第376章 第四册(32)

"Our old lessons taught us all this, but we can now go back to the force of cohesion to find the reason why certain bodies have these properties. When the cork, whalebone, lead, cane, and wire are bent and twisted, their molecules are forced out of their proper position. The whalebone, cork, and cane spring back to their former shape again, because each molecule takes up its old position.""I have been thinking about the metals, sir," said Willie. "Most of the metals are very tenacious, and it is this property which renders them so useful. Their tenacity, I suppose, is entirely due to the force of cohesion between their molecules.""It is, Willie," said Mr. Wilson. "A body is brittle when it possesses very little cohesion; it is tenacious when cohesion in it is strong. It is only the tenacious metals that are malleable and ductile. When a piece of metal is beaten out thin, or drawn out into wire, it really means that its molecules have been forced into different positions, and because they do this without losing their cohesion, themetal holds together. No metal can be malleable or ductile unless there is a very strong force of cohesion between its molecules.""I remember you told us too, sir," said Fred, "that cohesion will account for the different weight of different bodies. In a heavy body the force of cohesion is stronger than in a light one; the molecules are held more closely together.""Quite true," said Mr. Wilson, "and as a result of that, more matter is massed together into a given bulk, than if there were less cohesion."Lesson 10