H. Wurtz on the Indigo Test. 57 
when any interfering substance is present, as above recom- 
mended. If neutral, chlorid of sodium is added, instead of 
_ muriatic acid, to avoid dilution. If carbonate of soda has been 
used to precipitate iron, baryta, etc., it is better to 
trate somewhat before acidulating with chlorohydrie acid. In- 
digo solution (which should also be strong) is then mt until 
Jrom the operator on a piece of white paper cated. 
The mixture is then warmed, and concentrate oil of vitriol added 
cautiously (to avoid-explosive ebullition) with constant agitation. 
If but traces of nitric acid be present the bleaching ma 
e place for some minutes, or until a Spe pcp quantity of 
oil of vitriol has been added. If the oil of vitriol be free from 
otherwise pure. After the bleaching eo has once been reached, 
if a fresh portion of pre solution be added, it will seneeally 
must be concentrated after addition ot excess of carbonate of 
soda. The function of the sulphuric acid is not merely, as ap- 
peared to me at first, to appropriate the water present, and pro- 
duce a quasi-concentration, for chlorid of calcium cannot be suc- 
ostlifcy substituted for it It seems probable cae in a ms 
ussac’s 
* Before = the subject I must present some further nc Agee on whic hich - 
I cannot but hope to found a method. of ones for nite acd acid in pred ina ‘ 
T find that oxyd really even at 
reduces fe 
the ordinary rature, wwhilet nitric a (ree fom NO® NO®*) unless very concen- _ 
trated, does not reoxydize the protosalt. cate well known that it in 
i no ee Aecbey S Water — 
i- as in dilute sulphuric acid, and ferrous nitrate is Aas sper Tf then ines could = 
contrived to obviate wholly the interference of atmospheric oxygen during the 
operation of jiltration, it would be very easy, by eS the the liquid ¢ to be tested 
with chlorohydric acid, adding a little protosulphate 
cold, filtering, boiling the filtrate (during which last ped en ners 
acids, 
ekCONs BektE Ve. Lei No. 76.—JULY, eek. 
8 
