Quick lime is equally efficacious for this purpose, whether slacked or unslacked; and though the latter form is more convenient for stowage, by having less weight and bulk, yet the other is to be preferred for the sake of safety; for if water should by chance reach the unslacked lime, a great degree of heat is thereby produced, which has been known to give occasion to the most formidable accidents. The only other objection I know of to the use of quick lime is, that it converts the water into a lime water, rendering it thereby disagreeable to the palate and stomach: but the quantity necessary to preserve it makes but a very weak lime water; for part of the lime is precipitated by the mephitic air, or the aerial acid, as it is otherwise called, of which there is some contained in the water. The accidental exposure to the atmosphere, which also abounds with this sort of air, tends farther to lessen the acrimony of the quick lime[80]. There are other substances which have been found useful in correcting bad water. Alum and cream of tartar, as antiseptic bodies, have been employed for this purpose. Vinegar and the vegetable acid juices and fruits, such as tamarinds, may be used occasionally to take off the putrid offensive taste which may have arisen in case the use of quick lime has been neglected. In the fleet under Sir Charles Saunders, the water of the river St. Lawrence having been found to produce fluxes, this quality was removed by throwing four pounds of burnt biscuit into each cask before it was used. But there is nothing so effectual, and subject to so few inconveniences, as quick lime. The next method to be mentioned of purifying water is filtration, which not only separates the gross impurities, but removes the putrid smell and taste. It is performed with a dripping stone, which is a convenient contrivance for officers, but cannot furnish a supply for a whole ship’s company. When the water of wells or brooks is found loaded with mud, the following expeditious method of filtration, described by Dr. Lind, has been practised with success:--Let a quantity of clean sand or gravel be put into a barrel placed on one end, without the head, so as to fill one half or more of it, and let another barrel, with both ends knocked out, of a much smaller size, (or let it be an open cylinder of any kind) be placed erect in the middle of it, and almost filled with sand or gravel. If the impure water be poured into the small barrel or cylinder, it will rise up through the sand of both barrels, and appear pure above the sand of the large one in the interval between it and the small one.