MAXIMUM WORK. 123 



Thus they are manifested under the conditions where they 

 provoke either the change of physical state (liquefaction, vapori- 

 sation) of any one of the bodies experimented on, regarded 

 separately, or its isomeric modification, or its total or partial 

 decomposition. It is furthermore evident, generally speaking, 

 that a compound can only take part in a reaction, if it exist in 

 the isolated state under the conditions of the experiment, and 

 in the proportion in which it can exist. This remark rightly 

 understood, can, strictly speaking, be made to apply in practice, 

 for it is sufficient to regard each of the components and of the 

 products in a system, and to know its individual state of 

 stability or of dissociation, under given conditions, in order to 

 be able to apply the principle. It is, moreover, necessary to take 

 into account in calculations and reasonings all the compounds 

 capable of existing under the conditions of the experiment, such 

 as double salts, acid salts, perchlorides, hydrates, etc., and 

 secondary compounds of every kind, which are ordinarily 

 neglected in the general interpretation of reactions, but each 

 of which contributes its quota, and, so to speak, its weight to 

 the thermal balance of affinities. 



Lastly, let us note that in the calculation of the quantities of 

 heat liberated by a transformation, we should consider, as far as 

 possible, the corresponding bodies in the initial and final system 

 taking them under the same physical state. This mode of pro- 

 ceeding offers the advantage of putting aside, without further 

 discussion, a whole class of foreign energies, such as the 

 energies consumed in changes of physical state. 



We do not wish to enter here upon more extended develop- 

 ments ; it will suffice to refer the reader to the detailed discus- 

 sion which is to be found in " Essai de Mecanique Chimique." 1 

 There it will be seen how the third principle is deduced from 

 the experimental study of the phenomena of combination and 

 decomposition. 2 



The following theorems are given which are applicable to a 

 large number of phenomena : 



Theorem I. No endothermal reaction is possible without the 

 intervention of foreign energies. 



Theorem II. A system is the more stable, everything else being 

 equal, the larger the fraction of its energy which it has lost. 



Theorem III. Every chemical equilibrium results from the 

 intervention of certain dissociated compounds, that is to say t in 

 the state of partial and reversible decomposition, which act at 

 once by themselves, as compounds, and by their components. 



Under these conditions, there always intervene, in opposition 

 to the chemical energies properly so called, foreign energies, 

 electric or calorific, the latter especially. 8 



1 Tom. ii. pp. 421-471. 2 Ibid. pp. 424-438. 



3 " Essai de Mecanique," chap. ii. pp. 439 and following. 



