SIR W. THOMSON ON THERMODYNAMIC MOTIVITY. 743, 
hand, work is obtainable from B in virtue of some of these other causes, 
and if V denote its whole amount, then, 
Bt — Neem a ts) beth fa TOM 
is what I call the whole Motivity of B in its actual circumstances, according, 
to this more comprehensive supposition. 
We may imagine the whole Motivity of B developed in an infinite variety 
of ways. The one which is obvious from the formula (5) is first to keep every 
part of B unmoved and to produce all the work producible by perfect thermo- 
dynamic engines equalising its temperature to T; and then keeping it rigor- 
ously at this temperature to take all the work that can be got from it elastically, 
cohesively, electrically, magnetically, and gravitationally, by letting it come to 
rest unstressed, diselectrified, demagnetised, and in the lowest position to 
which it can descend. 
But instead of proceeding in this one definite 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 technical 
language of thermodynamics, provided all the operations be reversible, the 
same whole quantity of work will be obtained in passing from the same initial 
condition to the same final condition, whatever have been the order of procedure. 
Hence the Motivity is a function of the temperature, volume, figure, and proper 
independent variables for expressing 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 the centre of gravity above 
its lowest position). So also is the Hnergy 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 Mechanical 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 variables 
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 thermoelectric 
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 
