122 GENERAL PRINCIPLES OF THERMO-CHEMISTRY. 



Theorem II. In gaseous combinations and reactions effected 

 without condensation, the heat liberated is the same, whether at 

 constant volume or at constant pressure. 



Such is the case with the combustion of cyanogen, whether 

 by free oxygen or nitric oxide. 



Theorem III. In reactions effected with condensation the heat 

 liberated at a constant pressure,^?, at the atmospheric pressure 

 for example, and at a given temperature, t, is connected with the 

 heat liberated at constant volume, v, and at the same tempera- 

 ture, by the following relation 



Qtv = Qtp + 0-542 (N' - N) + 0'002*. 



N here expresses the quotient by 22'32 of the number of litres 

 occupied by the component gases reduced to and 0760 

 metres, and N' the same quotient for the resulting gases. 



This theorem is of great importance in calorimetric measure- 

 ments relative to explosive substances. It enables the difference 

 between the two quantities of heat at constant pressure and 

 volume to be calculated for every reaction of which the formula 

 is known (see p. 15). It is not only applicable to reactions 

 where all the bodies, components as well as products, are 

 gaseous, but also to those where some of them only possess the 

 solid or liquid state at the outset, or assume it at the end. 



THIRD PRINCIPLE MAXIMUM WORK. 



Every chemical change, effected without the intervention of a 

 foreign energy, tends towards the production of the body or of the 

 system of bodies liberating the most heat. 



The necessity of this principle may be seen by observing that 

 the system which has liberated the greatest possible amount of 

 neat, no longer possesses in itself the energy necessary for 

 effecting a new transformation. Every fresh change requires 

 work, which cannot be performed without the intervention of a 

 foreign energy. On the other hand, a system still capable of 

 liberating heat by a fresh change possesses the energy necessary 

 for effecting this change without any auxiliary intervention. 



The foreign energies here in question are those of physical 

 agents: light, electricity, heat; the energy of disaggregation 

 developed by solution ; lastly, the energy of chemical reactions, 

 simultaneous to that under consideration. Now, the interven- 

 tion of electric or luminous energies in a chemical phenomenon 

 is ordinarily apparent, and it is the same with chemical energy, 

 borrowed from a simultaneous reaction. The only cases which 

 call for discussion are those in which calorific energy and the 

 energy of disaggregation by solution intervene. They are dis- 

 tinguished by the following general character, that these energies 

 are exercised solely to regulate the conditions of existence of each 

 compound regarded separately without in any other way inter- 

 vening in the place of the reciprocal chemical actions. 



