Metallic Compounds of Chlorine, Bromine, and Iodine. 395 



of Zn CI, from the state of aggregation in which they exist, as 

 gaseous chlorine and metallic zinc, to that state in which they exist 

 in the dry chloride of zinc, 

 X =. heat due to the union of zinc and chlorine, 

 we shall have the following general equation : 



:r = A — B ± X. 



And, designating the corresponding values for bromine by a', b', x', af, and for 

 iodine by a", b", x", ar", we shall have 



0/ = a' - b' ± x', 



y'=A"-B"±x". 



5. The class of metals forming more than one compound with chlorine, bro- 

 mine, and iodine is very numerous ; but none of them present the same facilities 

 for this investigation as iron, to which accordingly I propose to confine my atten- 

 tion in the present paper. It is usually stated in chemical works that when 

 chlorine, bromine, or iodine act upon an excess of iron filings, suspended in water, 

 a solution of protochloride, protobromide, or protoiodide of iron is formed. But 

 such a description gives a very imperfect idea of the successive series of pheno- 

 mena which actually take place. We have only, indeed, to watch carefully the 

 progress of the experiment, in order to discover that a sesquicompound (Fe^Clj, 

 Fe^ Brj, Fej I3) is formed in the first instance, which afterwards, by combining 

 with an additional atom of iron, becomes converted into the protocompound 

 (FcaCla-j-Fe, &c.) To prove this, we only require to filter the liquid be- 

 fore the reaction has terminated, when a red solution will be obtained, having 

 all the properties of a solution of a sesquisalt of iron, and yielding by evaporation 

 a red deliquescent mass. Whether the experiment be made with chlorine, bro- 

 mine, or iodine, the same results will be obtained. An elegant illustration of a 

 similar train of changes is afiPorded by the action of chlorine gas on metallic tin. 

 If we agitate an excess of tin filings with a little water in a glass vessel of chlorine 

 gas, till the colour of the gas has scarcely disappeared, and instantly filter, the 

 liquid which passes through will produce only a faint opalescence, when dropped 

 into a solution of the bichloride of mercury ; but if the agitation be continued for 

 only a few seconds after the disappearance of the chlorine, the filtered liquid will 

 give a dense curdy precipitate when added to the same solution. 



3 E 2 



