WATER, in its liquid, aeriform, or solid state, is universally diffused through na ture. It was once considered as one of the four elements, and is in common language still frequently so termed. Water however is now known to be a compound substance con sisting of hydrogen and oxygen. It is colour less, transparent, inodorous, and insipid; it is an imperfect conductor of heat and electri city; it is very slightly compressible, yielding only n.bout 46.65 millionths of its bulk to the pressure of the atmosphere. Its specific gravity is 1, being the unit to which the den sity of all liquids and solids is referred, as a convenient standard, on account of the faci lity with which it is obtained in a pure state. Like all other fluids and substances it expands by exposure to an increase of temperature ; and with a curious exception, the dilatation within certain limits is proportionate to the degree of heat to which it is subjected. When water is heated to a certain point, which is arbitrarily fixed on the scale of Fahrenheit's thermometer at 212°, it acquires the greatest volume it is capable of assuming ; it then boils, and is converted into vapour. Steam at 212° occupies about 1700 times as much space as the water does from which it is generated. It is upon the elastic force of steam commu nicated by heat, and the instantaneous annihi lation of it by cold, that the working of the steam-engine depends.
Water is seldom found in a state of perfect purity, but, from its great solvent and ab• sorbent power, it is impregnated with a variety of saline substances, gases, and animal and vegetable substances, either living or under going a process of decomposition. The effect of these is to communicate different properties, and generally give it a peculiar taste, and not unfrequently an odour, which, if not cognisable by the blunted. senses of man, is sfs by animals, especially the camel, which can scent water at a great distance in the desert. The specific gravity is often much increased, especially that of sea-water and of mineral waters, from the saline ingre dients, and of some of the great rivers, from the quantity of mud and other matters which they contain.
.Rain-Water is commonly reckoned the purest : but it is by no means so free from accidental impregnations as is generally sup posed. Whatever foreign ingredients exist
in the atmosphere of any place are brought to the ground by the firstrain that falls. Rain water, from its great purity, has high solvent powers, which fit it well for the part it has to perform in the economy of nature, and also for many operations in the laboratory. In this respect it is nearly equal to distilled water. The surface water of sandy districts is the purest that can be obtained naturally.
Dew differs little from rain, save in con taining more atmospheric air. Ice-water differs when first obtained, from rain, in being destitute of atmospheric air, and hence it cannot sustain respiration in fishes ; it is for the same reason mawkish and insipid ; but by exposure to the air it speedily absorbs a due proportion. Snow-water is nearly similar.
Spring-water is of various degrees of parity, according to its source and the strata through which it passes. Its most common source is rain, which percolates through some of the superficial strata, and, meeting with some obstacle, is forced up to the surface. River water mostly originates in springs, augmented by rain-water. If it flows over sand or granite, it is found very pure, depositing in its course many earthy salts, especially the calcareous ones, from the escape of carbonic acid. This circumstance renders the water vapid, and less pleasant to drink than spring-water. It possesses however the property of absorbing much oxygen ; hence the surface-water both of rivers and the ocean holds more oxygen than the atmospheric air, to the amount even of 29.1 per cent. This contributes 'both to the maintenance of the respiration of fishes, and these, in their turn, to the growth of aquatic plants. Well-water is generally obtained from a greeter depth than spring water. It is also generally hard, or is apt to become so if kept in a reservoir lined with bricks, unless they be coated with an insolu ble cement. The water from old wells is more pure than from recent ones, the soluble particles being all gradually washed away. The pump and well waters in and about London, and chalky districts in general, are mostly hard.