Soap Bubbles
and the Forces which Mould Them

42 SOAP-BUBBLES, AND differences in the strength of the skin of water owing to the action of the camphor. I will give only one more example. If you are painting in water-colours on greasy paper or certain shiny surfaces the paint will not lie smoothly on the paper, but runs together in the well-known way; a very little ox-gall, however, makes it lie perfectly, because ox-gall so reduces the strength of the skin of water that it will wet surfaces that pure water will not wet. This reduction of the surface tension you can see if I use the same wire frame a third time. The ether has now evaporated, and I can again make it rest against the surface of the water, but very soon after I touch the water with a brush containing ox-gall the frame jumps up as suddenly as before. It is quite unnecessary that I should any further insist upon the fact that the outside of a liquid acts as if it were a perfectly elastic skin stretched with a certain definite force. Suppose now that you take a small quantity of water, say as much as would go into a nut- shell, and suddenly let it go, what will happen ? Of course it will fall down and be dashed against the ground. Or again, suppose you