IMPURITIES IN MEDICINES, &C. 
107 
course, propose to confirm or deny the assertion, but I do un- 
hesitatingly affirm that the commercial sugar of lead is unfit 
for medical use; and hence I wish to impress upon those 
whose business it is to dispense prescriptions, that it is a mat- 
ter of duty with them to re-crystallize and purify even the 
best looking salt which they can buy; the operation is at- 
tended with little expense, and trouble should be no consider- 
ation with the faithful pharmaceutist, who is fully sensible of 
the responsibility involved in the exercise of his profession. 
Sulphate of Zinc. This is a highly useful member of the 
Materia Medica, and one which is as frequently employed as 
almost any other; very frequently for internal use. Some 
experience has satisfied me that it can seldom, if ever, be 
bought pure, even in its best looking condition. Tincture of 
galls will almost invariably show the presence of iron in it. 
Hence the pharmaceutist who would vend a pure article must 
either make it for himself, or purify the commercial salt; if he 
adopt the former plan he will obtain, from the formula of the 
United States Pharmacopoeia, a handsome, pure article, by 
careful crystallization, rejecting the latter portions of his mo- 
ther waters. The latter mode, that of purifying the salt of com- 
merce, is attended with more trouble, and is perhaps quite as 
expensive; the iron must be separated either by immersing 
sheets of zinc in the solution required to be purified, which 
after a long time will decompose the sulphate of iron and pre- 
cipitate its oxide; or the same object may be accomplished in 
less time by adding small portions of chloride of lime to the 
solution, which, by its reaction on the water of the solu- 
tion, is converted into muriate of lime, and allows the iron 
to become peroxidized by combining with the oxygen set 
at liberty when the muriatic acid is formed; the resulting mu- 
riate of lime remains dissolved in the last mother waters, 
which must be thrown away; the solution must be heated, 
and the chloride added in solution, so long as red oxide of iron 
falls; if any sulphate of lime be formed, being insoluble, it is 
separated when the solution is filtered preparatory to crystal- 
lization. 
