§ 1. Mechanical Analogy.

which attack visible bodies and do not act uniformly on all molecules, must produce a relative motion of the molecules against one another. Because of the indestructibility of kinetic energy original: "lebendige Kraft" (living force); a historical term for kinetic energy ($mv^2$), this motion cannot cease but must continue into infinity.

In fact, experience teaches us that as soon as forces act perfectly uniformly on all parts of a body—as is the case, for example, in so-called free fall—all kinetic energy appears visibly. In all other cases, we have a loss of visible kinetic energy and the appearance of heat instead. The view suggests itself that this is the resulting motion of the molecules against each other, which we cannot see because we cannot see the individual molecules, but which communicates itself to the molecules of our nerves upon contact and thus produces the sensation of heat. It will always pass from the body whose molecules are moved more vigorously to the one whose molecules move only slowly. In doing so, because of the indestructibility of kinetic energy, it will behave like a substance A reference to the "caloric" theory, which treated heat as a fluid; the author is explaining why heat seems like a substance even though it is motion., as long as it does not arise from or transform into visible kinetic energy or work.

Now, we do not know the nature of the forces that hold the molecules of a solid body in their relative positions—whether they are forces acting at a distance Fernkräfte: forces that act across space without a physical medium, like gravity or whether they are mediated by a medium—and how they are influenced by thermal motion. However, since they resist both approach (compression) and further distancing (dilation), we clearly obtain a very rough picture if we assume that every molecule in a solid body has a position of rest. If it is brought closer to neighboring molecules, it is repelled by them; if it is moved away, conversely, attraction occurs. As a result of thermal motion, a molecule is initially set into pendulum-like oscillations in straight or ellipse-like paths around its rest position A (in the symbolic

A schematic diagram labeled "Fig. 1." illustrating the relative positions of molecules. Points representing molecules are labeled with letters: B is at the top, D and E are on the left, and a horizontal sequence of points shows A, A', and A''. A point C is located below A'.

Fig. 1, the centers of gravity of the molecules are drawn). If it reaches A' in this process, it is influenced by the neighboring molecules B and C