236 
N0R1>HAUSEN SULPHURIC ACID AS A REAGENT. 
ART, XXXIV. — ON SOME INCONVENIENCES WHICtI MAY 
OCCUR FROM THE USE OF THE SULPHURIC ACID OF 
SAXONY, OR OF NORDHAUSEN, AS A REAGENT. 
By M. A. Ddpasq&ier. 
In testing for minute traces of iodine in any liquid, sul- 
phuric acid affords the most sensible results by its addition to 
the liquid, with which a small quantity of a solution of starch 
has been previously mixed; at least this is the result of com- 
parative trials which I have made, and which have demon- 
strated that, to set free the iodine, sulphuric acid is preferable 
to the use of chlorine, the chlorides of the oxides, aqua regia, 
the voltaic pile, &c. 
Since making these experiments, chance has informed me 
that it is not a matter of indifference, in the detection of 
iodine, what kind of sulphuric acid is used, and for the 
following reason: In one of my latter lessons at the School 
of Medicine I wished to demonstrate the sensibility of this 
reagent, and was surprised at not obtaining the violet blue 
color as I expected. At the moment when I poured the acid 
into the liquid a faint violet tint was produced, but imme- 
diately disappeared. 
On investigating the cause of this unexpected result, I 
learnt that my assistant, not having the ordinary acid, had 
given me a bottle of that of Nordhausen. From this expla- 
nation i immediately concluded, that the non-coloration of 
the starch was due to the sulphurous acid, which is com- 
monly present in sulphuric acid, obtained by the distillation 
of sulphate of iron. In fact, we know that free iodine, in 
contact with a solution of sulphurous acid or a sulphite, im- 
mediately disappears, passing to the state of hydriodic acid. 
The following experiments have rendered certain this expla- 
nation of the non-coloration of starch. 
1. I poured ordinary sulphuric acid into a glass of water 
