Page:St. Nicholas, vol. 40.1 (1912-1913).djvu/539

1913.]

In order to explain the tendency of drops of liquid to become shaped like a sphere, it will be necessary to speak of a peculiarity which every liquid surface possesses. This is its power of contracting. If you will dip a camel’s-hair brush into water and then withdraw it, you will see that the slender bristles are all drawn down together to a point. This is due to the fact that the water clinging to the brush is drawn inward by the contracting of the outside surface film. We make use of this phenomenon when we moisten the end of a thread before putting it through the eye of a needle.

Now, the film of water which makes a soap-bubble, pulls the bubble into just as small a volume as it can. It contracts until the air within the bubble has such a shape as to require the least surface film to inclose it. Now this form is always a sphere. So the surface film of a water-drop will contract until it has forced the water inside into such a form as to have the smallest outside area. Hence the spherical form of drops.

Many interesting experiments can be made upon this remarkable characteristic of liquids. For example, cut a small, boat-shaped piece of cardboard or wood about an inch and a half long. In a V-shaped notch, cut in the stern, place a piece of camphor gum slightly larger than the head of a pin. Now lay the boat carefully upon the surface of the water, being sure the camphor touches the liquid. If everything is correct, the boat will move forward as though a tiny thread were pulling at its bow. The dissolving camphor lessens the contractive force of the water at the rear of the boat, and so the force of contraction at the bow pulls the boat ahead.

Another way is to float two wooden toothpicks or small bits of wood about half an inch apart on water, and then, with a medicine dropper, allow a small drop of alcohol to fall in the space between the toothpicks. These pieces of wood will immediately fly apart as though a miniature explosion had taken place. The alcohol, upon mixing with the water, weakens the contractive force of the water surface between the bits of wood. The force here is no longer able to balance the contractive force on the outside trying to pull the pieces apart. Hence the objects are pulled suddenly away from each other.

Our daily experience is filled with cases of the action of this force, which is known to scientists as . The action of water in laying dust is due to surface tension. The form of dew-drops and a small globule of water on a smooth piece of paper is due to this same force.—, Ypsilanti, Michigan.

The particles of which liquids are made up can move rather easily. This distinguishes liquids from solids, where the particles stick together more or less firmly. But even with liquids there is a little tendency for the particles to stick together, and to hold each other. Drops are formed when small amounts of liquid fall through the air because the particles hold together strongly enough to overcome the resistance of the air through which they are passing. Drops are spherical because all parts of the liquid in them attract each other equally, and because the particles are free to move. A drop cannot be made larger than a certain size, and this size varies with different liquids according to their stickiness. If you dip a match stick into water and then withdraw it, you will find that the drop at the end will fall when it reaches a certain size. If you use alcohol, you will find that the drops are smaller, and with a thick liquid, such as tar or honey, the drops will be larger. This shows that it is the stickiness of the liquid which helps the forming of drops, and that all liquids have some of this property of “stickiness.”—, New Haven, Connecticut.