available thermal energy. So the fallacious principle of the conservation of heat became merged into the doctrine of conservation of energy; the reconciliation between the first and second laws, which, like the Kantian antinomies, had seemed mutually contradictory, was effected, and Clausius reasoned that heat can not flow from a colder to a warmer body without compensation, that is, without the intervention of external forces. Meanwhile Lord Kelvin, to whom we owe our ideas and definitions of intrinsic and available energy, was able, in 1852, to shadow forth that comprehensive form of the second law afterwards stated by him as a physical law of irreversibility, according to which there is a universal tendency in nature towards irrevocable dissipation of energy. From this time on progress in the science was rapid. The mathematical part of the theory was improved by the introduction of the scalar value which Rankine called the "thermodynamic function" and Clausius the "entropy" of a body, a variable quantity, momentary increase or decrease of which indicates (in a reversible physico-chemical transformation) whether heat is leaving or entering the body at that moment, irrespective of its temperature or previous condition. The temperature of a body, although measured by arbitrary standards, is in reality a non-measurable "intensity" or quality of the body, depending upon whether it is capable of giving up or receiving heat, i. e., upon its dynamic potentiality; and, in practise, addition of heat to a body may change its physical state but does not necessarily alter its temperature; nor does a change of temperature, as Trevor has recently insisted, necessarily imply absorption or development of heat; but the entropy of a body is a definite measurable "capacity," and has been compared
- W. Thomson, Phil. Mag., 1852, IV., 304. "Available energy is energy which we can direct into any required channel. Dissipated energy is energy which we can not lay hold of and direct at pleasure, such as the energy of the confused agitation of molecules which we call heat." Maxwell, sub voce "Diffusion."
- Rankine, Phil. Tr., 1854, CXLIV., 126.
- Clausius, Poggend. Ann., 1855, CXXV., 390.
- "When a mass of air is adiabatically compressed or when it expands into a vacuum, the temperature of the mass changes, but no heat is added to it. When heat is added to a block of metal, the temperature of the block rises. When heat is added to a mass of liquid water and overlying water vapor supporting a constant pressure, the temperature of the mass is not altered. Heat may be added to a mixture of potassium sulphocyanate and water in the process of forming a mixture, and the temperature fall. . . . The quantity of 'heat' added to a body in a change of its thermodynamic state is the work absorbed or absorbable by the body through direct intervention of a change of the temperature of another body. This is all that a 'quantity of heat' means. To assume it to mean a quantity of an imponderable fluid, or a quantity of the kinetic energy of hypothetical and inaccessible particles is to replace direct statement of physical facts, made with the aid of clearly defined terms, by a hypothetical interpretation of the facts." J. E. Trevor, Jour. Phys. Chem., 1908, XII., 316.