The impurities of the Thames are not all we have to deal with--its hardness must cost the Londoners hundreds of thousands a year in the article of soap alone. The action upon lead is also marked; hence we find poisonous carbonates of that metal held in solution. Plumbers are well aware of this fact, and frequently meet with leaden cisterns deeply corroded. This corrosion may arise from either chemical or voltaic action. The junction of lead and solder, or iron, immersed in water impregnated with salts or acid of any kind, will cause erosion of the metal. A familiar instance of this is seen in the rapid manner in which iron railings rust away just where they are socketed in the stonework with lead. The presence of a piece of mortar on the lead of a cistern may even set up this action, and result in giving a whole family the colic. The pumps of the metropolis are liable to even more contamination than river-water, inasmuch as the soil surrounding them is saturated with the sewage of innumerable cesspools, and with that arising from the leakage of imperfect drains. Medical men entertained the opinion that the terrible outbreak of cholera in Broad Street, Golden Square, in 1854, arose from the fact that the people in the neighbourhood were in the habit of visiting a public pump which was proved to be foul with drain-water, and the handle of which was taken off, to prevent further mischief. Some of these public pumps appear to yield excellent water--cold, clear, and palatable; but the presence of these qualities by no means proves that they are pure. The bright sparkling icy water issuing from the famous Aldgate pump, according to Mr. Simon, the city officer of health, owes its most prized qualities to the nitrates which have filtered into the well from the decaying animal matter in an adjoining churchyard.