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Here the chemist has only succeeded in making a paste or glass having limpidity and a somewhat high refractivity, but wanting the hardness and “fire” of the real stone.

This low refractivity is noteworthy as strongly antagonistic to the view at one time favoured by eminent chemists that argon was a condensed form of nitrogen represented by N3.

Boron dissolves in molten aluminium, and on cooling, transparent, almost colourless crystals are obtained, possessing a lustre, hardness and refractivity near that of the diamond.

Glasses can now be made differing considerably both in refractivity and dispersive power.

In this case small temperature changes alter the refractivity of the liquid without appreciably affecting the quartz.