306 HISTORY OF CHEMISTRY. [LECTURE XV. 



that the ga^es had not been sufficiently heated prior to their 

 inter-mixture, 44 Deville afterwards repeats the experiment in a 

 manner which no longer permits of this objection being raised, 

 and again observes a rise of temperature, the amount of which, 

 however, he does not state. 45 He finds a further argument in 

 his favour in the fact that ammonia, when heated to 1100, 

 breaks up into nitrogen and hydrogen. In his opinion ammo- 

 nium chloride, after having been heated to this temperature, 

 ought to yield these two gases on cooling as evidence of the 

 formation of ammonia ; but this is not the case. 



In opposition to this argument, Than adduces the fact that 

 a gaseous mixture is much more difficult to decompose than a 

 pure gas, 46 and this is in complete agreement with Deville's 

 views regarding dissociation. 47 By diminishing the partial pres- 

 sure, the temperature at which dissociation begins is raised ; 48 

 or, the temperature remaining the same, the pressure due to 

 decomposition is diminished. Further, Than observed no rise 

 in temperature on mixing hydrochloric acid and ammonia at 

 360. Even if the errors were greater in his arrangement of 

 the experiment, and assuming that he was unable to measure 

 very small differences of temperature, still it is placed beyond 

 doubt by his statements that only inconsiderable quantities of 

 heat are liberated by the intermixture of ammonia and hydro- 

 chloric acid at 360. This is confirmed by an experiment by 

 Marignac, 49 who was able to show that just as much heat is 

 evolved in the formation of ammonium chloride from ammonia 

 and hydrochloric acid as is required for its volatilisation. Hence 

 it may be looked upon as fully proved that ammonium chloride 

 does not exist in the gaseous state, but that it breaks up, on 

 volatilisation, into its components. 



Similar facts, even if not always so convincing, have also 

 been observed in the cases of many other compounds whose 

 molecules in the gaseous state correspond to four volumes ; as, 



44 Comptes Rendus. 56, 1237. * Ibid. 59, 1057. * Annalen. 

 131, 129. 47 Deville, Le9ons. 364. 48 Compare Naumann, Annalen, 

 Supplementband 5, 341. 49 Comptes Rendus. 67, 877. 



