Soap Bubbles
and the Forces which Mould Them

THE FORCES WHICH MOULD THEM. 121 shadow on the screen. Now I tilt it gently to one side, and you see something pouring out of it, which is the vapour of ether. It is easy to show that this is heavy; it is only necessary to drop into the jar a bubble, and so soon as the bubble meets the heavy vapour it stops falling and remains floating upon the surface as a cork does upon water (Fig. 52). Now let me test the bubble and see whether any of the vapour has passed to the inside. I pick it up out of the jar with a wire ring and carry it to a light, and at once there is a burst of flame. Fig- 52- But this is not sufficient to show that the ether vapour has passed to the inside, because it might have condensed in sufficient quantity upon the bubble to make it inflammable. You remember that when I poured some of this vapour upon water in the first lecture, sufficient condensed to so weaken the water- skin that the frame of wire could get through