In place of nitrate of baryta, the nitrate of lead may be used (sulphate of lead being an insoluble salt), but the quantity required will be different. The atomic weights of nitrate of baryta and nitrate of lead are as 131 to 166; consequently 300 grains of the former are equivalent to 380 grains of the latter.
Iron, Perchloride of.
Symbol, Fe{2}Cl{3}. Atomic weight, 164.
There are two chlorides of iron, corresponding in composition to the protoxide and the sesquioxide respectively. The protochloride is very soluble in water, forming a green solution, which precipitates a dirty white protoxide on the addition of an alkali. The perchloride, on the other hand, is dark brown, and gives a foxy-red precipitate with alkalies.
Properties.—Perchloride of iron may be obtained in the solid form by heating iron wire in excess of chlorine; it condenses in the shape of brilliant and iridescent brown crystals, which are volatile, and dissolve in water, the solution being acid to test-paper. It is also soluble in alcohol, forming the tinctura ferri sesquichloridi of the Pharmacopœia. Commercial perchloride of iron ordinarily contains an excess of hydrochloric acid.
Litmus.
Litmus is a vegetable substance, prepared from various lichens, which are principally collected on rocks adjoining the sea. The coloring matter is extracted by a peculiar process, and afterwards made up into a paste with chalk, plaster of Paris, &c.
Litmus occurs in commerce in the form of small cubes, of a fine violet color. In using it for the preparation of test-papers, it is digested in hot water, and sheets of porous paper are soaked in the blue liquid so formed. The red papers are prepared at first in the same manner, but afterwards placed in water which has been rendered faintly acid with sulphuric or hydrochloric acid.