THERMODYNAMICS, the branch of theoretical physics which treats of heat as a mechanical agent, and is the basis on which the modern doctrine of energy is built. The second interpretation given by Newton of his third law of motion all but enumerates the principle of the conservation of energy. Ignorant, how ever, of the true nature of heat, he was unable to trace the mechanical loss caused by friction to its final issue. Not till more than a century after the publica tion of the "Principia" was any attempt made to fill the gap, and then the valu able experimental results of Rumford (1798) and Davy (1799), though con clusively disproving the accepted caloric theory, were given to an unappreciative world, and failed to excite any real inter est till 40 years later, when their discov eries were rediscovered. In 1812 Davy wrote, "The immediate cause of the phe nomenon of heat then is motion, and the laws of its communications are precisely the same as the laws of the communica tion of motion." Here then the dynamical theory of heat was enunciated, but it was carried no further, and not till the experi ments of Colding and Joule, executed in dependently about 1840, were published, can thermodynamics be regarded as established.
About the same time Seguin and May er approached the same object, and de duced from experiment values of the mechanical equivalent of heat. They, however, went to work on hypotheses or rather different forms of the same hypothesis which are now known to be false; so that their claims as the found ers of the doctrine of energy cannot be maintained against those of Colding and Joule, who went to work in a legitimate way. Mayer, however, deserves great merit for the manner in which he de veloped and applied the conservation principle. In the more restricted sphere of thermodynamics, Clausius, Rankine, and Thomson have been the great de velopers; and to the last-mentioned is due the modification of Carnot's forgot ten cycle of operations to suit the true theory, and the deduction therefrom of the doctrine of the dissipation of en ergy.
Thermodynamics is based on two laws. The first law enunciates heat to be a form of energy and subject to the conservation principle—an experimental truth rigor ously established by Joule. Clerk Max well gives it in the form: When heat is transformed into work or work into heat, the quantity of work is mechanically equivalent to the quantity of heat. A given quantity of work can always be transformed into an equivalent quantity of heat; but in transforming heat into work a certain limitation exists which is expressed in the second law of thermo dynamics. This law asserts that it is impossible, by physical processes, to transform any part of the heat of a body into mechanical work except by allowing heat to pass from that body to another at a lower temperature; or in Thomson's words, it is impossible by means of inanimate material agency to derive mechanical effect from any portion of matter by cooling it below the temper ature of the coldest of the surrounding objects. This is the law on which Car not's principle is based—a principle which has led to results of the highest con sequence. The principle is that the effi ciency of a reversible engine is the great est that can be obtained from a given range of temperature. Now a reversible engine in Carnot's sense is an altogether unrealizable heat engine, which can be made to go through a complete cycle of operations, either forward or backward. In other words, not merely is the engine able to do work while it transforms a given quantity of heat from the boiler to the condenser, but, by an expenditure of an equivalent quantity of work upon it, may be made to take back the same quantity of heat from the condenser to the boiler. In subjecting such an engine to a cycle of operations, the engine must be brought back to its original condition before any conclusion can be drawn re garding the relation between the heat which has disappeared and the work which has been done.