SirW. Thomson on Thermodynamic Motivity . 351 



be got from it elastically, cohesively, electrically, magnetically, 

 and gravitationally, by letting it come to rest unstressed, dis- 

 electrifled, demagnetized, and in the lowest position to which 

 it can descend. But instead of proceeding in this one defi- 

 nite way, any order of procedure whatever leading to the 

 same final condition may be followed ; and, provided nothing 

 is done which cannot be undone (that is to say, in the tech- 

 nical language of thermodynamics, provided all the operations 

 be reversible), the same whole quantity of work will be ob- 

 tained in passing from the same initial condition to the same 

 final condition, whatever may have been the order of pro- 

 cedure. Hence the Motivity is a function of the temperature, 

 volume, figure, and proper independent variables for express- 

 ing the cohesive, the electric, and the magnetic condition of 

 B, with the gravitational potential of B simply added (which, 

 when the force of gravity is sensibly constant and in parallel 

 lines, will be simply the product of the gravity of B into the 

 height of its centre of gravity above its lowest position). So 

 also is the Energy of a body B (as I first pointed out, for the 

 case of B a fluid, in Part V. of my Dynamical Theory of 

 Heat, in the Transactions of the Royal Society of Edinburgh 

 for December 15, 1851, entitled, " On the Quantities of Me- 

 chanical Energy contained in a Fluid in Different States as 

 to Temperature and Density "). Consideration of the Energy 

 and the Motivity, as two functions of all the independent vari- 

 ables specifying the condition of B completely in respect to 

 temperature, elasticity, capillary attraction, electricity, and 

 magnetism, leads in the simplest and most direct way to 

 demonstrations of the theorems regarding the thermodynamic 

 properties of matter which I gave in Part III. of the Dynamical 

 Theory of Heat (March 1851); in " Part VI. of Dynamical 

 Theory, Thermoelectric Currents," (May 1, 1854); in a paper 

 in the Proceedings for 1858 of the Royal Society of London, 

 entitled, " On the Thermal Effect of Drawing out a Film of 

 Liquid ;" and in a communication to the Royal Society of 

 Edinburgh (Proc. R. S. E. 1869-70), " On the Equilibrium 

 of Vapour at the Curved Surface of a Liquid;" and in my 

 article on the Thermoelastic and Thermomagnetic Properties 

 of Matter, in the first number of the Quarterly Journal of 

 Mathematics (April 1855); and in short articles in Nicholas 

 Cyclopaedia, under the titles " Thermomagnetism, Thermo- 

 electricity, and Pyroelectricity," put together and repub- 

 lished with additions in the Philosophical Magazine for Ja- 

 nuary 1878, under the title " On the Thermoelastic, Thermo- 

 magnetic, and Pyroelectric Properties of Matter." 



It would be beyond the scope of the present article to enter 

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