Metallic Compounds of Chlorine, Bromine, and Iodine. 407 



35. It must be remembered that each of the letters x, x', &c. represents two 

 unknown quantities ; first, the change of temperature due to the alteration of 

 aggregation of the particles of the metallic elements, in passing from their ordi- 

 nary form to that form in which they exist in the dry salt ; and, secondly, the 

 change of temperature arising from the like alteration of aggregation of the par- 

 ticles of the electro-negative element. The actual value of these quantities can- 

 not be determined by direct experiments, but it is probable that for the combi- 

 nations of the same metal, the differences between x, x', and x", and between 

 Y, \', and y" will arise chiefly from the alterations of aggregation of the electro- 

 negative, and not of the metallic element. Now, as the heat arising from the 

 condensation of chlorine from the gaseous to what may perhaps be termed tlie 

 saline solid state, must be far greater than that arising from the change of fluid 

 bromine, or solid iodine, to the same state, it would be an object of great interest 

 to determine the heat evolved or abstracted during the changes of these bodies 

 from one physical condition to another, which would enable us to compare the 

 heat of combination of each body in the same physical state. This I have only 

 attempted yet to effect for the case of the solidification of bromine; and, as the re- 

 sult of a very imperfect experiment, it may be stated, that the heat evolved during 

 the passage of that substance from the fluid to the solid state, would be sufficient 

 to raise an equal weight of water through 24°. This amount of heat is evidently 

 far too small to account for the differences observed in the values of x' and oc" , 

 and ofy andy ; from which it follows, that bromine and iodine, in the same 

 physical state, evolve very different quantities of heat when combining with the 

 metals. 



36. On comparing the numbers deduced from the foregoing experiments 

 (28, 29j 30) for the heat developed during the conversion of the sesqul-com- 

 pounds of iron into the corresponding proto-compounds, by combining with half 

 as much iron as they already contain, the very interesting general principle re- 

 sults, that, referred to the combining iron as unit, the heat evolved in all these 

 cases of combination is the same. In fact, we have 



. Fe^ClaAq-f Fe = 788°. 

 Fe,Br3Aq + Fe = 794°. 

 Fe, L Aq -f Fe = 783°. 



