=Lead= is an occasional contamination of slightly acid waters. The purest and most oxygenated waters act most readily on lead; as also those containing organic matter, nitrates or nitrites. Waters containing chlorides also act on lead, the chloride of lead being sufficiently soluble to produce poisonous symptoms. Upland surface waters derived from moorlands in certain districts, _e.g._ around Sheffield, have been found to be capable of dissolving considerable lead from lead service-pipes. The water taken first from the tap in the early morning is the most heavily charged with lead. Such waters are very soft; but other moorland soft waters do not dissolve lead. It is the water having a slightly acid reaction which possesses this property. The source of this acid, whether sulphuric acid from the products of combustion in a neighbouring town, or an organic acid, is uncertain. The plumbo-solvent action of such water is greatest in autumn, when the amount of acid is at its maximum. The property of dissolving lead is removed by passing the water on a large scale over filters of sand, spongy iron, chalk, or limestone. The addition of a small quantity of carbonate of soda has the same effect. In such districts the use of tin-lined iron pipes for domestic services has been recommended, but these are liable to fracture when bent. Pipes consisting of an outer case of lead and an inner pipe of tin with a layers of asbestos between have also been placed on the market. (See also page 68.) Hard waters have the least action on lead; a coating of insoluble carbonate of lead being formed on the interior of the pipe, which prevents any further action. Thus the use of lead pipes for water containing carbonates or sulphate, or calcium phosphate, is comparatively safe. Hard water containing carbonic acid gas under pressure will dissolve a small amount of carbonate of lead; this explains the cases of lead poisoning from soda water which was formerly supplied in syphon bottles with lead tubes. Lead is dissolved much more easily by water if other metals are in contact with it, as iron, zinc, or tin, galvanic action being thus set up. Zinc pipes containing some lead are very dangerous, especially with the distilled water used on board ships. =To determine the presence of lead in water=, place a given quantity, say 100 c.c. in a white dish, and stir with a rod dipped in a solution of ammonium sulphide; if the water becomes coloured, this is generally due to the presence of iron or lead. If the colour remains after adding a drop or two of hydrochloric acid, lead is present.