, but all the experi- ments, except the one with three bubbles, can be so carried out. The pipe must in every case be made to enter the highest point of a bubble in order to start an internal one. If it is pushed horizontally through the side, the inner bubble is sure to break. If the inner bubble is being blown with gas, it will soon tend to rise. The pipe must then be turned over in such a manner that the inner bubble does not creep along it, and so meet the outer one where penetrated by the pipe. A few trials will show what is meant. The inner bubble may then be allowed to rest against the top of the outer one while being enlarged. When it is desired