Soap Bubbles
and the Forces which Mould Them

THE FORCES WHICH MOULD THEM. 41 apply benzine the more the greasy benzine runs away carrying the grease with it. But if you follow the directions on the bottle, and first make a ring of clean benzine round the grease-spot, and then apply benzine to the grease, you then have the greasy benzine run- ning away from the pure benzine ring and heaping itself together in the middle, and escaping into the fresh rag that you apply, so that the grease is all of it removed. There is a difference again between hot and cold grease, as you may see, when you get home, if you watch a common candle burning. Close to the flame the grease is hotter than it is near the outside. It has therefore a weaker skin, and so a perpetual circulation is kept up, and the grease runs out on the surface and back again below, carrying little specks of dust which make this movement visible, and making the candle burn regularly. You probably know how to take out grease- stains with a hot poker and blotting-paper. Here again the same kind of action is going- on. A piece of lighted camphor floating in water is another example of movement set up by