The second law of thermodynamics (see THERMODYNAMICS) , which, like the first law, is an embodiment of the physical ex perience of generations of experimenters and observers of the first rank, denies that it is possible to derive mechanical energy from bodies by cooling them below the temperature of the sur rounding object, that is, it denies precisely that a perpetual motion of the second kind is possible. For us to be able to convert heat into work we must have a source of heat above the temperature of the surrounding bodies, or, in the language of the engineer, we must have both a boiler and a condenser (although, of course, we need not actually build the condenser, but can use the sur rounding air as condenser if for any reasons, such as exist in the case of the locomotive, a condenser is impracticable). The bigger the difference of temperature between the hot source of heat and the cold condenser, the bigger the fraction of the heat of the source which we can turn into work: when the temperature dif ference is zero we can turn none of the heat into work.
The impossibility of a perpetual motion, either of the first or of the second kind, is implicit, then, in the two great laws of Thermo dynamics, which express our knowledge of the behaviour of matter in such quantities as are handled in ordinary mechanical and chemical processes. They are statistical laws, and do not
apply to individual molecules, or to small collections of molecules, but to matter as we deal with it in engineering, where the smallest particle considered consists of millions of millions of millions of molecules ! When we come down to microscopic and ultra-micro scopic particles perpetual motion is possible, as exemplified by the ceaseless movement of tiny particles held in suspension in a fluid at a uniform temperature. (See BROWNIAN MOVEMENT.) Such movement of microscopic particles, however, does not enable us to derive useful work from the heat agitation of the liquid, that is, from the perpetual motion of the molecules themselves. It does not contradict, but supplements our knowledge of the behaviour of matter in bulk, which tells us that perpetual motion is a mechanical impossibility. (E. N. DA C. A.) PERPETUAL PENSIONS : see PENSIONS : PERPETUAL OR HEREDITARY.