Prof. Tiioinson on the Dynamical Theory of Heat. 217 



working with a source and refrigerator at the higher and lower 

 of the temperatures respectively. 



100. Convention for thermometric unit, and determination of 

 absolute temperatures of fixed points in terms of it. 



Two fixed points of temperature being chosen according to 

 Sir Isaac Newton's suggestion, by particular efi'ects on a parti- 

 cular substance or substances^ the difference of these tempera- 

 tures 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 ] 00 degrees. The determination of the absolute tempe- 

 ratures of the fixed points is then to be effected by means of 

 observations indicating the oeconomy of a perfect thermo- 

 dynamic engine, with the higher and the lower respectively as 

 the temperatures of its source and refrigerator. 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 conjunction 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 determi- 

 nation, which we have yet been able to obtain is, that the tem- 

 perature of freezing water is 273-7 on the absolute scale ; that 

 of the boiling-point being consequently 373-7. Further details 

 regarding the new thermometric system will be found in a joint 

 communication to be made by Mr. Joule and myself to the 

 Royal Society of London before the close of the present session. 



101. A corollary from the second general law of the dyna- 

 mical theory stated above in § 98, equivalent to the law itself in 

 generality, is, that if a material system experience a continuous 

 action, or a complete cycle of operations, of a perfectly reversible 

 kind, the quantities of heat which it takes in at different tem- 

 peratures are subject to a linear equation, of which the coeffi- 

 cients are the corresponding values of an absolute function of 

 the temperature. The thermometric assumption which has 

 been adopted is equivalent to assuming that this absolute func- 

 tion is the recipi'ocal of the temperature ; and the equation con- 

 sequently takes the form 



Hf H<, H<„ 



* " 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. 



t " 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 ; or Phil. Mag. vol. iv. p. 424. 



Phil. Mag. S. 4. Vol. 11. No. 71. March 1856. Q 



