Page:Popular Science Monthly Volume 74.djvu/478

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interest as the development of a science within a short time and by a small number of men, from the condition of a vague anticipation of nature to that of a science with secure foundations, clear definitions and exact boundaries."[1] Its development falls conveniently into three stages: (1) The derivation of the two laws governing thermal transformations of energy by Carnot, Rankine, Mayer, Joule, Clausius and Kelvin. (3) The deductive application of the second law to all physico-chemical phenomena by Gibbs. (3) The application of probabilities and statistical methods to the kinetic theory of gases by Clausius, Kelvin, Maxwell and Boltzmann and the final derivation of the theorems and equations of thermodynamics by statistical induction from the average behavior of mechanical systems by Gibbs.

"It must not be thought that heat generates motion or motion heat (though in some respects this is true) but the very essence of heat or the substantial self of heat is motion and nothing else.[2] In this sentence from the Novum Organum it is clear that Bacon, like Descartes, Count Rumford, Sir Humphry Davy and Young, had a more or less definite notion of the dynamic nature of heat and its convertibility into work. But the exact science which treats of heat as a mode of energy begins with the publication, in^ 1884, of the "Réflexions sur la puissance motrice du feu" of Sadi Carnot, whom Lord Kelvin calls the "profundest thinker in thermodynamic philosophy" in the first half of his century.[3] In this little work we have the first treatment of the heat engine as a reversible "cycle of operations," a mechanism which can be worked backward with its every action reversed; and such a system is now known everywhere as a "Carnot cycle." Carnot compared the motor power of heat to a fall of water.[4] As the power of the waterfall depends upon its height and the quantity of fluid employed, so the motor power of heat depends, not upon the nature of the working substance, but upon the quantity of heat employed and the difference in temperature between its source (the boiler) and the sink (or exhaust cylinder) to which it flows. A heat motor, then, requires a hot body and a cold body; the ideally perfect engine would be completely reversible and the efficiency of engines working between the same limits of temperature is the same. In other words, heat can not perform work except by spontaneous flow from a higher to a lower temperature. This is Carnot's principle, from which is derived

  1. Ibid., 1877-8, XVII., 257.
  2. Bacon, "Novum Organum," English translation of 1850, p. 165.
  3. Kelvin, "Popular Lectures," London, 1894, Vol. II., 460.
  4. "On peut comparer la puissance motrice de la chaleur à celle d'une chute d'eau:. . . la puissance motrice d'une chute d'eau dépend de la hauteur et de la quantité du liquide; la puissance motrice de la chaleur dépend aussi de la quantité de calorique employé, et de ce que nous appellerons le hauteur de sa chute, c'est à dire de la différence de température des corps entre lesquels se fait l'échange du calorique." Carnot, "Reflexions," 1824, 15.