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comes latent, is not lost; it has done work in producing a change of state in the substance-in loosening the atoms from the rigid bond of cohesion, and remains a fund of potential energy, like a bent bow, ready to become heat again, or be converted into mechanical power, as in the steam-engine. This is another instance of the conservation of energy.

SOURCES OF HEAT.

Next to the sun, chemical combinations are the chief sources of heat. When two substances unite chemically, their temperature is almost always raised. When the heat is evolved so rapidly as to render the substances luminous-which most substances become when heated to a certain degree-the process is called combustion. Fire is a solid rendered luminous or incandescent by combustion; flame is gas at a white heat.

Animal heat has the same source as the heat of a fire or of a candle; it arises from a species of combustion. The oxygen taken into the body by the lungs unites with the carbon and hydrogen of the waste parts of the blood and solids, and converts them into carbonic acid and vapour of water-burns them, in short, and thus produces heat.

Heat can also be produced by mechanical means, such as compression, percussion, and friction. A piece of iron may be rendered hot by hammering; and axles of carriages often ignite from friction. It is found that the amount of heat thus produced is always in proportion to the mechanical energy expended in the process.

98. Mechanical Equivalent of Heat.-The exact relation of the heat to the work that produces it is expressed in what is called the mechanical equivalent of heat.

It has been determined by accurate experiments

that 772 foot-pounds of work produce heat sufficient to raise a pound of water one degree in temperature; and 772 foot-pounds constitute the mechanical equivalent of heat. Another way of stating the same thing is to say, that a pound-weight falling from a height of 772 feet against the earth, generates heat sufficient to raise a pound of water one degree; or conversely, that the heat that raises a pound of water one degree in temperature, would, if it could be all used mechanically, raise 772 pounds a foot high.

CHEMICAL ACTION.

Chemical affinity, we have seen, differs from cohesion in this, that in drawing together two or more distinct substances, it makes out of them a new substance differing in kind from any of its components (see par. 21). A few examples, taken chiefly from operations going on daily under our eyes, will give a general idea of the action of this force.

Of

99. The Elements.-An element was defined as a substance that has not yet been shown to be a compound. Chemists reckon at present upwards of 64 elements. these, about 50 are metals, and 14 are non-metals. The non-metals are:

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Of the non-metals, five are gases at ordinary temperatures-namely, Chlorine, Fluorine, Hydrogen, Nitrogen, Oxygen. Bromine is a liquid, and with the exception of the metal mercury, is the only element that is liquid at an ordinary temperature.

The greater number of the metals are rare and of little importance except to the professional chemist; those occurring more commonly are:

Combining

Names. Symbols. Weight.

Combining

Names. Symbols. Weight. Aluminium.....Al..........274 Magnesium.....Mg..... .24 ..Sb.........122 Manganese.. .Mn.... ...55

As..........75

Antimony.

Arsenic...

Barium..

Ba...

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Mercury... .Hg... ..200

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Calcium... ......Ca..........40

Chromium... ...Cr..........52.5 Silver......

Potassium..

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.Co..........587 Sodium......

Au........197 Tin..

Zinc...........

.Na...

..23

Copper... ....Cu..........63.5 Strontium.......Sr................. .87.5

.Sn.. ..118

..Zn..........65.2

Cobalt...

Gold.

Iron..........

Lead...

.Fe..........56

.Pb........207

All the matter forming our globe is made up of these few elements, either combined to form compounds, or uncombined. Few of the elements occur uncombined, or free, in nature. The most prominent are oxygen, nitrogen, carbon, sulphur, gold, sometimes copper, more rarely silver.

100. Compound Substances.-A substance formed of two elements is called a binary compound; of three, a ternary; of four, a quaternary.

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101. Binary Compounds.-The water is an instance of a binary compound. It is a combination of two gases-oxygen and hydrogen. These two elements play a most important part in nature, and their combinations with one another and with other

elements furnish typical instances of chemical action. We therefore notice them more at length than the other elements.

102. Oxygen.-Oxygen exists in a free state in the atmosphere, of which it forms one-fifth by bulk (see par. 210). Oxygen is most readily obtained pure from a substance called chlorate of potash (potassium chlorate), which is made up of chlorine, potassium, and oxygen, the oxygen being more than one-third of the whole weight. When this substance is heated, the oxygen is driven off, and may be collected in jars and bottles. To the eye, oxygen is not distinguishable from air; but it is slightly heavier, and has much more active properties.

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In its diluted condition in the atmosphere it supports ordinary combustion and animal life. But if an extinguished taper with the wick still glowing is plunged into a jar of pure oxygen, it is suddenly rekindled, and burns with extraordinary brilliancy. If the end of a fine iron wire is tipped with burning sulphur and immersed in oxygen, the iron continues to burn with brilliant scintillations after the sulphur is consumed. Similarly, when pure oxygen is breathed instead of air, it excites the vital actions to hurtful activity.

103. Hydrogen.-Hydrogen is rarely found free in nature, but is easily prepared by the chemist. It is a

colourless gas, and has neither taste nor smell. It is the lightest substance known, being 14.5 times lighter than air, and 16 times lighter than oxygen. Hydrogen does not support combustion; but it burns in air with a blue flame, having little light but great heat. Compounds of hydrogen with carbon are the chief ingredients in coalgas.

104. Combination of Oxygen and Hydrogen.-If now a mixture of hydrogen and oxygen is inclosed in a dry glass tube, in the proportion of one measure of oxygen to two of hydrogen, and an electric spark is sent through the mixture, an explosion takes place. The two gases disappear, and the sides of the glass are covered with drops of water. The same proof of the composition of water is afforded by burning a fine jet of hydrogen in air, and holding a dry glass vessel over it. The combination of the oxygen of the air with hydrogen forms steam, which condenses on the glass and trickles down as water. The same thing is proved when the galvanic current is used to split up water into its constituent gases (see par. 68); hydrogen is found at one pole of the circuit, and oxygen at the other, and always two measures of the hydrogen to one of the oxygen.

105. Oxides.-All the other elements combine with oxygen, and the binary compounds thus formed are styled oxides. The process, which is constantly going on in nature, is called oxidation. A familiar instance is when iron rusts in the air. Oxidation is always attended with the evolution of heat; and when the heat is so great as to produce light, it is called combustion. In ordinary combustion the fuel, or substance that burns, consists of carbon or compounds of carbon with hydrogen; and the products are an oxide of carbon (carbonic acid), and an oxide of hydrogen (water).

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