118 
ON THE PROTO-IODIDE OF IRON, &C. 
Remark.-^-As soon as the combination begins to be effected, 
the proto-iodide of iron formed completely dissolves the 
iodine, and the liquid becomes of a reddish brown. If we 
continue for some minutes to heat the solution, and shake the 
mixture, the brown color disappears, which indicates that 
the iodine and iron is completely combined. We may ac- 
quire an absolute certainty by filtering the liquid, which 
should be perfectly colorless, or at least have an almost im- 
perceptible greenish hue. When the liquor retains a red 
color, or only slight yellow, or even a deep green, it is in- 
dispensable' to replace the solution on the iron, and retain it 
in contact until, on filtering, the solution shall pass without 
color. 
The quantity of iron indicated in the formula, far exceeds 
what is necessary to saturate the iodine, since the proto-iodide 
of iron is composed of 
1 atom iron, or 17.8 
2 " iodine, or 82.2 
100.0 
But this is of no consequence, as the iodine can dissolve 
only a limited proportion of the metal. My object in employ- 
ing a great excess of iron, is to render the combination more 
rapid, and render certain in every case the complete satura- 
tion of the iodine, even in inexperienced hands. Expe- 
rience has demonstrated, that even when equal parts of 
iodine and of iron are used, it is difficult to obtain a perfectly 
colorless solution. It is then more frequently of a deep 
rate in this manner, we should proceed as follows: shake the mixture, 
then allow it to stand for some minutes; the iodine then reacts upon the 
iron, the vessel becomes strongly heated and combination takes place. 
We may facilitate this by two or three agitations, when the mixture has 
reached its highest temperature. On the contrary, if we constantly shake 
the mixture from the commencement, the mixture does not attain the 
same degree of heat, and the combination is longer in taking place. 
