"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." "Heat relaxes or overpowers the cohesive attraction of bodies." "What is cohesive attraction?" "It is that force which binds together the atoms of matter in simple substances, that is, bodies like iron or copper or silver, composed of but one kind of substance, or in compound bodies it is the force which unites the compound molecules of matter." "Give us now some illustrations of the effect of heat in overcoming cohesive attraction." "The blacksmith heats his iron in order to overcome its cohesive attraction and render it soft, that he may easily hammer it. The founder heats his metal till its cohesion is so far destroyed that it becomes fluid and can be poured into the mould. Heat relaxes the cohesive force of ice and changes it to water, and by farther heating its cohesion is entirely overcome and the water is changed to a gas." "We use heat also in cooking our food," spoke up Peter: "is it not because heat destroys the cohesive attraction, and thus softens it?"