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which hydrogen and oxygen combine at a high temperature. If a mixture of hydrogen and atmospheric air be used, a similar explosion will take place. Hence, in preparing hydrogen for experiments, great care must be taken to expel all air from the apparatus before any of the gas

is collected. If a small rubber balloon be filled with hydrogen, it rises readily in the air, showing that the gas is lighter than air. It is, in fact, the lightest substance known, being 14.5 times lighter than air, -and 16 times lighter than oxygen. It was formerly used in filling balloons; but coal-gas, which, though not so light, is much cheaper, is now commonly employed instead.

The lightness of hydrogen and the energy with which it combines with oxygen, are its two most important characteristics.

22. The Oxy-hydrogen Blowpipe. - The energy with which hydrogen and oxygen combine develops intense heat. This may be shown by means of the oxy-hydrogen blowpipe, represented in Figure 8. The hydrogen is Fig. 8.

forced through the tube H, and the oxygen through the tube 0; and it will be seen that the two can mix only at the mouth of the jet where they are burned,

so that there is no danger of an explosion. The gases may be kept in a gasometer (Figure 9), or, more conveniently, in gas-bags of india-rubber.

Hold a copper or iron wire in the flame of this blowpipe, and it burns as readily as a pine shaving held in the flame of a lamp. A steel watch-spring burns with brilliant scintillations. If a bit of zinc be placed on a

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piece of charcoal hollowed out for the purpose, and the flame of the blowpipe be directed upon it, the metal quickly melts and burns. Antimony, bismuth, and many other metals will burn in the same way, each with a characteristic light.

Cast - iron burns with a shower of bright sparks Platinum, which is one of the most infusible of substances, is readily melted, and even converted into vapor. A small cylinder of lime

OXYGEN HYDROGI placed in the blowpipe flame becomes white-hot, and glows

FO with most intense brilliancy, forming what is known as the lime light or Drummond light. When the rays of this light have been gathered and reflected by a mirror, it has been seen at a distance of one hundred miles in full daylight.

WATER.

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23. The Properties of Water. - The most important compound of hydrogen is water, which, as we have already learned (2), contains hydrogen and oxygen.

At ordinary temperatures, water is a clear, colorless ransparent liquid, without taste or smell. It freezes at 32°, and under the ordinary pressure of the atmosphere it boils at about 212°.

It is the most universal solvent known. There are but few substances which are not dissolved by it. Even those which we call insoluble, generally differ from the rest

only in degree. Spring water dissolves lime-rocks, and almost all other mineral substances.

The beautiful crystals found in the rocks are almost invariably deposited from a solution of the mineral in water, though in many cases it may have taken thousands of years to form them.

Water dissolves almost all gases as well as solids. It is on the gases dissolved in the water that all aquatic plants and animals live.

24. Various kinds of Natural Waters. - Owing to its solvent powers, water is never found in nature in a state of purity. Rain water, collected after long-continued wet weather, comes nearest to it; but even this always contains small quantities of atmospheric air, and the gases floating in the air.

Spring water always contains more or less of saline matter, which it has dissolved from the soil. The salts most frequently found in it are common salt, calcic carbonate and sulphate (carbonate and sulphate of lime), and magnesic carbonate and sulphate (carbonate and sulphate of magnesia). Most spring water contains carbonic acid, to which it owes its sprightly taste.

Mineral waters are waters which contain an unusually large proportion of any of the salts just named, or of other and rarer substances. In many cases they have medicinal qualities, which vary with the salts held in solution. They sometimes contain salts of iron, and are then called chalybeate springs. In other instances, carbonic acid is so abundant as to give them an effervescent character. Less frequently sulphur is the chief ingredient, giving the water a nauseous taste and smell.

Water is familiarly called hard or soft, according to its action on soap.

Hard waters contain salts of calcium or magnesium, which cause the soap to curdle, that is, to become insoluble. Soft waters do not contain these salts, and dissolve the soap without difficulty. Many hard

sea.

waters become soft by boiling, the salts held in solution being deposited, as a fur or crust, on the inside of the boiler.

Sea water contains a large amount of common salt and of magnesic chloride (chloride of magnesium), to which it owes its saline, bitter taste. Smaller quantities of many other salts are found in it. All this matter has been washed out of the soil by the rivers which flow into the

It remains in the sea, since the water which evaporates from the surface is almost perfectly pure ; but the surplus is continually taken up by marine plants and animals, so that the sea becomes no salter.

25. Water in Plants and Animals. - Water co stitutes the greater part of all plants and animals. The human body is four-fifths water. In many of the lower animals the proportion is much greater. From a sun-fish weighing 30 pounds, only 240 grains of solid matter were obtained; so that water makes up about .999 of the weight of such animals. The vegetable substances which we use for food contain almost as large a percentage of water. In potatoes, the fraction is -75; in apples, .80; in turnips, .90; in watermelons, .94; and in cucumbers, .97

26. Water of Crystallization. - Many salts, in crystallizing from their solutions, unite with a definite quantity of water, which is then called water of crystallization. If the salt be heated, the water is driven off, and the crystals fall to pieces; but the chemical properties of the substance are not altered. Many salts part with this water by mere exposure to the air, and effloresce; that is, crumble into white powder. Sodic carbonate (carbonate of soda) is an efflorescent salt. Other salts, on the contrary, like potassic carbonate (carbonate of potash), absorb water from the air, and become moist, or even dissolve; in which case they are said to deliquesce.

27. Uses of Water.—“On the uses of water it is almost needless to enlarge, for they are universally felt and appreciated. In each of its three physical conditions, the blessings which it confers on man are inestimable. As ice, it furnishes in northern lands, for months together, a solid bridge of communication between distant places : in the liquid condition, it is absolutely necessary to the existence of vegetable and animal life ; in this shape, too, it furnishes to man a continual source of power in the flow of streams and rivers ; it supplies one of the most convenient channels of communication between places widely separated; and further, it is the storehouse of countless myriads of creatures itted for use as food: in the state of vapor, as applied in the steam-engine, it has furnished a power which has, in late years, done more than any other physical agent to advance civilization, to economize time, and to ameliorate the social condition of

In each and all of these points, if rightly considered, we must perceive the entire adaptation of this compound to the ends which it was designed by the Creator to fill.

“Glancing at the physical condition of our planet, we cannot fail to be impressed with the important effects produced by the movements of water at periods anterior to the existence of man, as well as in more recent times. To such causes must we refer the formation of sedimentary rocks, and their arrangement in successive strata. upon the surface of the earth : even now, observation shows that denudation is proceeding at some points, elevation and filling-up at others; whilst the accumulation of drift, and a variety of other extensive geological changes, must be traced to the same ever-acting and widely-operating agency.

further be observed, that there is no form of matter which contributes so largely as water to the beauty

men.

It may

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