50 H. Wurtz on the Indigo Test. 
others, and certainly, when manipulated as there directed by 
Rose, Richemont’s test is scarcely surpassed in delicacy by any 
other ; and when the liquid to be tested is colorless, it is ex-_ 
tremely convenient; but unfortunately the color produced has — 
too little about it that is characteristic, and is therefore not cer- 
tainly recognizable in the presence of many other colors that 
are of very frequent occurrence. When the quantity of nitric 
acid is minute, this test requires also a very considerable length 
of time, an important objection. 
. The todine test, or method of Higgin,* dependent upon the 
setting free of the iodine of iodid of potassium by free nitric 
acid, and formation of the blue starch compound. When no 
other free acid than the nitric is present, this mode may be made 
very delicate and reliable, but when in preaesiaion, the nitric 
must be set free by the addition of sulphuric acid, and as Gay- 
Lussac long ago observed, sulphuric acid aan gives with a so- 
lution of iodohydric acid, or, which is the same ie with pas 
of potassium, f iodine, by the following reaction, SO*+H 
SO! 4 +HO+L+ Even in dilute solutions this eae a 
place after a time, and when but traces are to be tested for, it is 
cult to = certain whether the free iodine indicated is due to 
ni to this reaction. With great care, however, I be- 
tieve that this m method may in some cases be made useful, particu- 
larly when /ree nitric acid is to be tested for. 
Tn the presence of the least trace of a sesquisalt of iron, this 
method is especially fallacious, a fact which has I believe hereto- 
fore escaped attention. I find that a deep blue color is immedi- 
ome produced when but a minute trace of any such compound 
is present. In fact, the conclusion that I have drawn from ex- 
— is that a mixture of iodid of potassium and starch so- 
tions when used as a test for sesquioxyd of iron in solution, 
yields in delicacy and sensitiveness to no other mode now in 
use, and I wish therefore to prope it as a new method of de- 
tecting ane presence of small quantities of iron. The-reaction 
ace both in neutral and acid solutions; with sesquichlo- 
rid of i iron it is of course as follows, 
| Fe*Cl?+KI=2FeC1+KC1+1. 
Scheeffer’s test ; BE en esented to this Association by G. C. 
Schieffer at the New Haven meeting in 1851.t It depends upon 
leat of the nitric into nitrous acid by the ie of 
le then treating with yellow prussiate of potas 
a acid, when a deep golden yellow color is devels 
he eee indicating, according to Schee! 
arts of water. id howe 
