VARIATION IN HEATS OF COMBINATION. 121 



equality exists in fact for compound gases formed without conden- 

 sation. It is admitted that it should exist in principle for per- 

 fect gases, if the combination were effected at constant volume, 

 hence the definition (p. 114) of the molecular heat of combination. 



Theorem IV. Combinations referred to the solid state. The 

 same equality exists approximately for solid bodies ; the specific 

 heat of these compounds, referred to equivalent weights, being 

 nearly the same as the sum of those of their components. The 

 heats of combination can therefore be referred to the solid state, 

 as validly as the atomic specific heats already are by Dulong's 

 law; which shows the importance of the expression S given 

 above. The liquid or dissolved state does not present the same 

 advantages ; for instance, the heat liberated in the reaction of 

 dilute hydrochloric acid on dilute soda, these two bodies being 

 taken at a given degree of concentration, varies from -f- 14*7 Gal. 

 to 4- 10-4 CaL, between and 100, that is to say, nearly by 

 half the latter's value. 



Theorem V. The heat liberated or absorbed during solution of 

 an anhydrous salt changes continually in amount with the tempe- 

 rature of solution; since the specific heat of saline solutions 

 differs, generally speaking, from the sum of the specific heats of 

 the salt and water taken separately. 



It is smaller with the majority of the dilute solutions formed 

 by the inorganic salts. But the contrary holds good with the 

 solutions of various organic salts. The heat of solution of 

 anhydrous salts changes as a rule even in sign, for an interval 

 in temperature not exceeding 100 to 200; sometimes this 

 change of sign occurs near the surrounding temperature, and 

 can be determined by direct experiments. 



Hence it follows that those of the inorganic salts which pro- 

 duce cold when dissolving in water, at the ordinary temperature, 

 produce on the contrary heat at a higher temperature, whence it 

 also follows that there exists a temperature for which no thermal 

 variation is produced during solution. 



These results, of which the development and demonstration 

 will be found in the author's " Essai de Mecanique Chimique," 

 torn. i. p. 123, et seq., prove that solution has hitherto errone- 

 ously been assimilated to fusion. 



5. Theorems relative to the Variation of the Heat of Combination 

 with the Pressure. 



Theorem I. In gaseous combinations and reactions the heat 

 liberated is independent of the pressure, operating at a constant 

 volume. 



This statement is no other than Joule's law, and is only true 

 for slight pressures and on the assumption that there is no 

 appreciable internal work in the gases, this work being in fact 

 negligible for gases remote from the point of liquefaction and at 

 a low pressure. 



