"Well," said he, "what do you think of the force of cohesion now?""I can see, sir," said Fred, "that it has more to do with solid bodies and their properties than I thought it had. In our early lessons we learned something about the properties of bodies. We are now able to say not only that a body has certain properties, but to tell the reason why it has these properties. If I were to scratch a piece of lead, for instance, with an iron nail, a boy in one of the lower classes might say that I was able to do it because the iron is harder than the lead. So it is, but he would not be able to say why the iron is harder than the lead.
"We can now tell this from the force of cohesion. When the force of cohesion in a solid is great, the molecules are held very closely together, and the body is hard. When there is little cohesion, the molecules are more loosely held together, and the body is soft. It is because the particles of the soft body are held loosely, that a hardsubstance is able to force them aside, as it does when it scratches them.
