196 Petit and Dulong on some important Points [Sept, 



It would be very difficult to reconcile these facts with the ideas 

 generally received respecting the production of heat in chemical 

 phenomena ; for in order to do so, it would be necessary to 

 admit the very improbable supposition that heat exists in 

 bodies in two very different states, and that the portion which 

 We consider as united to the particles of matter is entirely inde- 

 pendent of the specific heats. Besides, there is so much vague- 

 ness and incoherence in the explanations relative to the kind of 

 phenomena of which we speak. There exist with respect to 

 them opinions so different that they cannot be subjected to a 

 regular discussion, nor exposed to a complete refutation. But, 

 perhaps, it will not be useless to recall in a few words the prin- 

 cipal facts and the inductions belonging to this important part 

 of science. 



Of all the chemical actions considered as sources of heat, none 

 has been recognized till very lately, except combustion. It would 

 be useless to look for a plausible theory for this mode of the 

 production of heat before the epoch marked by the memorable 

 discoveries of Lavoisier. This illustrious chemist having more 

 particularly studied the action of oxygen in the state of gas, he 

 formed an opinion respecting the cause of the phenomenon in 

 question naturally suggested by the observations of Black on 

 latent heat. Hence the idea that the heat disengaged during 

 combustion comes from the change of state of the oxygen. The 

 determination which he made, together with M. Laplace, of the 

 quantities of heat disengaged by the combustion of several sub- 

 stances appeared to furnish a powerful argument in favour of his 

 conjectures. Experiment showed that when the same quantity 

 of oxygen was united successively with phosphorus, hydrogen 

 and carbon, it disengaged more heat in the first case than in the 

 second, and more in the second than in the third. This was 

 what might have been concluded from the theory, since the 

 result of the first combustion is solid, that of the second liquid, 

 and that of the third gaseous. But on considering that the two 

 elements which concur to form water lose both the gaseous state, 

 and that notwithstanding the heat developed is less than what 

 results from the combustion of phosphorus naturally solid, it 

 was necessary to conclude that the latent heat of oxygen must 

 be superior to that of the other elastic fluids. Another difficulty 

 soon after presented itself. Nitric acid in which the oxygen has 

 already lost the form of an elastic fluid, and still more nitre, 

 which is in a solid state, produce, when decomposed by combus- 

 tibles, quantities of heat very little different from that which 

 would be produced by a weight of gaseous oxygen equal to that 

 which they contain. This observation, which ought to have 

 excited doubts respecting the primitive explanation, only 

 restricted its generality. It was then supposed that in certain 

 combinations the oxygen was capable of retaining a dose of heat 

 almost as great as that which it contains when in the elastic 



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