Spring water is to be preferred to running or stagnated water; for, unless it is taken at the source, or near it, it is apt to be impregnated with decayed vegetable and animal substances, such as leaves, grass, wood, and dead insects. This inconvenience is greatest in a hot climate, where every thing teems with life, and where the materials of putrefaction are both more abundant and more prone to corruption. This is the most pernicious kind of impurity; for the mineral impregnations common in springs are seldom, in any degree, unwholesome, and do not tend, like the other, to make the water corrupt. At many of the West-India watering places the water is found stagnated just above high-water mark; and care should be taken to go higher up to take it where it is running. The purest water is apt to spoil by producing a putrid glare upon the inner surface of the cask which contains it. There is a great difference in this respect between a new cask, especially if made of moist wood, and that cask which has been hardened and seasoned by age and use. Several contrivances have been proposed for preparing the vessels that hold the water; but none have been found by experience so effectual as letting them stand for some time full of sea water; and it is a great advantage of this method, that it is so easily practicable. It is in few places we meet with water such as that of Bristol, which, in clean vessels, may be kept for any length of time. We may consider all water kept in wooden vessels as more or less liable to putrefaction; but there is a substance, which is neither rare nor costly, that effectually preserves it sweet. This is _quick lime_, with which every ship should be provided, in order to put a pint of it into each butt when it is filled. It has the advantage of not being injurious to health; but, on the contrary, is rather friendly to the bowels, tending to prevent and check fluxes. In the year 1779 several ships of the line arrived in the West Indies from England, and they were all afflicted with the flux, except the Stirling Castle, which was the only ship in which quick lime was put into the water. Nor does it spoil the water for any culinary purpose. Its action in preventing putrefaction consists, in part at least, in destroying vegetable and animal life. An addition of putrescent matter is produced in water by the generation of small insects; and the glare that collects on the sides of casks, and also what collects on the surface of the water, is a species of vegetation of the order called by naturalists _algæ_[79]. Quick lime is a poison to this species of vegetable life as well as to insects: but upon whatever principle it depends, the property of it in preserving water sweet is so well ascertained, that it is inexcusable ever to neglect the use of it.