Chapter 15 of 18 · 3788 words · ~19 min read

Part 15

Globular lightning is seen on rare occasions, when the electrical discharge takes the form of a ball of fire, and descending with less rapidity, is visible for several seconds. In certain conditions of the atmosphere, globes or spires of electrical light, called St. Elmo’s fire, are seen tipping the extremities of bodies in contact with the earth, like church spires, or masts of ships.

All the conditions which give rise to electrical excitement in the atmosphere are much more intense in warm than in cold latitudes; hence the thunder storms of the tropical regions greatly exceed, both in frequency and in violence, those of temperate and cold climates.

THE AURORA BOREALIS, OR NORTHERN LIGHTS

This phenomenon is frequently observed in the northern heavens. It occurs in many forms, but the most common is that of a luminous arch whose summit is in the magnetic meridian of the place of observation, and from which vivid flashes of light dart towards the zenith. A like phenomenon in the southern heavens is denominated the Aurora Australis. Auroras are most frequent and brilliant in the polar regions, and diminish in intensity towards the equator.

RAINBOWS, HALOS AND CORONAS

Rainbows are arches of prismatic colors, formed by the reflection of rays of light from within drops of water. The rays, which are refracted in entering the drops, are reflected from their posterior surfaces, and again refracted as they re-enter the air, the colors being separated by their unequal refrangibility.

Halos and coronas are circles of prismatic colors which, in certain states of the atmosphere, surround the Sun and the Moon.

Halos are supposed to be occasioned by the presence, in the atmosphere, of small ice crystals which act as minute prisms, decomposing and refracting the light which passes through them.

Coronas are seen when a light mist is floating in the air, and are supposed to be formed by reflection from the external surface of the globules of vapor.

COLORS OF THE SKY AND CLOUDS

The azure tint of the cloudless sky is due to the decomposition and refraction of light, as it passes through layers of air successively increasing in density. The blue and violet, being more refrangible than other colors of the solar spectrum, are diffused through the atmosphere; and being reflected from its particles, they impart to it their own color.

The clouds, floating in the atmosphere, absorb the more refrangible rays, and reflect the less. At sunrise and sunset, when the light traverses the greatest depth of atmosphere, all the colors are absorbed except the red and the yellow; and these, being deflected from the particles of vapor, produce the brilliant coloring of sunrise and sunset.

THE MYSTIFYING MIRAGE

The mirage is an optical phenomenon in which images of distant objects are seen, reflected beneath, or suspended in the heavens above. Occasionally, also, objects are seen double, being repeated laterally instead of vertically.

The mirage is caused by the refraction and reflection of light as it passes from denser to rarer strata of air. It is most frequent in arid plains, where the soil, exposed to the burning rays of the sun, becomes intensely heated, and, in consequence, the strata of air near the ground are less dense than those above.

In this case rays of light passing from any distant object, as a tree, to the ground, are refracted more and more towards the horizontal, until finally they are reflected from a horizontal layer of the heated air, and reach the eye from beneath. Then an image of the object is seen as if mirrored in the tranquil waters of a lake.

[Illustration: =THE MAGNIFICENT CURTAINS OF LIGHT THAT FORM THE AURORA BOREALIS=]

USEFUL MINERALS OF THE EARTH

HOW MINERALS ARE IDENTIFIED

Minerals can be identified and distinguished by various physical properties and by ascertaining their chemical composition. The chief distinguishing physical properties are crystalline form, cleavage, hardness, and specific gravity.

Each mineral or special class of minerals has its own definite geometrical shape or crystalline form. The crystals of each mineral have also a tendency to break or cleave most readily in a particular direction. The term hardness, as applied to minerals and other solid bodies, is used to indicate resistance to being scratched or the power to scratch. The harder of two bodies is the one which will scratch the other, and which resists being scratched by that other.

CRYSTALS THE MOST BEAUTIFUL OF MINERAL FORMS

There are three general classes of crystals--calcareous, silicious and gypsum--but by far the most important are the silicious crystals because of their great hardness. These include quartz or rock crystal--which is quite common--and the so-called _precious stones_, among which are the diamond, rubies, sapphires, etc., a description of which will be found in the Dictionary of Minerals.

To find the relative hardness of substances, a scale has been arranged, beginning with the softest mineral (talc) and ending with the hardest (diamond). The minerals of the scale, therefore, are so arranged that each will scratch any other mineral of lower number in the scale, or be scratched by any of higher number.

=SCALE OF HARDNESS=

MINERAL CHEMICAL NAME 1. Talc. }Can be scratched{ 1. Magnesium silicate. 2. Gypsum (or rocksalt). } by the { 2. Calcium sulphate or } finger-nail { Sodium chloride.

3. Calc-spar. } { 3. Calcium carbonate. 4. Fluor-spar. } Can be { 4. Calcium fluoride. 5. Apatite. } scratched { 5. Calcium phosphate. 6. Felspar. } by knife or { 6. Potassium and } file { aluminum silicates.

7. Quartz (rock-crystal).} Cannot { 7. Silica. 8. Topaz. } be scratched { 8. Aluminum } by { fluosilicate. 9. Corundum (sapphire, } knife { 9. These gems are ruby). } or { crystallized alumina. 10. Diamond. } file {10. Crystallized carbon.

As a first inquiry into the chemical composition of a mineral, dilute hydrochloric or sulphuric acid is tried. All _carbonates_ effervesce when placed in acid or when acid is dropped upon them, while quartz and all the _silicates_ show no effervescense when so treated.

The table on pages 104-7 contains a brief description of the distinctive physical features of a number of the very common or important minerals.

DICTIONARY OF IMPORTANT MINERAL PRODUCTS

=Aluminum=, a metal which does not occur in nature in the free state, but for the most part in combination with silica, as a silicate of aluminum, in clay and many minerals. As extracted from clay by a series of very difficult chemical operations, it forms a white metal, very ductile and malleable, and susceptible of a high polish. On account of its lightness, aluminum is highly valued; it forms excellent alloys.

_Bauxite_ (aluminum hydrate) is the only ore. It is mined in France, Ireland, Austria, Arkansas, Alabama and Georgia, and is refined by electric processes. It is used largely as an addition to iron and steel, preventing bubbles and waste in castings; in electrical work, and for purposes where a light, strong metal is necessary, as in certain machinery, hulls for small boats, etc. Refineries are located in Switzerland, France, Great Britain and United States.

_Cryolite_ (fluoride of aluminum and sodium), a mineral mined only in Greenland, was formerly used as an ore but is now utilized in the manufacture of alum and soda.

_Alum_ (a sulphate) is made from cryolite or clays.

_Corundum_ (aluminum oxide) is, next to the diamond, the hardest natural mineral. Canada, North Carolina, Alabama and India have mines of corundum. Emery is produced chiefly in Greece and Asia Minor. Corundum and emery are powdered for use as abrasives in wheels, sharpening stones, polishing powder and cloth.

_Emery_ is an impure form of corundum.

_Feldspar_ is a silicate of aluminum with other metals. It is mined in Canada, Pennsylvania, Connecticut, New York, Maine and Norway, and ground up for use in pottery making.

_Clay_ is chiefly silicate of aluminum and other metals. _Kaolin_ is its purest form. The properties of clay vary with its composition, as china clay, fire clay, pipe clay, brick clay. Clays are found in all parts of the world as a result of the decomposition of other rocks.

The location of manufacturing centers of pottery of all kinds and of bricks, is dependent on clay deposits. In pottery making, Ohio, New Jersey and Pennsylvania lead the United States. Abroad, fine china is made in France, Germany, Austria, England, Japan, and China.

=TABLE FOR THE IDENTIFICATION OF COMMON MINERALS; THEIR SCIENTIFIC AND COMMON NAMES AND CHIEF CHARACTERISTICS=