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72. Now, it is important to remember that we must treat cohesion and chemical affinity exactly in the same way as gravity has been treated; and just as we have energy of position with respect to gravity, so may we have as truly a species of energy of position with respect to cohesion and chemical affinity. Let us begin with cohesion.

73. We have hitherto regarded heat as a peculiar motion of the molecules of matter, without any reference to the force which actuates these molecules. But it is a well-known fact that bodies in general expand when heated, so that, in virtue of this expansion, the molecules of a body are driven violently apart against the force of cohesion. Work has in truth been done against this force, just as truly as, when a kilogramme is raised from the earth, work is done against the force of gravity. When a substance is heated, we may, therefore, suppose that the heat has a twofold office to perform, part of it going to increase the actual motions of the molecules, and part of it to separate these molecules from one another against the force of cohesion. Thus, if I swing round horizontally a weight (attached to my hand by an elastic thread of india-rubber), my energy will be spent in two ways—first of all, it will be spent in communicating a velocity to the weight; and, secondly, in stretching the india-rubber string, by means of the