NORDHAUSEN SULPHURIC ACID AS A REAGENT. 237 
to which one drop of a solution of iodide of potassium and 
some solution of starch had been previously added; the liquid 
immediately assumed a violet blue color, 
2. The same experiment was repeated, some drops of a 
solution of sulphurous acid being added before the sulphuric ; 
the color was not manifested, even on the addition of a large 
excess of this latter acid. 
3. The same experiment repeated, some drops of a solu- 
tion of sulphite of soda being substituted for the sulphurous 
acid, and with the same result; the liquid remained unco- 
lored. 
4. The same experiment with a large proportion of iodide 
of potassium ; the same result, only at the moment when the 
sulphuric acid was added, faint traces of violet appeared, but 
immediately faded away. 
From these experiments we must conclude, that the presence 
of sulphurous acid in the sulphuric acid of Nordhausen, is 
the true cause of the non-coloration of the starch, when this 
acid is used to detect the presence of an iodide in a liquid. 
This influence of sulphurous acid on iodine gives a satis- 
factory explanation of the following fact, which is analogous 
to that just pointed out. 
In my chemical experiments, made during five or six 
years, on the employment of protiodide of iron, I have taken 
care to direct that the urine of the patient should be pre- 
served, to ascertain, by a chemical test, whether this remedy 
had been really taken. The test consisted in diluting the 
urine with an equal bulk of water, then adding a little solu- 
tion of starch, and finally some sulphuric acid. The liquid 
immediately assumes a blue color, whenever the patient had 
taken the iodide of iron, a short time before voiding the 
urine. The addition of water is indispensable; for the color 
is not manifested, or is hardly sensible, when the urine is very 
much charged with, or is not rendered more clear, or less abun- 
dant in organic matter, by means of this liquid. Is it not 
evident that the non-coloration of the amidon, under this cir- 
cumstance, of a highly charged urine, is due to the formation 
