Detection of Sulphurous Acid, fyc. 223 
simples et composes, (page 17) consists in saturating the hy- 
drochloric acid with barytes water, after having diluted it 
with three or four times its weight of distilled water. A white 
precipitate of sulphite and sulphate of barytes is formed, 
which washed several times to separate the chloride of ba- 
rium, and then sprinkled with concentrated sulphuric acid, 
exhales the odour of sulphurous acid. Independently of the 
time and the manipulations required by this plan, which are 
of themselves sufficient to prevent its use, it has another se- 
rious defect; which is that it requires a great sensibility of 
the organ of smell, which is seldom found among manufac- 
turing chemists. 
Another mode has been proposed by M. Chevreul, in his Le- 
cons de chimie appliquee a la teinture (xi. Lecon, page 15.) 
This learned chemist has ascertained that by adding a solu- 
tion of sulphite of potash to a salt of the deutoxide of cop- 
per, a yellow precipitate is formed, consisting of the double 
sulphite of potash, and protoxide of copper; and that if this 
precipitate be heated with water it is decomposed into sulphite 
of potash which dissolves, and sulphite of the protoxide of 
copper, which is insoluble, and of a red colour. From this 
fact, M. Chevreul concluded, that when a hydrochloric acid 
contained a notable quantity of sulphurous acid, that it might 
be verified, by saturating the former with potash, and adding 
a solution of sulphate of copper, thus producing a yellow pre- 
cipitate, which would become red by ebullition. But these 
theoretical views are not confirmed in practice. In fact, the 
plan of M. Chevreul, though well calculated to distinguish sul- 
phurous acid when free or combined with bases, does not an- 
swer when this acid is mixed with the hydrochloric. We have 
often applied this process to hydrochloric acids surcharged 
with sulphurous acid, and have never obtained the reaction 
indicated by M. Chevreul. The addition of the sulphate, or 
any other salt of copper to these acids, neutralised by potash, 
does not occasion any precipitate, and when they are concen- 
trated, it only produces a light bluish deposit which does not 
change on ebullition. 
Gay Lussac was the first to recommend (in 1813, Ann. de 
