DYNAMICAL THEORY OF HEAT. 125 



and if all the localities of the system by which heat is either emitted or taken in 

 be at one or other of two temperatures the aggregate amount of heat taken in or 

 emitted at the higher temperature, must exceed the amount emitted or taken in 

 at the lower temperature always in the same ratio when these temperatures are the 

 same, whatever be the particular substance or arrangement of the material system, 

 and whatever be the particular nature of the operations to which it is subject. 



99. Definition of Temperature, and General Thermometric A.ssumption.~lf 

 two bodies be put in contact, and neither gives heat to the other, their tempera- 

 tures are said to be the same ; but if one gives heat to the other, its temperature 

 is said to be higher. 



The temperatures of two bodies are proportional to the quantities of heat 

 respectively taken in and given out in localities at one temperature and at the 

 other, respectively, by a material system subjected to a complete cycle of perfectly 

 reversible thermo-dynamic operations, and not allowed to part with or take in heat 

 at any other temperature : or, the absolute values of two temperatures are to one 

 another in the proportion of the heat taken in to the heat rejected in a perfect 

 thermo-dynamic engine working with a source and refrigerator at the higher and 

 lower of the temperatures respectively. 



100. Convention for thermometric unit, and determination of absolute tempe- 

 ratures of fixed points in terms of it. 



Two fixed points of temperature being chosen according to Sir Isaac New- 

 ton's suggestion, by particular effects on a particular substance or substances, 

 the difference of these temperatures is to be called unity, or any number of units 

 or degrees, as may be found convenient. The particular convention is, that the 

 difference of temperatures between the freezing and boiling points of water under 

 standard atmospheric pressure shall be called 100 degrees. The determination 

 of the absolute temperatures of the fixed points is then to be effected by means 

 of observations indicating the economy of a perfect thermo-dynamic engine, with 

 the higher and the lower respectively as the temperatures of its source and refri- 

 gerator. The kind of observation best adapted for this object was originated by 

 Mr Joule, whose work in 1844* laid the foundation of the theory, and opened 

 the experimental investigation; and it has been carried out by him, in conjunc- 

 tion with myself, within the last two years, in accordance with the plan proposed 

 in Part IV. f of the present series. The best results, as regards this determina- 

 tion, which we have yet been able to obtain is, that the temperature of freezing 

 water is 2737 on the absolute scale ; that of the boiling point being consequently 

 373 - 7. Farther details regarding the new thermometric system will be found in 



* On the Changes of Temperature occasioned by the Rarefaction and Condensation of Air. See 

 Proceedings of the Royal Society, June 1844 ; or, for the paper in full, Phil. Mag., May 1845. 



j- On a Method of discovering experimentally the Relation between the Heat Produced and the 

 Work Spent in the Compression of a Gas. Trans. R.S E., April 1851. 



