204 ACTION OP CERTAIN CHEMICAL REAGENTS. 
necessary, as otherwise the reagent will not indicate the pre- 
sence of even l-1000dth magnesia in solution. This is pro- 
bably the reason that M. Roth fixes the delicacy of this rea- 
gent at l-4000dth of magnesia. 
A solution of ammonia produced, after some minutes, a 
slight precipitate in a solution, containing l-6000dth mag- 
nesia. 
For Protoxide of Iron. — Tincture of galls, and a solution 
of ferro-prussiate of potassa, acidulated with a few drops of 
hydrochloric acid, indicated, after some minutes, the presence 
of protoxide of iron, in a solution containing l-440,000dth of 
crystallized sulphate of iron. 
For Peroxide of Iron. — Tincture of galls indicated the 
presence of l-300,000dth of peroxide of iron, in solution °f 
sulphate of peroxide of iron, by rendering it of a slight violet 
color. 
A solution of ferro-prussiate of potassa indicated the pre- 
sence of l-420,000dth of the same salt. 
For Copper. — A solution of ammonia gave, after several 
hours, a slight blue color to a solution of sulphate of copper, 
containing l-9400dth of oxide of copper, 
A solution of prussiate of potassa rendered the presence of 
l-7800dth of the same salt visible. 
Polished iron showed the presence of l-125,000dth of oxide 
of copper, or l-156,000dth of metallic copper, if the solution 
was acidulated by a drop of nitric acid. 
For Lead. — A piece of zinc precipitated lead from a so- 
lution of the nitrate, when l-3000dth of oxide was present. 
An excess of sulphuric acid occasioned a precipitate in a 
solution of the same salt, containing l-20,000dth of oxide. 
A solution of chromate of potassa occasioned a cloudiness 
in a solution containing l-70,000dth of the same oxide. 
A solution of hydrosulphuric acid, blackened a solution 
containing l-350,000dth. 
For Silver. — Chromate of potassa produced a slight red pre- 
cipitate in a solution of nitrate of silver containing l-10,000dth 
