110 
MEANS FOR DETERMINING THE PURITY OF DRUGS. ETC. 
has been present, probably starch or gum. To decide, treat an- 
other portion of the suspected calomel, with hot water, and test 
with solution of iodine and sub-acetate of lead — the former will 
produce a blue coloration, with starch ; whilst the latter will 
cause a white precipitate with gum. 
Sulphate of Zinc. — White vitriol is in colorless and translucent 
crystals of varying size, having the form of four-sided prisms. 
Slightly efflorescent, but losing six-sevenths of their water of crys- 
tallization at 212° ; readily soluble in water,but insoluble in alco- 
hol. The caustic alkalies and carbonate of ammonia produce in a 
solution of sulphate of zinc white precipitates, soluble in an ex- 
cess of precipitant. The impurities found in sulphate of zinc, are 
iron, copper, cadmium and arsenic. The two former may be de- 
tected by adding, to a solution of the salt, a solution of ammonia 
in excess, w T hen the presence of iron will be indicated by dark 
or red flocculi, and of copper by the smalt blue color of the solu- 
tion. Cadmium and arsenic, by acidulating the solution with 
sulphuric acid, and passing a stream of sulphuretted hydrogen 
through it, when if either metal be present, it is deposited as a 
yellow sulphuret. 
From these it may be purified by immersing a clean plate of 
zinc in its solution, and exposing it to the action of air until it 
ceases to deposit a yellowish-brown sediment. Sulphate of magne- 
sia from similarity of form, may be used as an adulteration. It 
may be recognized by an excess of caustic alkali not redissolving 
the precipitate which it produces in its solution. 
Sulphate of Magnesia. — This salt occurs in small acicular co- 
lorless crystals, which effloresce on exposure to air. Its taste is 
saline, bitter and nauseous. It is soluble in its own weight of 
cold water, but insoluble in alcohol. The alkalies and their car- 
bonates produce white gelatinous precipitates in solution of sul- 
phate of magnesia, insoluble in excess of the precipitant. The 
most common impurity found in epsom salt is iron. This may be 
detected by ferrocyanuret of iron producing a greenish or blue 
color. When, however, the iron is in the state of protoxide, or 
in small amount, the coloration is not immediately evident, but 
will appear gradually on exposure to action of the air; the quan- 
