THEBMO CHEMISTRY 347 THEBMODYNAMICS to indicate some natural law which is not, at present, fully understood. The Heat of Fwmation of a substance is the amount of heat liberated or ab- sorbed when the substance is produced by a direct combination of the elements of which it is composed. It is customary to determine the heat of formation on gram-molecular weights; so that, quan- titatively, the heat of formation of any substance is expressed as the amount of heat liberated or absorbed when a gram- molecular weight of the substance is formed by combination of its elements. In those cases where compounds cannot be formed from direct combination of elements, it is necessary to determine the heat of formation indirectly. This is commonly done by first burning the elements in oxygen, then burning the compound in oxygen, and determining in each case the heat liberated. The prod- ucts of combustion will, in each case, be the same, so that the difference in the heat liberated in the two cases will give the heat of formation of the compound. THERMOCROSIS, in physics, heat coloration. A red flame, looked at through a red glass appears quite bright, but through a green glass it appears dim or scarcely visible. So in like man- ner heat which has traversed a red glass passes through another red glass with little diminution, but is almost complete- ly stopped by a green glass. To these different obscure calorific rays Melloni g^ve the name of thermocrosis. THERMODYNAMIC ENGINE, any form of heat engine (as gas or steam engines) by means of which a percent- age of the heat lost by one body called the source, on account of its connection with another body called the refrigera- tor, is converted into kinetic energy or mechanical effect, and made available for the performance of work. The ef- ficiency of a heat engine is the ratio of the heat available for mechanical ef- fect to the total heat taken from the source. A reversible engine is called a perfect engine, because it is the most efficient engine between the tempera- tures of its source and refrigerator. 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 thermodjmamics 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
