Composition of Water

the minute living forms called micro-organisms or
germs, some of which are the cause of disease. A filt
of porous stone or procelain, in which the water filt
slowly, is more effective. A good filter is shown
Figure 5.

Water which has strained or filtered through sev feet of earth is often much improved, but the e filter itself may become contaminated after a while more harm than good result. A thick layer of and rock, however, removes germs effectively, and co sequently water from deep driven wells is afe.

Water was long considered an elementary or sin substance, but towards the end of the last century was found to consist of two quite different substance so intimately joined together that the identity of each is lost. If we pass an electric current through water in the proper way, we see a gas rising in bubbles fre the end of the wire by which the current enters an like appearance at the wire by which the current leaves the water. The two gases have evidently e Croni the water and are the substances out of w made for the water begins to disappear. By placi inverted glass filled with water over each wires*. gases are easily collected. See Fig. 6. Whore bottle is full of gas, the other will be only half ful on decomposing the whole of a given amount of water this proportion holds true.

ed

If we test these gases, we shall find them que dia ferent. The bottle which is full contains a gas called:

hydrogen. There is evidently twice as much of this by 'ume in water as of the other gas which is called oxygen. These two gases were tied together by what nown as chemical force, but the electric current rated them and gave us an opportunity to make cquaintance of each by itself. We would hardly ose this clear, colorless iiquid to be composed of naterial. On decomposing pure water from any

.in

'itio.

. 6. Decomposing Water Into Oxygen and Hy-
drogen Gas.

the proportion of oxygen to hydrogen is always
e, and in fact, all chemical compounds have a
composition which never varies under any con-

T name hydrogen comes from two Greek words, ean water and to produce. Hydrogen is interesting as being the lightest common substance. It is an wisible gas like air, but unlike air will burn. If a

Hydrogen

lighted candle be placed in a bottle of hydrogen, the flame will be at once extinguished, though the hydrogen will take fire at the mouth of the bottle. Fig. 7. Hydrogen will unite with other substances besides. oxygen; that is, it will join with other substances by chemical force. It forms a part of most animal and vegetable substances.

Oxygen

Fig. 7.

Hydrogen Will Burn
in Air.

Oxygen, as well as hydrogen, is a tasteless, colorless, odorless gas. The weight of a given volume is sixteen times that of the same volume of hydrogen. It is very abundant and the most important substance to mankind. Should we test this gas with a lighted candle, as we did the hydrogen, we would find that the oxygen would not give a flame, but that the candle. would burn far more vigorously. Fig. 8.

When substances burn in oxygen they really unite with it chemically, forming new substances called oxides. Water is hydrogen united with oxygen and its chemical name might therefore be oxide of hydrogen. When water is heated in an open vessel, evaporation from the surface of the liquid is more rapid as the temperature increases. Soon vapor is formed on the sides and bottom of the vessel and bubbles begin to rise which are at once condensed by the cooler parts of the liquid, thus making the familiar "singing" noise. Finally the liquid becomes so hot that the bubbles reach the surface without condensing, and then the water boils and goes off into the air as steam, an invisible gas. This occupies the small space between the spout of the tea-kettle and the cloud of vapor which is commonly called steam, but is really finely divided drops. of water. A cubic inch of water makes about a cubic foot of steam.

The temperature at which pure water begins to boil at sea level is 212° Fahrenheit (or 100° Centigrade) and this temperature remains the same while the boiling continues. Increasing the heat simply increases the violence of the boiling. The steam given off is of the same temperature as the boiling liquid. Most pure liquids have a definite boiling point; ether boils at 100° F, alcohol at 173° F, turpentine at 315° F.

When the pressure of the atmosphere on the surface of the liquid is less than at the sea level, as on a mountain, where there is not so much air above pressing down on the surface of the liquid, the temperature of

Effect of

Heating Water

Boiling Point

Latent

Heat

boiling is less. For example, the boiling point of water in Denver, Colorado, is about 202° F, and on the top of some of the mountains in the Himalayas, 180° F. People living in high mountain regions have difficulty in cooking with water or steam.

Increasing the pressure on the surface of the liquid, on the other hand, raises the boiling point. This is seen when water boils in a confined space, as in a steam boiler. Under five pounds pressure of steam, water boils at about 227° F and at 100 pounds pressure, at 337° F.

An increase in the boiling point of water is caused by dissolved substances. A very strong solution of common salt boils at about 226° F, and a solution of sugar syrup or molasses-boils at an increasing temperature as the water is lost.

The temperature at which a syrup boils, is a measure of its thickness or density. In many modern cookery books temperature tests are given for boiling sugar in making confections, which vary from 215° for a thin syrup, up to 350° for caramel. In making maple sugar a "sugar thermometer" is often placed in the boiling syrup. At a given temperature, which is higher for sugar cakes than for soft sugar, the proper con

centration is reached.

Considerable heat is absorbed by the process of boiling. It requires 966 times as much heat to change a pound of water at the boiling point into steam as it does to raise it one degree Fahrenheit. The heat