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names beginning with the same letter. The symbol of an element always stands for one atom of the element.

The symbol of a compound indicates its composition. It is formed by writing together the symbols of the elements of the compound, with a small figure after each symbol expressing the number of atoms of that element found in a molecule of the compound. The symbol of a compound always stands for one molecule of the compound.

The chemical changes, or reactions, which substances undergo, are indicated by equations made up of these symbols.

The compounds of oxygen with the elements are called oxides. These oxides are either acid, neutral, or basic. The oxides of the non-metallic elements are generally acid; those of the metallic elements are generally basic.

The acid oxides combined with water form acids.
The basic oxides combined with water form bases.

When a metal takes the place of the hydrogen of the acid, the compound is called a salt.

A hydracid is made up of hydrogen and a non-metallic element.

The names of binary salts take the ending -ide; those of ternary salts, the endings-ate or -ite, according as the name of the acid ends in -ic or -ous.

In general, the name of a binary compound takes the ending -ide.

Oxygen is the most abundant element, and is usually prepared from potassic chlorate. It has a wider range of affinities than any other element.

Ozone is an allotropic form of oxygen, and is more active than ordinary oxygen.

Hydrogen is the lightest element, and is usually obtained by the action of zinc on dilute sulphuric acid. Its most important compound is water.

Nitrogen is one of the most inert of elements. It is remarkable for the indirect way in which it enters into combination, and for the variety and the nature of its compounds.

Carbon is an infusible solid, and has three allotropic states.

Sulphur is an inflammable solid, forming sulphides which in their composition resemble oxides. It has several remarkable allotropic states.

Chlorine is most readily prepared by the action of black oxide of manganese on muriatic acid. It is distinguished by its color, its odor, and its affinity for hydrogen and the metals. Its bleaching and disinfecting power is due to its strong affinity for hydrogen.

Bromine, iodine, and fluorine are elements closely resembling chlorine in their chemical properties.

Phosphorus is a very inflammable substance, and is mainly used for making lucifer matches. It is obtained from bones.

Phosphorus and arsenic resemble nitrogen in the compounds which they form.

Silicon resembles carbon in its allotropic states.

Boron combines directly with nitrogen at a high temperature.

Nitric acid is remarkable for its oxidizing power.

Sulphuric acid is the strongest of acids, and is remarkable for its affinity for water.

Sulphurous acid is a bleaching agent.
Nitrous oxide is used as an anæsthetic.

Carbonic acid is a heavy gas, and is most readily prepared by the action of muriatic acid on marble.

The most important non-metallic sulphides are hydric sulphide and carbonic bisulphide.

The most important of the non-metallic chlorides is muriatic acid.

Cyanogen is a remarkable compound of nitrogen and carbon, and plays the part of a non-metallic element.

Ammonium is a remarkable compound of nitrogen and hydrogen, and plays the part of a metal.

Ammonia, although it is not an oxide, has very powerful basic properties. It is known as the volatile alkali, and is extensively used in the arts.

THE METALS.

GENERAL PROPERTIES OF THE METALS.

71. Physical Properties of the Metals. The metals differ widely from one another, both in their physical and chemical properties. Those metals which are lightest have the greatest affinity for oxygen, whilst the heavier metals are oxidized with difficulty.

The following Table gives the specific gravities of the most important metals :: Iridium 21.8 Iron

7.8 Platinum

21.5
Tin

7.3 Gold

19.3
Zinc

7.1 Mercury 13.596 Antimony *

6.7 Lead

11.3
Chromium

5.9 Silver 10.5 Aluminium

2.56 Bismuth * 9.8 Strontium

2.54 Copper 8.9 Magnesium

1.75 Nickel

8.8 Calcium Cadmium 8.6 Sodium

0.972 Cobalt 8.5 Potassium

0.865 Manganese

Lithium

0.593

1.58

8.0

more

The melting points of the metals differ even widely than their densities, as will appear from the following Table, which is mainly compiled from Miller :

* See in Appendix the chapter on Chemical Philosophy, $$ 19

and 20.

Mercury
Cadmium
Tin
Bismuth
Lead
Zinc

-39° Antimony

+ 737° +242 Silver

1873 442 Copper

1996 507 Gold

2016 617 Steel

2350 to 2500 733 Wrought Iron 2700 to 2850

Some metals can be easily converted into vapor, or volatilized ; thus mercury boils at 662°, while potassium, sodium, magnesium, zinc, and cadmium can be distilled at a red heat. Even the more infusible metals, such as copper and gold, are not absolutely fixed, but give off small quantities of vapor when strongly heated.

72. Metallic Ores. — Only a few of the metals occur in a free state in nature; in general they are found combined with oxygen, sulphur, or some other non-metal. These metallic compounds are most variously distributed throughout the earth's crust; some are known to occur in only one or two localities, and even then only in minute quantity, while others are found widely distributed in

masses. Aluminium, iron, calcium, magnesium, and sodium form, when united with oxygen and silicon, the whole mass of granitic rocks composing our globe ; but it is not from these sources that they can be obtained for the purposes of the arts. For this object we employ other combinations, found in smaller quantity, termed metallic ores, from which the metals can be more easily extracted.

The heavy metals and their ores are interspersed throughout the older rocks, in the form of veins or lodes, which are cracks or fissures of the rock filled

up

with Other ores, such as ironstone, are found in the more recent rocky formations, having been deposited in large masses, probably from aqueous solution.

enormous

the ore.

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