we now proceed to trace them, from their faintest indication to their most permanent state, through the different conditions of their appearance, which for easier survey we here at once summarily state.
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First condition.—The contact of two smooth surfaces of hard transparent bodies.
First case: if masses or plates of glass, or if lenses are pressed against each other.
Second case: if a crack takes place in a solid mass of glass, chrystal, or ice.
Third case: if lamellæ of transparent stones become separated.
Second condition.—If a surface of glass or a polished stone is breathed upon.