and Pyroelectric Properties of Matter. 5 



one case is the excitation of thermal motions, and in the other 

 the overcoming of resistances. The body must itself be alter- 

 ing in its circumstances, so as to contain a less store of energy 

 within it, by an amount precisely equal to the aggregate value 

 of the mechanical effects produced ; and conversely, the ag- 

 gregate value of the mechanical effects produced must depend 

 solely on the initial and final states of the body, and is there- 

 fore the same, whatever be the intermediate states through 

 which the body passes, provided the initial and final states be 

 the same. 



2. The total intrinsic energy of a body might be defined as 

 the mechanical value of all the effect it would produce, in heat 

 emitted and in resistances overcome, if it were cooled to the 

 utmost, and allowed to contract indefinitely or to expand inde- 

 finitely according as the forces between its particles are attrac- 

 tive or repulsive, when the thermal motions within it are all 

 stopped ; but in our present state of ignorance regarding per- * 

 feet cold, and the nature of molecular forces, we cannot deter- 

 mine this "total intrinsic energy '■' for any portion of matter ; 

 nor even can we be sure that it is not infinitely great for a 

 finite portion of matter. Hence it is convenient to choose a " 

 certain state as standard for the body under consideration, and 

 to use the unqualified term intrinsic energy with reference to 

 this standard state ; so that the " intrinsic energy of a body in 

 a given state " will denote the mechanical value of the effects 

 the body would produce in passing from the state in which it 

 is given, to the standard state — or, which is the same, the me- 

 chanical value of the whole agency that would be required to 

 bring the body from the standard state to the state in which it 

 is given. 



3. In Part V.* of a series of papers on the Dynamical Theory 

 of Heat, communicated to the Royal Society of Edinburgh, 

 a system of formulae founded on propositions established in 

 Part I.j of the same series of papers, and expressing, for a 

 given fluid mass, relations between its pressure, its thermal 

 capacities, its intrinsic energy (all considered as functions of 

 its temperature and volume), and Carnot's function of the 

 temperature, were brought forward for the purpose of pointing 

 out the importance of making the intrinsic energy of a fluid in 

 different states an object of research along with the other 

 elements which have hitherto been considered, and partially 

 investigated in some cases. In the present communication a 

 similar mode of treatment, extended to include solid bodies, 

 unmagnetic [and unelectrified] , or magnetized [or electrified] 



* Trans. Roy. Soc. Edinb. December 15, 1851. 

 t Ibid. March 17, 1851. 



