Page:Treatise on poisons in relation to medical jurisprudence, physiology, and the practice of physic (IA treatiseonpoison00chriuoft).pdf/421

 *lar waters, it may in some cases be impossible to prevent it by any attainable means. But the inquiries detailed above suggest two modes by which a remedy may be generally found. It appears that, where a crust of carbonate is allowed to form slowly and quietly on the surface of lead, even distilled water ceases to have any material action; and that the action is reduced almost to nothing if a crust be thus formed in a solution containing a minute quantity of some powerfully protecting salt, such as phosphate of soda. It appears to me then that a remedy may be often found in the instance of unusually pure spring waters—either by leaving the new pipes filled with the water for a few months, care being taken not draw any water from them in the interval,—or perhaps even more effectually by filling the pipes for a similar period with a solution containing about a 25,000th of phosphate of soda. I had determined to try the latter plan with the pipes in the Dumfries-shire case mentioned above, but recommended that in the first instance the pipes should be left for a few months full of the water of the spring, and the stopcocks kept carefully shut; and on this being done for three or four months, it was found that the water afterwards passed with scarcely any impregnation of lead, and what little was contracted at first gradually diminished in the course of time.—Probably neither of these methods will be of more than temporary use, when the chief or only salt present is chloride of sodium, even though the proportion be considerable. Both plans seemed to answer for a time in the instance which occurred at Lord Aberdeen's (p. 411); but after a while the action recommenced, probably owing to the deposited carbonate being slowly dissolved. At the time of publication of my paper in the Transactions of the Royal Society of Edinburgh, the cure appeared complete, and was there represented to be so.

I should add that an effectual remedy has been lately introduced by a patent invention for covering lead pipes both externally and internally with a thin coating of tin.

In the remarks now made on the action of water on lead no account has been taken of the effect of the galvanic fluid in promoting it. This, however, is a most important co-operating agent, or rather perhaps it ought to be considered a distinct power; for it acts with energy where water alone acts least, namely, when there is saline matter in solution, because then a galvanic current of greater force is excited. In general it is necessary that two different metals be present in the water before galvanic action be excited;. but a very slight difference may be sufficient. For example, it seems enough that the lead contain here and there impurities, constituting alloys slightly different from the general mass of the pipe or cistern. It is probable that galvanic action may be thus excited by the joinings being soldered with the usual mixture of lead and the more fusible metals. At least I have seen pipes deeply corroded externally, when made of sheets of lead rolled and soldered; and the action was deepest on each side of the solder, which had itself entirely escaped corrosion. Even inequalities in the composition of the