These tests are amply sufficient for determining the presence of lead in a solution, provided they act characteristically. Others have been also used, however; and it is therefore right to notice them cursorily.
The alkaline carbonates throw down a white precipitate in a very diluted solution of lead. This test is ineligible, because the alkaline carbonates cause a white precipitate with many other salts. It might be rendered decisive, however, by washing the precipitate thoroughly, suspending it in pure water and transmitting sulphuretted-hydrogen, which blackens it. No other white carbonate is similarly altered except those of bismuth and silver, which are rare. The soluble sulphates likewise cause with solutions of lead a white precipitate, the sulphate of lead. To this test the same objections apply as to the carbonates of the alkalis. The ferrocyanate of potash causes a white precipitate, the ferrocyanate of lead. This is an objectionable test, as many other substances besides lead are similarly acted on by it. 4. Goulard's Extract.
Goulard's extract, the diacetate of lead, is easily distinguished from the acetate or sugar of lead by the effect of a stream of carbonic acid, which throws down a copious precipitate of carbonate of lead. The proper method of analyzing it is to transmit this gas till it ceases to act any longer, and then to subject the precipitate and solution to the tests for carbonate of lead, and acetate of lead. Solutions of the common acetate usually give a scanty white precipitate with carbonic acid, in consequence of containing a faint excess of oxide.
The presence of vegetable or animal matters may either decompose the salts of lead, or materially alter the action of the preceding reagents.
It appears from the experiments of Orfila, that most vegetable infusions possess the power of decomposing them more or less. The acetate furnishes, for example, an abundant precipitate with infusion of galls, or with infusion of tea. Almost all animal fluids, with the exception of gelatin, possess the same property; albumen, milk, bile, beef-tea, all give with it a copious precipitate. In fluids which do not decompose it altogether, the colour of the precipitate formed by the tests is so materially altered, that they cannot be relied on for the detection of lead. The test, however, which undergoes least alteration is hydrosulphuric acid.
Before proceeding to the detection of lead in complex organic mixtures, some remarks will be required on its relations to medical police. Here the various ways in which it is apt to be insidiously introduced into the body, chiefly by the action of chemical agents on metallic lead itself, will come under consideration.
Of the Action of Air and Pure Water on Lead.
When lead is exposed to the air it becomes tarnished. This
