396 Dr. Andrews on the Heat developed during the Formation of the 



6. From these observations It follows, that the primary form of combination, 

 into which the molecules of chlorine, bromine, and iodine enter with iron, is that 

 represented by the formulas Fcj CI3, Ye^ Br 3, Fe^ I3, and that the so-called proto- 

 compounds are, in reality, secondary combinations, formed by the union of the 

 sesquicompounds with an additional atom of iron (Fcj CI3 + Fe, &c.). This con- 

 clusion is farther confirmed by the well-known fact, that when these substances 

 unite at elevated temperatures, the red or sesquicompounds are always formed.* 



7. Let us now make 



c =z heat evolved during the reaction of chlorine, iron (in excess), and 



water. 

 D = heat evolved during the solution of Fcj CI3 in a similar proportion 



of water. 

 E = heat evolved during the combination of Ye^ CI3 in solution with Fe. 

 Y = heat evolved or absorbed during the change of aggregation of the 



constituents of Fe^ CI3. 

 y =: heat due to the union of Fej with CI3. 

 Let us also, as before, represent the corresponding values for bromine by c', d', 

 e', y', y, and for iodine by c", d", e", y",/'. The following equations will 

 then give the values oi y, y', andy. 



8. Having thus endeavoured to lay down general formulas for the heat of 

 combination, I proceed to describe the experiments by which the values of a, b, c, 

 &c. have been determined. 



9. The apparatus employed in these experiments consisted of several distinct 

 parts. The combination was effected in a thin glass vessel of the form repre- 

 sented in fig. 1. When chlorine was the subject of experiment, this vessel was 



* If the view, which regards Fe CI as the primary form of combination, be preferred, it will be 

 necessary to suppose that three successive changes occur, — first, the formation of the compound 

 Fe, + Clj ; secondly, its conversion intc Fe, CI3 by combining with CI ; and thirdly, the reconver- 

 sion of the latter into Fej CI3 by its union with Fe. 



