when put into water, while a piece of greasy glass does not? If we hold a sheet of glass down against a water surface, and then try to pull the glass away, we find that we must exert more force than is needed to lift the glass against gravity alone (Fig. 71). Another force resists our tearing the glass away. The force that holds the water particles together is called cohesion, while the force that holds two different substances, as water and glass, together is called exerted to tear the glass away adhesion. A postage stamp adheres to the envelope. The adhesion of water to glass is a greater force than the cohesion of water, or the water would not wet the glass.

FIG. 71.

Force must be

from the water.

Have you ever thought why liquids form drops? We know that a large surface of water is flat (horizontal), because gravity pulls down on all parts of the surface. But when the body of water is small (a drop), the effect of gravity

is also small, and cohesion is able to pull the water into the form of a sphere. The water surface acts like a tightly stretched, elastic covering, say of rubber. That there is such an elastic surface is shown by the trick by which we can float a needle upon water (Fig. 72). We grease the needle slightly, so that water will not wet it, and then put it down carefully on the water's surface. A fork will help us to do this. Note how the needle stretches the elastic water surface, without breaking it. Is the needle heavier, or lighter, than water?

FIG. 72.- The greased needle stretches the water surface without breaking it.

108. Why Does a Blotter Absorb Ink? - Did you ever wonder why, if you touch a drop of ink with a blotter, the whole drop flows into the blotter? Or have you ever left one corner of a dry towel in a dish of water and found the water rose into the towel? The force exerted is called capillary action. Capillary means hairlike. The action is so called because it

takes place best in tiny tubes.

Capillary action takes place between water and glass tubes because the water wets the inside of the tube and because the surface of the water is elastic. If we have some water in a dish, or in a large tube, the water is raised only at its edge (Fig. 73). The elastic surface does not have

FIG. 73. The elastic water surface has

force enough to lift force enough to lift the water, against gravity,

the water in a column.

up into the tube and around the side of the dish.

But if we use a tiny tube- a capillary tube — the weight of water is smal and the force of the elastic surface pulls the column of water up in the tube. In blotters, cloth, string, soil, and other loose materials, the spaces are so small that they act as a multitude of fine tubes.

In watering a potted plant we often set the pot in a dish of water; how does the water get up to the plant?

109. Exercises. - 1. What do we call the force which you must overcome in driving a nail into wood?

2. What causes water, mercury, kerosene, and other liquids to form drops? Why does a drop of water not remain spherical on a table while a drop of mercury does?

3. Why is it possible to ride on a bicycle and not fall to one side or the other?

4. Why does a steel ship float?

5. Why does an engine begin to slow up long before it is near the station?

6. How could you find out which is the heavier, gasoline or water; olive oil or water; marble or gold; iron or lead; salt brine or water? 7. If dropped from the same height at the same time, which will strike the ground the sooner, a pound bag of feathers or a pound bag of iron nails?

8. Why is it hard for a two-legged chair to stand? Which is harder to push over, a three-legged or a four-legged stool? Has a three-legged stool any advantage over a four-legged one on rough ground?

9. When you pitch a baseball, or bat a tennis ball, why do you have to "allow for the wind"? Does an apple fall straight down on a windy day? Why? Does this mean that gravity is not pulling straight down on such a day?

10. Why should a brick or stone wall be built "plumb"? Why does not the "leaning tower" fall over?

11. Would it be easier for you to float in fresh water or salt water? Why?

12. Would iron float on mercury? See Appendix, Table III About how much of the iron would sink into the mercury?

CHAPTER XIII

SUBSTANCES

110. What is a Substance? Substances are the different kinds of matter; have you ever thought what a wonderful variety of them there is in this world of ours? Some of them are gases, some are liquids, and some are solids. Some are colorless, some have color; some dissolve, others do not; some are light and some are heavy ; some we can see through (they are transparent) and others we cannot see through (they are opaque); some we can set on fire and others refuse to burn. Then, too, some substances, as limestone, air, and water, are very abundant in the earth, while others, like gold and diamonds, are rare. Thousands of different substances are known and many more will probably be discovered as time goes on. The qualities by which we tell substances from one another are called the properties of those substances. Thus, one of the properties of salt and sugar is their taste; the color, odor, and density of a substance, whether it will dissolve or burn, and many other qualities are also properties of the substance.

Tell all the properties you know of pine wood, soap, sugar, glass, iron, and milk.

111. Can Substances Be Changed? It is easy for us to see that many substances change, for they gain new properties or lose old ones. Liquid water becomes

a gas (water vapor) or a solid (ice). A "tin " can rusts away. Fruit juices ferment; milk sours; cut grass becomes hay. If we examine a cliff, we see heaps of chips and dust at its base; these were broken off from the cliff. Wonderful changes take place in our own bodies. Men eat many different kinds of food, but in some way these foods are changed into the material of which man's body is built up.

If we put iron into a fire, it becomes hot. It may become red or white-hot and give off light. But if it is removed from the fire, it gradually gives off the heat it gained and finally looks just as at first. Men may break off coal in a mine hoist it above the surface, and haul it to our coal bin, but it is still coal. These changes have not really altered the coal. We call such changes physical changes. But if the iron poker is left in a damp cellar, it rusts (cf. § 34). If we burn the coal, its carbon disappears and carbon dioxide is formed. The iron of the poker and the carbon of the coal have each combined with oxygen to form new substances: rust and carbon dioxide. Such changes as these are called chemical changes, because they are studied in chemistry.

112. Can Water Be Changed? Of course we know that water can be changed to a gas or a solid and then changed back into liquid water. But can it be broken up into other substances? If you can get the apparatus, the following experiment (Fig. 74) will give you the

answer:

Let two wires from a battery of several cells, or some other source of the electric current, pass into a vessel