526 HISTORICAL INTRODUCTION TO CHEMISTRY CH/ 



its colour immediately when cooled ; nor can Pebal's method 

 of diffusion be applied to a substance which gives only one 

 product of decomposition. But the dissociation, indicated by 

 the changing vapour-densities and made visible by changes 

 of colour, is revealed in a striking way by the large 

 absorption of heat which accompanies it. To raise the 

 temperature of 92 grams of the gas through i C. requires 

 (Berthelot and Ogier, Ann. Chim. Phys., 1883 [v], 30, 392) 



Between 27 and 67 C 747 calories 



67 ,, 103 57-0 



- 103 150 27-0 



,, 150 ,, 198 9-1 



As the heat-capacity of a gas usually increases with rising 

 temperature, the remarkable absorption of heat in the lower 

 ranges must be due to a chemical change : a direct com- 

 parison has shown that the heat-capacity is greatest just 

 when the degree of dissociation (as calculated from the 

 vapour-densities) is increasing most rapidly and that the two 

 changes run parallel throughout ; thus the percentage of 

 dissociation occurring over three consecutive ranges of tem- 

 perature were calculated to be as follows (Berthelot and 

 Ogier, loc. at. p. 398) : 



From vapour density. From heat capacity. 



27-70 C. 35 -6% 27-67 C. 40-0% 



70-100 23-6% 67-103 26-3% 



100-136 9-5% 103-150 13-2% 



Deville (1867) does not accept the dissociation of sal- 

 ammoniac, acetic acid, phosphorus pentachloride, and 

 nitrogen peroxide. It is remarkable that Deville found 

 himself unable to accept the view that sal-ammoniac vapour 

 contained free ammonia and free hydrogen chloride : these, 

 he held, were formed from the undecomposed vapour by 

 the process of diffusion. Further, he did not consider that 

 dissociation was responsible for the change of vapour-density 

 with temperature, and concluded that : 



