Soap Bubbles
and the Forces which Mould Them

THE FORCES WHICH MOULD THEM. 33 If you again consider these two cases, you will see that a plate that is wetted tends to move towards the higher level of the liquid, whereas one that is not wetted tends to move towards the lower level, that is if the level of the liquid on the two sides is made different by capillary action. Now suppose one plate wetted and the other not wetted, then, as the diagram imperfectly shows, the level of the liquid between the plates where it meets the non-wetted plate is higher than that outside, while where it meets the wetted plate it is lower than that outside; so each plate tends to go away from the other, as you can see now that I have one paraffined and one clean ball floating in the same water. They appear to repel one another. You may also notice that the surface of the liquid near a wetted plate is curved, with the hollow of the curve upwards, while near a non- wetted plate the reverse is the case. That this curvature of the surface is of the first import- ance I can show you by a very simple experi- ment, which you can repeat at home as easily as the last that I have shown. I have a clean glass bead floating in water in a clean glass