Page:Popular Science Monthly Volume 74.djvu/482

Helmholtz-Kelvin statement of the first and second laws as conservation and dissipation of energy enables us to apply these principles in the broadest and most philosophical way. Lord Kelvin extended the application of the second law to cosmic physics and, with Boltzmann, to predictions as to the ultimate thermal death of the earth. Meanwhile Clausius, Maxwell and Boltzmann began to apply the second law to the kinetic theory of gases, a phase of the subject which belongs essentially to the last stage of its development. Maxwell in particular emphasized the important point that since the heat of a body is the kinetic energy of its molecular motions, the second law is in reality not a mathematical but a statistical truth. It can not, says Maxwell, be reduced to a form as axiomatic as that of the first law, but stands upon a lower plane of probability, because it depends upon the motions of millions of molecules of which we can not get hold of a single one.^[1] Could we reduce ourselves to molecular dimensions, and with the gift of molecular vision trace the movements of individual molecules, the distinction between work and heat would

Thomson's "available energy," with the statement that Clausius meant by it that part of the energy which can not be converted into work. As Gibbs pointed out, this is entirely incorrect. The entropy of a body is a definite physical property of the body itself, and can not be measured by the same unit as energy. If $dQ$ represent the amount of heat imparted to a body at any point and $T$ its absolute temperature at that point, Clausius has shown that $dQ/T$ represents the infinitesimal change of entropy at that point for any given moment. The total (fliange of entropy of any reversible chemical system in passing from an initial state $a$ to a final state $b$ would then be

$\Sigma \ \int _{a}^{b}\ {\frac {dQ}{T}},$

and for a reversible (Carnot) cycle the mathematical statement of the second law is the "Carnot-Clausius equation":

$\int \left({\frac {dH}{T}}\right)=0.$

This means that the positive and negative entropies of the system in passing from $a$ to $b$ and in reversing backwards from $b$ to $a$ must balance each other. Or as Gibbs has expressed it, "The second law requires (for a reversible cycle) that the algebraic sum of all the heat received from external bodies, divided, each portion thereof, by the absolute temperature at which it is received shall be zero." The criterion of irreversible processes is the "inequality of Clausius"

$\int \left({\frac {dQ}{T}}\right)<0$

which implies that the phenomenon will proceed irrevocably or irreversibly in a definite direction, entropy increasing or available energy dissipating to a maximum until a final state of rest or equilibrium (uniformly distributed temperature) is attained. Reversible thermodynamics deals, then, with equations; irreversible thermodynamics with inequalities, because in reversible processes the total entropy of a system remains unchanged while in irreversible processes it continually increases.

1. Maxwell, Nature, London, 1877-8, XVII., 279.