英文科学读本 第四册·Lesson 17 Cohesion in Liquids

Lesson 17 Cohesion in Liquids

We have seen something of the effect which the force of cohesion has on the molecules of a solid body, said Mr. Wilson. "I want now to study this same force in its effects on the molecules of liquids. In a solid the force of cohesion is so great that the molecules cannot shift about. Those, which today form the extreme end of this iron rod, will not tomorrow move into the middle, but will remain where they are.

In liquids there is very little cohesion. The molecules of a liquid are free to move about, roll one over the other, and change their places. Here is a tumbler of water. See, I can move this stick about through the water with scarcely any effort, because the molecules of the water are easily pushed aside. Come to the front, Fred," he continued, "and do the same with this basin of peas. The peas yield readily to the pressure and give way. You have only to imagine the water made up of extremely fine round molecules, many million times smaller than a pea, and you will quickly understand it all. The molecules of the water, having little cohesion between them, move freely as the stick pushes them aside.

I have here another tumbler half full of treacle. Notice what happens when I slant it on one side. Take the tumbler of water in one hand and the tumbler of treacle in the other and slant both of them. You see the treacle moves very slowly. Now stir the two. It is much more difficult to move the stick through the treacle than through the water. Why is this?"

I suppose it is because there is more cohesion between the molecules of the treacle than between the molecules of the water, said Fred, "and therefore they separate more slowly."

Quite right, said Mr. Wilson. "We call liquids, which are like treacle in this respect, viscous liquids. Can you think of any of them?"

Tar, glycerine, liquid gum, and castoroil would all act in the same way, said Fred.

There are some liquids, on the other hand, said Mr. Wilson, "such as alcohol, ether, and benzoline, that have less cohesion than water. Their molecules move about very freely indeed. We call them mobile liquids.

Now let us take another step. Your very early lessons taught you that one of the differences between a liquid and a solid is that the liquid has no shape of its own, but always takes the shape of the vessel which holds it. We are now in a position to see the reason for this. The reason is two-fold. First, the molecules of the liquid, having little cohesion, are free to move about. Secondly, the force of gravity acts upon each individual molecule, and draws it downwards towards the bottom of the vessel. The molecules cannot stand in a heap, because those above press on those below, and push tightly in to every corner of the vessel. They follow one another downwards and outwards in this way until they meet with the bottom and sides of the vessel. This is why a liquid can have no shape of its own, but at once takes the shape of the vessel which holds it.

Those same early lessons taught you that although flour, or sawdust, or sugar may be piled up in a heap in a basin, it is impossible to make water stand in a heap. The reason is that the force of gravity draws the molecules of the water downwards, and as they have little cohesion, they do not resist, but roll and tumble one over the other, until the surface is level. The force of gravity is greater than the force of cohesion. The sawdust, on the contrary, stands piled up in a heap, because the force of cohesion between its particles is greater than the force of gravity."