CAPILLARY ATTRACTION. If you drop the corner of a towel in the bathtub, what happens? The water climbs up the threads and soon the whole towel is wet.
If you take a lump of sugar and dip one end of it in cocoa, in a few seconds the whole lump is soaked.
Blotting paper takes up ink, and lampwicks raise oil to the burner in the same way.
Men first studied the action of this force in tiny tubes little larger than a hair, and so they called it " capillary attraction" or "capillarity," from the Latin word capillus meaning "hair." They found that when one end of a very small tube is put in a glass of water, the water will rise just a little higher in the tube than its level in the glass. And the finer the tube the greater the height to which the water will be drawn upwards.
Sugar, salt, starch, sponges and the fibers of plants all have a multitude of tiny tubes and hollows that pull up—or, as we say, "soak up"— water or other fluids just as the glass tube does.
This principle of capillarity is very important in farming, for on it depends the power that water has to rise above the water level in the soil toward the surface. Here it is the soil particles that pull the water up, just as a lampwick or the towel does; and just as the water rises highest when the tube is small est, so the smaller the particles of dirt are, and the closer they are together, the more freely the moisture rises. When we plant small seeds we press the earth firmly down about them, so that the moisture will rise more readily and help the seeds to sprout. Sometimes after a summer shower the upper layer of earth is packed in a hard crust. Then the moisture is quickly drawn through this tightly packed earth to be evap orated by the air, and thus lost to the soil and the plant roots.
Let us take another lump of sugar and, before dip ping it into the cocoa, sprinkle a little granulated sugar over the top. The cocoa rises quickly up to this layer of loose sugar, but it takes considerable time to pass through that. In those regions where there is a great deal of rainfall in the early spring, but very little through the hot summer months, the farmers take advantage of this principle in what is known as "dry farming." First, the soil is plowed deep and fine, and packed down pretty firmly so the moisture can come up readily around the roots of the plant.
But on the surface the soil is stirred and worked to make a layer of dry pulverized earth, called a "mulch." This checks the moisture, so it will not evaporate into the air, just as the dry sugar checked the cocoa. By harrowing it is renewed as often as is necessary.