Soap Bubbles
and the Forces which Mould Them

THE FORCES WHICH MOULD THEM. 12? in the very centre of the large bubble, and finally, on breaking the outer one the inner floats away, none the worse for its very- unusual treatment. There is a pretty variation of the last experi- ment, which, however, requires that a little green dye called fluorescine, or better, uranine, should be dissolved in a separate dish of the soap-water. Then you can blow the outer bubble with clean soap-water, and the inner one with the coloured water. Then if you look at the two bubbles by ordin- Fig. 58. ary light, you will hardly notice any difference; but if you allow sunlight, or electric light from an arc lamp, to shine upon them, the inner one will appear a brilliant green, while the outer one will remain clear as before. They will not mix at all, showing that though the inner one is apparently resting against the outer one, there is in reality a thin cushion of air between. Now you know that coal-gas is lighter than air, and so a soap-bubble blown with gas,