454 M. H. Dcvillc on the Decomposition of Bodies by Heat, 



When a liquid is boiled, there remains in the vapour no trace 

 of the properties which characterize cohesion, and the quantity 

 of latent heat absorbed during the passage of a gas into a gaseous 

 state is often considerable. Similarly, when by the simple action 

 of a high temperature a gas has been decomposed into its ele- 

 ments (this temperature is evidently higher than that which 

 these elements develope during their combination), there remains 

 in the mixture no trace of this effect of the force of affinity which 

 constitutes the stability of a compound ; and as the gaseous volume 

 generally increases at the moment of the separation of the ele- 

 ments, there ought to be an absorption of a certain quantity of 

 latent heat of decomposition. 



Thus water melts at 0°, and its volume varies little; it boils 

 at 100°, and its volume increases considerably ; it dissociates at 

 1000° without any material deviation in its volume from Gay- 

 Lussac's law of expansion ; it decomposes above 2500°, and its 

 volume suddenly increases in the ratio of 2 to 3. 



In other words, at 0° the cohesion of water approaches its 

 maximum j above 0° it is nil, or almost nil ; at 100° it is ne- 

 gative. At 100° the stability of aqueous vapour is at its maxi- 

 mum up to 1000°, a temperature at which it almost disappears, 

 and suddenly, in dissociated vapour ; lastly, above 2500° it is 

 negative in decomposed vapour : the constituent molecules then 

 repel each other as if they were of the same kind. 



If heat acts by expansion a3 well on the .constituent as on the 

 integrant molecules of compound bodies, we see that stability, 

 like cohesion in solid bodies, may exist in gases with a positive 

 value, which is the condition of combinations at temperatures 

 at which we usually work ; that it may exist with a volume almost 

 nil, like cohesion in liquids (this is the state of dissociation, con- 

 firmed in the case of water, potass, soda, chloride of magnesium 

 at temperatures of 1000° to 1500°); that, lastly, stability may 

 become negative, like cohesion in gases, in bodies entirely decom- 

 posed by heat, like anhydrous carbonate of ammonia at 60°, 

 ammonia at a red heat, water at 2500°. 



This conception may be rendered palpable by assuming that in 

 gaseous compounds in, a stable state the constituent molecules 

 are solid; that in dissociated bodies the molecules are liquid*; 

 lastly, that in decomposed bodies the molecules are endowed with 

 an indefinite mobility — in other words, the molecules themselves 

 are gaseous. 



* On this hypothesis, the constituent molecule might have a tension 

 varying with the temperature, and which is greatest at the temperature of de- 

 composition. The quantities of water decomposed in the experiments of 

 Mr. Grove and of myself, during the state of dissociation, would be the quan- 

 tities of uncondensed vapour corresponding to this state of tension. 



