four substances present which are essential to its existence. But between air and water there is this important chemical distinction, that in the former the constituents are merely mixed together, while in the latter they are chemically combined. When nitrogen and oxygen are mixed together to form common air, each of them retains its gaseous form, and all its properties unaltered; but when hydrogen and oxygen are combined to form water, they severally lose both their origi nal gaseous form, and all their distinctive properties, both physical and chemical. Water is not light, like hydrogen, nor will it burn as that gas does; neither will bodies burn in it as they do so readily and brilliantly in oxygen gas.

Now, when bodies combine chemically, they always form a new substance different in its properties from those which have been employed in producing it; and, indeed, it is one of the wonders which modern chemistry has made known to us, that hydrogen, which burns so readily, should form so large a part of water, our great extinguisher of flame; and that oxygen, so indispensable to animal life, should form eight-ninths of a liquid in which few terrestrial animals can live for more than three or four seconds of time.

That water is indispensable to animal and vegetable life, appears both from its forming so large a proportion of the bodies of living animals and plants, and from some other considerations which have been stated in the preceding chapter. But many of the properties which water possesses are wonderfully conducive to our comfort, to the supply of our daily wants, and to the maintenance of the existing condition of things.

1o. Thus, even the unheeded property of its freedom from smell and taste is important to animal comfort. Sweet odours are grateful to our nostrils at times, and pleasant savours give a relish to our rarer kinds of food. But health fails in an atmosphere which is ever loaded with incense and

PROPERTIES OF PURE WATER.

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perfumes, or where the palate is daily pampered with highseasoned dishes and constant sweets. The nerves of smell and taste do not bear patiently a constant irritation, and the whole body suffers when a single nerve is continually jarred. Hence it is that water and air, which have to enter so often into the animal body, and to penetrate to its most delicate and most sensitive organs and tissues, are made so destitute of sensible properties that they can come and go to any part of the frame without being perceived. Noiselessly, as it were, they glide over the most touchy nerves; and, so long as they are tolerably pure, they may make a thousand visits to the extremest parts of the body without producing the most momentary irritation or sense of pain. Externally, also, they can be applied to the most delicate, inflamed, or skinless parts of the body, not only without irritating, but generally with the most grateful and soothing effects. These negative properties, which are common both to air and water-though, as I have said, they are rarely thought of-are nevertheless most essential to our daily comfort.

2o. Again, water possesses a cooling property, which is very grateful to all living things. The priceless value of water in "a dry and thirsty land" arises mainly from the necessity of constantly supplying that which, in a dry and warm atmosphere, is constantly evaporating from the skin and the lungs. But in all climates water has a cooling power, which gives it a new value to the hot and fevered animal. When taken into the mouth and stomach, or when poured over the inflamed skin, it cools more than an equal weight of any other liquid or solid substance we could apply. This arises from the circumstance, that it takes more heat to give a sensible warmth to water than to an equal weight of any other common substance. Thus the same quantity of heat which is required to raise the temperature of 1 lb. of water

a single degree (from 60° to 61° for example), would give an equal increase of temperature to 30 lb. of quicksilver; and so, again, to convert water into vapour, requires more heat than an equal weight of any other liquid we consume. Hence, when water evaporates from the skin, it serves as a constant cooler of the surface; while the vapour, which escapes with the breath, cools equally the interior of the body. It is really very interesting to observe how the great capacity of liquid water for heat makes it so gratefully cooling as it enters the body; and how its still greater capacity for heat, when passing from the liquid state to the state of steam, enables it so constantly to bear away from us the germs of fever, as it escapes from our bodies in the form of insensible vapour.

3°. But the peculiar composition of water is also a very important circumstance to animal and vegetable life. It consists of oxygen and hydrogen; and all the solid parts of animals and plants contain these same elements in large proportion. In the dry wood of the tree, for example, and in the dry flesh and bone of the animal, both are present. Now, as the plant and animal increase in size, oxygen and hydrogen are required for the formation of their growing parts, and water is everywhere at hand to supply these necessary ingredients. This is a chemical duty which no other liquid but water could equally perform. Water, in discharging this duty, is not merely the drink, as we usually call it, but is really part of the food both of animal and plant.

4°. Further, pure water possesses the property of mixing with some other fluids, such as alcohol (strong spirits) in all proportions, merely weakening or diluting their strength. With others, again-as with oil-it refuses to mingle. Solid substances it has the property of dissolving; and upon this property depend many of the most useful purposes served by water, in reference both to animal and vegetable life.

WATER NEVER PURE IN NAUURE.

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If a piece of sugar and a piece of glass be put together into a quantity of water, the former will dissolve and disappear, while the water will remain for any length of time in the water unaltered in form or in weight. Water does not dissolve all bodies therefore. Sugar is soluble-glass is insoluble in this liquid.

Again, if into two equal quantities of water we introduce loaf sugar and common salt-the sugar into the one and the salt into the other-as long as they are respectively dissolved and disappear, we shall see that 1 lb. of water will dissolve perhaps 2 lb. of sugar, forming a thick syrup, while it will only dissolve 54 oz. of common salt. Thus, of those substances which dissolve in water, some are much more soluble-disappear, that is, in larger quantity than others do.

In nature, water is never found perfectly pure; that which descends in rain is contaminated by the impurities it washes out of the air; that which rises in springs, by the substances it meets with in the earth itself. In rivers, the impurity of the water is frequently visible to the eye. It is often of a red colour as it flows through rocks of red marl which contain much oxide of iron in their composition; it descends milky from the glaciers of Iceland and the slopes. of the Andes, because of the white earth it holds in suspension; it is often grey or brown in our muddiest English rivers ; it is always brown where it issues from boggy lakes, or runs across a peaty country; it is sometimes black to the eye when the quantity of vegetable matter is excessive, as in the Rio Negro of South America; and it is green in the Geysers of Iceland, in the Swiss lakes, among the islands of the South Sea, and around our own islands, because of the yellow matters which it everywhere holds in suspension or solution. Only in clear and deep waters-like those of the Bay of Naples, and in parts of the Pacific, where minute objects

may be seen on the bottom some hundreds of feet down-is the real blue colour natural to water, in large masses, distinctly perceptible.

But among the rocky and other materials which water meets with in and upon the earth, there are many which it can dissolve, as it does salt and sugar, and the presence of which cannot be detected by the sense of sight. Hence the clearest and brightest of waters-those of springs and transparent rivers, even when filtered-are never pure; they all contain in solution a greater or less quantity of saline matter, sometimes so much as to give them a decided taste, and to form what are hence called mineral waters.

Among the purest natural waters hitherto examined is that of the Loka, in the north of Sweden, which flows over hard impenetrable granite and other rocks, upon which water produces little impression. It contains only of a grain (0.0566) of solid mineral matter in the imperial gallon. Some waters in the granite regions of the north of Scotland, and even some springs which rise through the green-sand in Surrey, contain as little as 4 or 5 grains in the gallon. The water which is supplied to the city of Edinburgh contains 7 to 14 grains in the gallon, and that of the Thames, near London, about 21. These are both comparatively pure waters, and are very good for general consumption. That of the river Wear, which supplies the city of Durham, contains 15 grains in the gallon, and is still a good water for domestic use. That which is used in the town of Sunderland, and is obtained from the lower new red sandstone, contains 27 grains in the gallon. Some of the other waters supplied to and used in London and its neighbourhood,

* This is the blue which is seen in the azure grotto of the Isle of Capri, în the Bay of Naples, and in the deep, indigo-like waters of some parts of the Mediterranean and Adriatic seas.

+ This is 1 to 2 parts by weight in 10,000 of the waters-a gallon of pure water at 60 Fahr. weighing 70,000 grains.