“2 1 Wert on the Indigo Test. 53 
_quichlorid.* Excess of chlorohydrie acid does not affect the 
result. A solution of pure ferric sulphate made by evaporatin 
_ the sublimed sesquichlorid with excess of pure sulphuric aci 
until free from chlorine, also bleached indigo powerfully ; 
nd 
pom a small excess of indigo was added so that the liquid had 
reeptibly bluish tint after long boiling and cooling, crystals 
of pure chlorid of sodium being added produced no further 
change after renewed ebullition, thus showing that the bleach- 
ing oe hei of the sesquisulphate is little, if any, inferior to that 
e sesquichlorid. Red aR indicated also in this case 
a etm to ferrous sul 
When iron then is ested in a liquid to be tested by indigo, 
it is necessary to precipitate it first, and test in the filtrate after 
addition of excess of acid. For this precipitation I have recom- 
mended in a previous paper the use of ammonia, but since penn 
was written the idea has occurred that the presence of amm 
acal salts may possibly interfere in some way with the feaitions 
although it may be difficult to understand how. An experi riment 
however, in a whic two = trials were made, baneidecs 
nitric acid, gave a solution which bl mda tn very See 
when. boiled Ais it, gold being reduced and deposi ot 
as a metallic mirror upon the sides, and partly as a black pow- 
der at the bottom, of the test tube. When boiled with a lar; 
excess of indigo so as to remain still blue, then filtered and 
blue color destroyed by nitric acid, the liquid was not affected 
es oe acid, and no longer contained therefore any 
~ §0 
Bichlorid of platinum, freed in the same way from all nitric 
: acid, bleached indigo powerfully, with production of a very deep 
* It was observed that such a solution, after exposure to the light for — weeks 
an observation of Osann to the same 
effect (Lieb. and spe prone: 1855, p. 206). Such a solution must thet be 
kept in the dark. I may rem ark here also that it is advisable not to Jilter such a 
i — i 
x = pa 
+ It may be added here that some other vegetable colors were tried: such as acid 
solutions of litmus, logwood and cochineal, and found to be likewise readily decolor- 
sesquichlorid of iron. 
