Marsh Water, being the most stagnant, is the most impure of all water, and is generally loaded with decomposing vegetable matter. There can be no doubt that numerous diseases have sprung up from its use. 237. It is, however, in vain that pure water is discovered, if proper means be not adopted to convey it for the use of the inhabitants. In ancient times this was clone by means of aqueducts of extraordinary magnificence; and the materials of which they were composed were even then acknowledged to be capable of affecting the water which flowed through them. Pal-ladius testifies his aversion to the use of lead, as apt to become covered with ceruse, and thereby rendered poisonous; and Vitruvius and Columella recommend pipes of earthenware, as not only cheaper, but more wholesome than those of lead. Dr. Lambe, to whom we are indebted for an important work 1 upon this subject, states, that there is a great diversity in the corrosive powers of different waters; in some places the use of leaden pumps has been discontinued, from the expense entailed upon the proprietors by the perpetual want of repair.

Dr. Lambe states an instance where the proprietor of a well ordered his plumber to make the lead of a pump of double the thickness of the metal usually employed on such occasions, to save the charge of repairs; because he had observed that the water was so hard, as he called it, that it corroded the lead very soon. If any acidity be communicated to the water, from the accidental intrusion of decayed leaves or other vegetable matter, its power of dissolving this metal will be increased to a very dangerous extent. The noted colic of Amsterdam is said by Tronchin, who has written a history of the epidemic, to have been occasioned by leaves falling and putrefying in leaden cisterns filled with rain water. Van Swieten has also related an instance of a whole family who were afflicted with colic from a similar cause2; and Dr. Lambe entertains no doubt, but that the very striking case recorded in the Medical Commentaries3 proceeded more from some foulness in the cistern, than from the solvent power of the water. In this instance, the officers of a packet vessel used water out of a leaden cistern; the men also drank the same water, except that the latter had been kept in wood: the consequence was, that all the officers were seized with colic, while the men remained healthy.

Sir George Baker has furnished the following striking illustration of this subject: - The most remarkable case that now occurs to my memory," says he, "is that of Lord Ashburnham's family in Sussex; to which spring water was supplied from a considerable distance in leaden pipes. In consequence, his lordship's servants were every year tormented with colic, and some of them died. An eminent physician of Battle, who corresponded with me on the subject, sent up some gallons of that water, which were analyzed by Dr. Higgins, who reported that the water had contained more than the common proportion of carbonic acid; and that he found in it lead in solution, which he attributed to the action of the carbonic acid. In consequence of this representation, Lord Ashburnham substituted wooden for leaden pipes; and from that time his family have experienced no particular complaints in their bowels." As timber pipes are liable to decay, and to impart a bad taste to the water, those made of cast-iron are to be greatly preferred.

Before quitting this subject, I ought to state that I have found the presence of iron, as for instance a pump, in contact with lead to accelerate the solution of the latter metal; by forming a simple voltaic circuit, and thus effecting the decomposition of the saline ingredients, and transferring the alkaline element to the lead, by which it is dissolved. (Pharmacologia, edit. 8th, p. 394).

1 Researches into the Properties of Spring Water, with Medical Cautions against the Use of Lead.

2 Van Swieten and Boerhaav. Aphor. 1060, Comment.

3 Duncan. Med. Comment. Dec. 2, 1794.

238. For the purification and preservation of water, numerous methods have been adopted. The mechanical impurities may be removed by filtration, a process which is suggested by nature herself; for all springs arising through sand, gravel, etc, must undergo this process. Hence it occurred, that if waters of a putrid, marshy, or unwholesome nature, were filtered through a factitious bed of sand, or a vessel made of porous stone1, they might be deprived of their bad qualities. As that peculiar property of water which constitutes what is termed hardness, generally depends upon the presence of sulphate of lime in solution, it cannot be removed by simple filtration; but the addition of an alkaline carbonate2, twenty-four hours before it is used, will be found to restore it; or, if it should depend upon super-carbonate of lime, long ebullition, without any addition, will be found sufficient for its cure. Another mode of improving water, and one that has been most recently discovered, is by means of charcoal, a substance which enjoys, in an eminent degree, the property of preserving water from corruption, and of purifying it after it has been corrupted: hence the filtration of water through alternate layers of sand and charcoal offers a ready and effectual mode of abstracting its impurities, especially when they consist of animal or vegetable matter.

Where we have reason, however, to suspect much injurious contamination, the process of boiling should never be omitted; after which it may be strained and filtered, and lastly agitated in contact with the atmosphere, in order to restore to it its natural proportion of air. In China, water is never drunk until it has been boiled. The mischievous effects of impure Mater, where it cannot be corrected by any chemical process, are said to be best counteracted by some bitter vegetable. Virey supposes that this circumstance first induced the Chinese to infuse the leaves of the tea plant. The brown tinge so generally seen in ponds and slow streams, depends upon a minute quantity of extractive matter, from the decomposition of fallen leaves and other vegetable bodies, and will be retained by the water in solution, after it has undergone the process of depuration; nor can it be removed by boiling, but alum and certain metallic salts, as sulphate of iron, especially if heated with it, throw down a precipitate, and leave the water colourless.

1 Various machines have been constructed for this purpose; hut the most modern are those known by the name of, "Bennet's Patent Filtering Machine," and Robins's Royal Filter.

2 In the proportion of from ten to fifteen grains to every pint.

239. The juices and infusions of vegetable and animal matter constitute the second division of drinks.