Page:Popular Science Monthly Volume 35.djvu/620

596 out, and all the superior liquid will spread in an infinitely thin layer over the other. This is what happens to a drop of oil when it is thrown upon water. When a liquid is brought in contact with a solid, as when a first drop of water is let fall upon a horizontal plate of glass, the inclosing sac is flattened where it is in touch with the glass, and bulges where it is in contact with the air. The form of the sac and the angle of its junction with the glass are determined by the fact that the two tensions of the envelope.



the upper and lower, should balance the traction of the exterior glass upon the cordon separating them. In the case of a drop of alcohol, the tensions being much weaker, can not resist the traction of the glass, and the liquid spreads out at once, as also happens with water when the plate has already been moistened. Mercury opposes a very strong tension, and is hardly flattened at all on striking the glass. A drop of water cast upon a hot plate also exhibits a superior tension, and assumes the spheroidal state, which was first analyzed in 1850 by M. Boutigny, of Evreux. He said, "Bodies in a spheroidal state are bounded by a film of matter, the molecules of which are so connected that we can compare them to a solid, transparent, very thin, very elastic envelope, probably less dense than the rest, that protects the liquid within it against any too considerable heating."

This force of superficial tension exists and is manifest in all liquids, but in different degrees. It is stronger in water than in any other of the common liquids except mercury. Its value has been measured, and is usually expressed, in milligrammes per