LECTURE XV.] HISTORY OF CHEMISTRY. 303 



combination of two substances. But further, in exactly the 

 same way that evaporation begins below the condensing point, 

 the decomposition of substances can be observed below the 

 true combining temperature. As every degree of the thermo- 

 metric scale corresponds to a definite vapour pressure, so, in 

 certain cases at least, the pressures of the products of decom- 

 position can be stated. Deville distinguishes between decom- 

 position by the action of heat and decomposition by chemical 

 means. He applies the name dissociation to the former only. 19 

 It is characterised by the facts that its different phases can be 

 observed ; that it begins at one definite temperature and is com- 

 pleted at another ; and that between these limits the pressures 

 increase from o to 760 mm. and more of mercury, so that a 

 definite pressure, due to the gaseous products of the decom- 

 position, corresponds to every temperature. 



Subsequent experiments on this subject (of a very detailed 

 character) have confirmed Deville's views, in general at least. 

 Only those decompositions are now regarded as examples of 

 dissociation which take place in opposition to the chemical 

 forces and are accompanied by the absorption of heat. 20 The 

 comparison of these phenomena with evaporation, even if it is 

 not quite generally applicable, still holds in the decomposition 

 of solid substances with the formation of gaseous constituents, 

 as was shown by Debray in the case of calcium carbonate, 21 by 

 Naumann in that of ammonium carbonate, 22 of Isambert in that 

 of ammonium hydrosulphide, 23 and by others. Investigations 

 of the compounds of silver chloride with ammonia 24 and of 

 compounds containing water of crystallisation, came to be of 

 special importance, because in the cases of these substances the 

 different compounds with ammonia and the different stages of 



19 Annalen. 105, 383. 20 Compare Horstmann, Theoretische Chemie. 

 666. 21 Comptes Rendus. 64, 603 ; Bull. Soc. Chim. 7, 194. ^ Berichte. 

 4, 779. ' 23 Comptes Rendus. 92, 919; 93, 731. " 4 Isambert, Laden- 

 burg's Handworterbuch der Chemie. 3, 400 ; Ilo-stmann, Berichte, 

 9> 749- 



