“The blacksmith heats the wagon-tire in order that it may easily slip over the wheel. If a kettle be filled with cold water, by heating it the water is expanded and runs over. I have noticed that the spaces between the ends of the successive iron rails upon the railroad are larger in winter than in summer, showing that the rails are shorter in winter than in summer. While skating during the cold winter evenings upon the mill-pond, I have seen cracks in the thick ice start and run across the mill-pond with a roar almost like thunder. The ice was contracted by the cold till it could no longer fill the whole space between the banks, and being frozen fast to the banks, it was torn asunder. The mercury in the tube of a thermometer is constantly expanding or contracting by every change of temperature.”
“Yes, those are all good illustrations, and we might go on to mention others equally good by the score. In cold countries, during the intense cold of winter, the surface of the earth cracks by shrinkage, just as you have seen the ice upon the mill-pond torn in two. The Britannia iron tubular bridge over the St. Lawrence at Montreal rises and falls two and one-half inches on account of greater expansion of the upper surface when exposed to the heat of the sun, while a loaded freight train causes a depression of but one-fourth of an inch. A few years since, in order to make some philosophical experiments connected with the rotation of the earth upon its axis, a ball was suspended by a wire in the interior of Bunker Hill monument. By this means it was accidentally discovered that the heat of the sun, expanding the sides of the monument exposed to its rays, caused the whole monument to sway back and forth daily.”
“What is it, Ansel?”
“I was going to mention the belief of geologists that the mountain ranges were thrown up by the contracting of the earth’s crust on account of cooling.”
“That is an illustration of contraction by loss of heat on an enormous scale. The materials which form our globe may have existed in the beginning in a nebulous or gaseous state. There is certainly very good reason for believing that the earth was once in a fluid state, the whole of its substance molten by intense heat. It is certain that the interior is now hot, and portions of it molten. It is by very many believed that the whole interior is molten. The crust of the earth may have been formed by cooling. If after an outer crust had been formed, and its temperature had fallen so low as to become nearly stationary, the interior mass continued to cool, the molten mass would tend to sink away from the crust and the crust would sink in upon it by wrinkling. Thus mountains may have been formed. Along the line of fracture the easiest vents would be formed for volcanoes. But this carries us somewhat aside from our subject, and as the expansion of bodies by heat has been sufficiently illustrated, we will leave it. Will some one now state the manner in which the dynamic theory of heat explains this expansion?”
Samuel answered: “I think you have already given us the explanation.”
“I have briefly referred to it, but you may give it again.”
“The atomic motion which is supposed to constitute what we call heat, whatever that motion be, whether a vibration or rotation or revolution, requires that the atoms of bodies shall not be packed in absolute contact, and the more intense the agitation or the wider the swing of the vibration or revolution, the greater must be their separation. Hence heat expands bodies by thrusting their atoms farther apart.”
“That will do,” said Mr. Wilton. “Let us look now at some of the secondary effects of heat. You may mention some of them, Ansel.”