1. The lead exists in some unknown state of combination in solution in water. We are greatly indebted to Dr. Lambe[[390]] for the able directions which he has afforded us for ascertaining the presence of minute portions of lead in water; and we recommend the practitioner, who may be engaged in such an investigation, to peruse his work with attention. The following are the reagents through which our analysis must be conducted.
(a) Sulphuretted hydrogen. A solution of this gas in distilled water is a very delicate test for lead, throwing down a precipitate of a very dark brown colour, approaching to black. The competency, however, of this test to the discovery of very minute quantities of lead, in certain states of combination, has been questioned by Dr. Lambe; who was enabled to detect the presence of this metal, by other methods, in water that manifested no indication with sulphuretted hydrogen. He detected it, for instance, in the precipitate occasioned in such water by the carbonate of potass or soda. In operating on these waters, he noticed the following appearances.
1. The precipitate, produced as above stated, when re-dissolved in nitric acid, formed a dark cloud with sulphuretted hydrogen.
2. Although the sulphuretted hydrogen formed no cloud, the precipitate itself became darkened by it.
3. The precipitate re-dissolved in nitric acid, (as in 1) formed, with sulphuretted hydrogen, a white cloud.
4. Sulphuretted hydrogen neither formed a cloud, nor darkened the precipitate.
5. In the cases 2, 3, 4, if the precipitate be heated to redness, in contact with an alkaline carbonate; and after dissolving out the carbonate, it be redissolved in nitric acid; then sulphuretted hydrogen will form a dark cloud with the solution. In these experiments it is necessary that the acid used to redissolve the precipitate be not in excess; if it should so happen, the excess must be saturated, before the test is applied. It is better to use so little, that some precipitate may remain undissolved. The nitric acid, used in these experiments, should be perfectly pure; and the sulphuretted hydrogen test should be recently prepared by saturating distilled water with the gas.
(b) Sulphate of soda, or potass. This test will produce a white precipitate in water, containing one hundred-thousandth of its weight of lead; and is considered by Dr. Thomson as the most unequivocal reagent of that metal which we possess. “The precipitate is a fine dense powder, which speedily falls to the bottom, and is not re-dissolved by nitric acid; no other precipitate can be confounded with it, except sulphate of baryta, and there is no chance of the presence of baryta in solution in water.”[[391]]
(c) Muriate of soda. One of the methods of analysis proposed by Dr. Lambe, consists in precipitating the lead by common salt; but as the muriate of lead is partly soluble in water, this test cannot be applied to small portions of suspected water. The precipitate must, therefore, be collected from two or three gallons, and heated to redness with twice its weight of carbonate of soda. The alkaline carbonate is then to be dissolved out, and nitric acid added, in order to saturate any superfluity; the sulphuretted hydrogen test will then produce its indication.
(d) Reduction of the metal. This is undoubtedly the most satisfactory of all the tests; and, except the trouble of collecting a large quantity of precipitate, is not embarrassed with any difficulty. The precipitate may be mixed with its own weight of alkaline carbonate, and exposed either with, or without, the addition of a small proportion of charcoal, to a heat sufficient to melt the alkali. On breaking the crucible, a small globule of lead will be found reduced at the bottom. The precipitate from about fifty gallons of water yielded Dr. Lambe about two grains of lead.