Lake-water varies much in its composition. The main difference depends on the lake pos sessing an outlet or being destitute of one. The water of the former generally corresponds with that of the rivers which flow into it; but the flow becoming slower, there is more scope for the devekipment of animal and vegetable matters, and for the decomposition of organic remains.
Marsh-water is stagnant, and abounds in animal and vegetable remains, either in a state of decomposition or passing into new combinations, generally of a low grade, as the lowest members of the vegetable kingdom and those of each section of the animal are mostly aquatic. These waters are for the most part unwholesome, both from the gases they emit, and also when used as drink. Impure or putrid water may be rendered pure by adding alum or recently prepared charcoal, or by simply pouring it from one vessel into another in the sun.
Sea-water abounds in saline matters so much, that it is unfit for use internally, except in small quantity as a medicine. Sea-water may be rendered fit for drinking by pressure, filtration, and freezing, or simply by boiling it, and condensing the steam as it arises. , For many chemical, pharmaceutical, and even dietetical purposes, water must be of greater purity than it is generally found. For this end it is directed to be distilled, in which process never more than two-thirds of the water put into the still should be allowed to pass over.
_Mineral Waters are generally characterised i by possessing some principle different from what is found in common water, or some of the ordinary principles in unusual proportion ; yet among these are reckoned certain springs which have no claim to repute beyond what is due to their extreme purity, such as Malvern and Holywell, or to having a higher tempera ture throughout the year than the mean of the latitude where they are situated. These last
are classed among the thermal springs, which are properly divided into two sections, the mineralised hot springs and the immineralised, among which are some only tepid, such as Matlock, where some springs, are 66° the lowest of the class in Britain, and others cold, presenting this peculiarity, that the tepid springs arise from fifteen to thirty yards above the level of the river Dement, whilst those which arise either above or below this range are cold.
For practical purposes mineral-waters may be classed under four heads, each susceptible of secondary heads, according as they are hot or cold, or have other peculiarities, viz.: saline, alkaline, chalybeate, and sulphureous.
It has never ceased to be an object of interest to determine whether sea-water can be so distilled on board a ship as to yield drinkable fresh-water. Many processes have been devised for this purpose ; hut all have been wanting in some one or more desired qualities. Some of the government ships have lately been provided with Grant's Dis tilling and Cooking Galley ; during the period it is required to keep the fires alight in these galleys for purposes of cooking, the distilla tion of salt water is going on so as to yield one gallon of distilled water per man per day. Like all distilled water, it is vapid at first from the loss of oxygen during the process ; but the motion of the ship is said to aerate the dis tilled water in the tanks, by agitation. Expe riments have been made in one of the govern ment ships, to see whether the oxygen might be restored to distilled water by an electrical current passed through it; but this seems too complicated an arrangement for the rough usages of a ship.