126 
Lord Kelvin on 
[r.S.E., sess. 
On Thermodynamics founded on Motivity and Energy. 
By the Rt. Hon. Lord Kelvin, G.C.Y.O., E.R.S., F.R.S.E., 
etc. 
(Read March 21, 1898.) 
§ 1. In a verbal communication to the Royal Society of Edin- 
burgh in 1876, under the title “Thermodynamic Motivity,” I 
suggested the name motivity to express energy, whether thermal or 
of any other kind, available to generate velocity in molar matter, 
or to move molar matter against resisting force. By molar matter 
I mean matter as we know it, consisting, as we believe, of vast 
numbers of atoms or molecules. Only the title of this communi- 
cation was published in the Proceedings of the Royal Society : 
a short report of it was published in the Philosophical Magazine 
for May 1879, in which it was pointed out that a “very short and 
simple analytical method of setting forth the whole non-molecular 
theory of thermodynamics ” might be founded on the consideration 
of Motivity and Energy as two functions of all the independent 
variables specifying a body, or a system of bodies, or some definite 
apparatus under consideration : — apparatus, as I shall call it for 
brevity, to include every case, even such as a single crystal. The 
object of the present communication is to carry out this proposal. 
§ 2. Let the apparatus be given all at one temperature t. De- 
note by g v g 2 , y 3 , etc., the other variables by which its condition is 
specified. These will in many cases be geometrical, specifying 
elements or co-ordinates, such as strain-components, expressing 
change of bulk or shape of a piece of crystal or other elastic solid 
under stress ; or positions of pistons in a pneumatic apparatus ; or 
area, or curvature, of a free liquid surface in an application to 
theory of capillary attraction ; or positions of electrified bodies, or 
electrostatic capacities, in an electrostatic system. Or, considered 
as generalised co-ordinates, the independent variables may be 
physical qualities, such as proportion of vapour to liquid in an 
enclosure, with or without a piston ; or quantities of electricity on 
