"We have seen that heat causes bodies to expand," said Mr. Wilson, "and that when the heat is removed the bodies contract again to their original bulk. It will be our business now to learn why this happens. Look at this piece of iron. It is so hard that we cannot even scratch it. The smith, by heating it in his forge, would make it soft and pliable, so that he could beat, cut, weld, and pierce it, and work it up into any shape he pleased. Now in the first place, why is the iron such a hard substance?""Its molecules are held closely together by a very strong force of cohesion, sir," said Fred. "The stronger the cohesive force, the denser and harder is the substance.""Very good, Fred. Now can you guess why the iron becomes soft when it is put into the furnace? I think I must tell you. The heat overcomes the force of cohesion. Cohesion is not so strong between the molecules as it was before the iron was put into the fire. It is the same with all metals.
"What happens to these metals when they are still further heated?""They melt, sir; they change from the solid to the liquid form.""Quite right, Fred," said Mr. Wilson, "and you can now explain what it all means.""I suppose the force of cohesion becomes still further weakened by the heat, sir, so that the melted particles are free to move about.""Just so. The heat changes the solid to a liquid by overcoming the force of cohesion between its molecules.
"Now let us take another step. The force of cohesion in liquids is very slight, and is thus easily overcome. What happens when we apply heat to a liquid?""The liquid is converted into a gas, sir, and the particles of a gas, having no cohesion, fly from each other in all directions.""Exactly. The solid ice becomes liquid water; the water becomes vapour or water-gas, and all by the application of heat.
"Workmen often make great use of these expanding and contracting forces in the materials which they use. The wheelwright, for instance, in fitting the tire of a wheel, first makes it red-hot, and while it is in this expanded state, fixes it round the wheel. It is then plunged into cold water,and as it cools it contracts and presses the wheel so closely, that it binds all parts of the wheel together.
"In building a furnace, too, the workman always leaves one end of the iron bars free. If both ends were built fast into the bricks or masonry, their expansion by reason of the heat would tear away the brickwork.
"In laying down a railway line, the iron or steel rails are not placed with their ends close together. There is a little space left between them. The rails expand with the heat in the summer, and if they were close together the expansion would cause them to bulge and curve.
"In riveting the iron and steel plates of ships and boilers, the rivets are always fastened and fitted red-hot, and thus as they cool, they draw the plates together with great force by their own contraction. Heated iron bars are often used to draw bulging walls upright. The barsare secured and screwed up while heated, and as they cool they contract, and so draw the wall into the proper shape."Lesson 47