86 Notes on Chemistry, &c. 
A scale of the salt introduced into a drop of the acid no larger than 
a pin’s head will show the effect distinctly. 
The acids in which I have found the salt to redden are, sulphuric, 
hydrochloric, hydrofluoric, chromic, phosphoric, (if slightly diluted,) 
and the common vegetable acids, such as oxalic, tartaric, citric and 
acetic acids. 
I have found it to blacken with chlorine gas, solution of chlorine, 
(recently prepared,) bromine, sulphuretted hydrogen, nitrous acid 
vapors, and nitric acid. 
It is highly probable, that it would be blackened by bromic acid 
and chloric acid, and possibly by iodic acid, but I have not at pres- 
ent these acids ina free state to determine their action; the method, 
however, in which I use the test will prevent any fallacy from the 
presence of chloric, bromic, iodic or chromic acids, and of sulphuret- 
ted hydrogen. It is to evaporate the supposed nitrate to dryness, 
and introduce into a tube retort A, (see the figure,) a small portion 
of the salt, on which a few drops of sulphuric acid are to be poured ; 
then on applying moderate heat, by means of a spirit lamp, a portion 
of the volatile products are to be driven over into the receiver B, in 
which a few scales of the salt are previously placed. If these are 
blackened, the salt is to be considered as a nitrate, provided the pres- 
ence of those few substances which might cause the same result has 
been guarded against. Now by the very method proposed, viz. 
evaporating to dryness and adding sulphuric acid, the presence or 
absence of chromic, chloric or iodic acid and sulphuretted hydrogen 
will be determined ; for the color of a chromate, the evolution of per- 
oxide of chlorine from a chlorate, the liberation of iodine from an 
jodate, and the odor from a sulphuret, will at once decide with re- 
gard to each. As iodic and bromic acids, even if they are found to 
blacken the salt, are not sufficiently volatile to be driven over by the 
heat to be employed, no error could arise from their presence. 
I have observed, that if the salt used above, or the bi-iodide of 
mercury itself, be introduced into a test tube, with strong sulphuric 
