Soap Bubbles
and the Forces which Mould Them

THE FORCES WHICH MOULD THEM. 169 to take little enough. As much as can be picked up on the last eighth of an inch of a pointed pen-knife will be, roughly speak- ing, enough for a wine-glassful of the soap solution. If the quantity is increased beyond something like the proportion stated, the fluor- escence becomes less and very soon disappears. The best quantity can be found in a few minutes by trial. To blow bubbles containing either coal-gas or air, or a mixture of the two, the most convenient plan is to have a small T-shaped glass tube which can be joined by one arm of the T to the blow-pipe by means of a short piece of india-rubber tube, and be connected by its vertical limb with a sufficient length of india-rubber pipe, one-eighth of an inch in diameter inside, to reach to the floor, after which it may be connected by any kind of pipe with the gas supply. The gas can be stopped either by pinching the india-rubber tube with the left hand, if that is at liberty, or by treading on it if both hands are occu- pied. Meanwhile air can be blown in by the other arm of the T, and the end closed by the tongue when gas alone is required. This