Page:Popular Science Monthly Volume 74.djvu/480

476 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