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.
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.