And the only thing that prevents all this from actually happening right now is that there is a force called _cohesion_ that holds things together. It is the pull which one particle of anything has on another particle of the same material. The paper in this book, the chair on which you are sitting, and you yourself are all made of a vast number of unthinkably small particles called _molecules_, each of which is pulling on its neighbor with such force that all stay in their places. Substances in which they pull the hardest, like steel, are very hard to break in two; that is, it is difficult to pull the molecules of these substances apart. In liquids, such as water, the molecules do not pull nearly so hard on each other. In a gas, such as air, they are so far apart that they have practically no pull on each other at all. That is why everything would turn to a gas if the force of cohesion stopped. Why things would turn cold will be explained in Chapter 4. Cohesion, adhesion, and capillary attraction, all are the result of the pull of molecules on each other. The difference is that capillary attraction is the pulling of particles of liquids up into fine spaces, as when a lamp wick draws up oil; adhesion is the pull of the particles of one substance or thing on the particles of another when they are very close together, as when water clings to your hand or when dust sticks to the ceiling; while cohesion is the clinging together of the particles of the same substance, like the holding together of the particles of your chair or of this paper. When you put your hand into water it gets wet because the adhesion of the water to your hand is stronger than the cohesion of the water itself. The particles of the water are drawn to your hand more powerfully than they are drawn to each other. But in the following experiment, you have an example of cases where cohesion is stronger than adhesion: EXPERIMENT 16. Pour some mercury (quicksilver) into a small dish and dip your finger into it. As you raise your finger, see if the mercury follows it up as the water did in Experiment 14. When you pull your finger all the way out, has the mercury wet it at all? Put a lamp wick or a part of your handkerchief into the mercury. Does it draw the mercury up as it would draw up water? [Illustration: FIG. 23. The mercury does not wet the finger, and as the finger is lifted the mercury does not follow it.]