Houston[7], in an extensive and very full report on the effect of water upon lead, especially undertaken for the purpose of inquiry into the contamination of supplies of drinking water by means of lead, distinguishes two species of action--namely, plumbo-solvency, which is brought about by the acidity of the water in contact with lead; and a second kind of action, erosion, determined to some extent by the dissolved air in the water. He came to the conclusion that the plumbo-solvency and erosive action of water on metallic lead differed considerably, and that the protective layer or plumbo-protective substance did not always protect lead pipes from the solvent action of water. =Chemical Characters of Lead Salts.=--A short summary of the chemistry of lead salts may not be out of place. A soluble salt of lead, such as the acetate or nitrate, is precipitated by (1) hydrogen sulphide or alkaline sulphide as a brown or black precipitate, which is insoluble in ammonium sulphide. In dilute solutions this sulphide is, however, appreciably soluble in mineral acids, and may introduce errors in analysis, especially as the solubility is distinctly increased by the presence of certain earthy salts. The sulphide produced through the action of alkaline sulphide on a soluble salt of lead is less soluble than is the corresponding acid sulphide. Soluble salts of lead are at once precipitated by albumin or peptone; the resulting precipitate has no stable composition. Under certain conditions definite colloidal precipitates are formed, particularly in the presence of sulphide of copper or mercury. (2) Sulphuric acid or soluble sulphates produce a precipitate of lead sulphate insoluble in excess of the precipitating salt or sulphuric acid, and only slightly soluble in alkaline solutions. This method is the one generally adopted for gravimetric determination of a lead salt. (3) Potassium chromate produces a precipitate of chromate of lead very little soluble in acid, but soluble in caustic alkali. (4) Potassium iodide produces a yellow lead iodide, soluble on heating, and reprecipitating and crystallizing on cooling. (5) Alkaline chlorides and hydrochloric acid produce needle-like crystals of lead chloride soluble on heating, and reprecipitating on cooling. (6) Potassium nitrate in conjunction with a copper salt (copper acetate) produces a precipitate of a triple copper, lead, and potassium nitrate, crystallizing in characteristic violet-black cubes. This reaction is one made use of in the qualitative determination of small quantities of lead in organic fluids (see p. 167).