PHARMACEUTICAL NOTICES. 
279 
a preference to the decision of experience alone. The prin- 
cipal one is that of mutual decomposition between two solu- 
tions, formed of nitrate of protoxide of mercury, on the one 
part, and iodide of potassium on the other. 
Unless he be acquainted with the reaction which takes place 
between these two substances, according to their chemical com- 
position, the operator will be surprised to find his precipitates 
of a different hue from the one he wishes, which will oblige 
him to go on decomposing and re-composing till he has suc- 
ceeded. But this difficulty will be avoided upon learning 
that the influence of the acid, in his mercurial solution, governs 
the product. If the solution be made with a neutral salt, he will 
have a green precipitate ; if the acid predominates, a yellow 
one, and, if still more acid, a scarlet precipitate; but, if in 
great excess, this last will be blended with some free iodine. 
By pursuing the following directions, the operator will be 
repaid with an abundant precipitate of a rich yellow color, 
which will be pure sesquiodide of mercury. 
Take an excess of purified mercury; pour upon it a certain 
quantity of pure nitric acid, and dissolve without the assistance 
of heat. Separate from the mercury the crystals obtained, 
which dissolve in a small quantity of water, with the addi- 
tion of a little nitric acid. On the other hand, make a con- 
centrated solution of ioduretted hydriodate of potash, (one 
part iodine and two of the salt, with about as much water as 
makes the Lugol's Solution.) This being done, mix the 
two solutions rapidly together. The sesquiodide will be in- 
stantly formed in a state of almost purity. Throw upon a 
filter to separate the precipitate, and wash well with pure 
water. Test the water afterwards to ascertain if any excess 
of nitrate of mercury, or iodide solution remains. Upon the 
recurrence of this preparation I shall take a note of the weight 
of substances in order to facilitate future practical operations. 
Syrup of Digitalis. 
Take of European digitalis leaves, §iv. 
Water, q.s. 
Sugar, lib. 
