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or the Me Chanical Theory of Heat Thermodynamics

engine, temperature, mechanical, water, energy, quantity, unit, law, equivalent and reversible

THERMODYNAMICS, or THE ME CHANICAL THEORY OF HEAT, that branch of p'hysical science which treats of the relation of heat energy to energy of other kinds, and particularly of the convertibility of heat energy into mechanical energy, and the converse. In order to discuss, quantitatively, the conversion of one Icind of energy into another lcind, we must first have a definite method of measuring each of them. Mechanical energy (see ENERGETICS) is measured by. de termining the amount of work that a given quantity of it can perform; the customary unit employed for this purpose being the °foot pound* or the "metre-kilogram') in engineering practice and the "erg° in scientific work; the °ere being defined as the quantity of work done in overcoming a resistance of one dyne, through a distance of one centimeter. The unit employed in the measurement of heat is.almost universally the quantity of heat reqtured to raise. the temperature of some definite mass of water through one degree, on some stated part of the thermometric scale. The ordinary "British thermal unit,)) whict is used in engi neering practice in English-spealcing countries, is the quantity of heat required to raise the temperature •of one pound of water by one Fahrenheit degree; and in countries that use the metric system, the engineering unit is the quantity of heat required to raise the tem perature of one lcilogram of water through one centigrade degree. As the specific heat (q.v.) of water varies slightly at different tempera tures, these definitions are not absolutely defi nite, unless the part of the thermometric scale at which the experiment is to be performed is specified. Unfortunately there is no general agreement among engineers on this point; and for most purposes in practical engineering it is customary to ignore the slight variation in the specific heat of water and to consider the foregoing definitions to be sufficiently precise as they stand. For scientific purposes, where the greatest possible accuracy is required, this course is not permissible, and it becomes neces sary to specify the particular degree through which the temperature of the water is to be raised. Even here there is no definitely estab lished convention; but there appears to be a growing tendency to adopt the degree that ex tends from 14.5° C. to 15.5° C. In scientific work, too, it is customary to define the thermal unit in terms of a gram of water, instead of a kilogram; and the scientific heat unit (which is called the "small calorie? to distinguish it from the "greater calorie') that is used in engineer ing) may be defined as the quantity of heat re quired to raise the temperature of one gram of water from 14.5° C. to 15.5° C. The quan tity of heat required to raise the temperature of one gram of water from 3.5° C. to 4.5° C.— 4.0° being the temperature at which water has its maximum' density—is also known as the atherm.)) The science of thermodynamics is founded upon two general, fundamental laws, which, so far as we are aware, are absolutely rigorous and which are respectively known as the "first') and °second" laws. These we shall consider in order.