52 H. Wurtz on the Indigo Test. 
ture a solution of the sesquichlorid, standing for several days in 
contact with gold leaf, gave a green tinge with the red prussiate, — 
indicating reduction. An acid solution of the sesquichlorid is i 
partially reduced by digestion for a few hours on the sand- — 
bath with clippings of platinum foil. The solutions of gold are 
not affected by refrigeration, concentration, dilution or filtration. 
I cannot find that this action of gold and platinum upon ferric 
solutions has been announced before. If observed, it may have 
been attributed to traces of nitric acid or chlorine still remain- 
ing in the liquid, a cause of fallacy not existing when the sub- 
limed sesquichlorid is experimented with. Metallic silver, how- 
ever, has been observed to reduce ferric salts. Gmelin says, 
“metallic silver takes up oxsgen from a boiling aqueous solu- 
tion of the sulphate of sesquioxyd of iron so that sulphate of 
silver and sulphate of protoxyd of iron are formed; in the cold, 
metallic silver sie ares again and the Solution contains only 
sulphate of sesquioxyd of iron as before 
tis evident that this behavior of cold = entirely the 
gold test for nitric acid in the presence o 
6. The indigo test. This familiar ae ig needs no descrip- 
tion. When chlorohydric acid (or chlorid of sodium, as ea 
mended by Liebigt) is added, the bleaching is undoubtedly du 
to the production of free chlo orine, just as the solution of eel 
under the same circumstances was proved to be by Gay-Lussac.t 
= ne five methods gels ates mentioned depend evidently 
e production of a color not previously existing in the 
Tout, and which in many cases tay be produced by the pres- 
ence of other substances besides nitric acid, whilst the indigo 
test depends upon the destruction of a color previously existing, 
this color being at the same time a very characteristic one, sel- 
dom found in “solutions, and belonging to a very stable com- 
pound that is well known to be affected only by the most potent 
oxydizing agents. It seemed to my mind therefore that, other 
things being equal, the indigo test possesses elements of greater 
certainty than the others, and furnishes stronger positive evidence 
of the presence of nitric acid, though some others may give 
stronger proof of its absence, 'T set myself at work therefore to 
ae, the pro test, and ascertain the circumstances which 
