THERMIDOR, i.e., the "hot month," formed, in the calendar of the first French republic, the 11th month, and lasted from July 19 to Aug. 18. The 9th Thermidor of the Republican year 2 (July 27, 1794) is historically memorable as the date of Robes pierre's fall, and the termination of the Reign of Terror. The name Thermidorians was given to all those who took part in this fortunate coup d'etat, but more particularly to those who were desirous of restoring the monarchy.—See Duval's Souvenirs Thermi doriens (2 vols., Par. 1844.) or the DYNAMICAL THEORY ON HEAT, though literally merely the science of the relations of heat and work, is now very generally employed to denote the whole science of ENERGY. See FORCE. We propose in this place to give a general sketch of this grand modern generalization, supplementary to what will be found in the article just referred to; but, for the sake of continuity, we must repeat a little of what was there given, though in a somewhat different form.
Energy is strictly defined as the power of doing work (q.v.), and is of one or other of two kinds—potential or kinetic. A raised weight, a wound-up spring, gunpowder, and the food of animals, are instances of stores of potential energy. A missile in motion, wind, heat, and electric currents are instances of kinetic energy. Sound, light, and other forms of wave-niotion (see WAVE), are all instances of mixed potential and kinetic energy.
The CONSERVATION OF ENERGY is the statement of the experimental fact, that energy is, like matter (q.v.), indestructible and uncreatable by any process at the command of man.
The TRANSFORMATION OF ENERGY is the statement of the experimental fact, that any one form of energy may in general be transformed wholly or partially into any other form. This used to be known as the CORRELATION OF FORCES. But it is subject to the condition derived from the first fact, that the portion transformed retains its amount unchanged. It is also subject to the law of DISSIPATION, or degradation, which is a statement of the experimental fact, that energy generally tends at every transformation to at least a partial transformation into heat; and that, once in that form, it tends to a state of uniform distribution, in which no further transformation is possible. .
The original energy of the universe, therefore, though still of the same amount as at creation, being in a state of ceaseless transformation, has been in great part frittered down into heat, and will at length take wholly that final form.
The history of the grand discoveries which are briefly summarized in these few lines, has been much discussed of late—especially in the Philosophical Magazine—and is now pretty clearly ascertained.
Newton took the first great step. Ina scholium to his third law of motion (q.v.), he lays down in a few clear words the conservation of energy as the embodiment of the experi mental results known in his day with reference to forces and visible motions. Part of this statement of Newton's was afterward reinvented under the name of conservation of vis-viva; but all that Newton really wanted to enable him to complete the conservation of energy was an experimental knowledge of the nature of heat, electricity, etc. That heat is motion of some kind, not matter, and that the laws of its communication are the same as those of the communication of visible motion, was experimentally proved at the very end of last century by Davy. Rumford had almost completed a proof a year or two before; but he had also made a very fair attempt to determine the " mechanical equivalent" of heat—i.e., the quantity of heat which is equivalent to a given amount of mechanical work. That there is such an equivalent is at once evident by looking at Davy's discovery in the light of Newton's scholium already referred to. But though the dynamical theory of heat was thus really founded in 1799, it was not generally received. The first to recall attention to it was Seguin, nephew of the celebrated Montgolfier (from whom lie states that he derived his views), who, in 1839, distinctly enunciated the equiva lence of heat and mechanical work, and sought to prove by experiment that heat dis appears, or is put out of existence, in the production of work from a steam-engine.